#picodexed — Public Fediverse posts

picoDexed + MiniDexed EuroRack

Since attempting my picoDexed + StackyPi + MiniDexed EuroRack build and failing, I’ve found another Pico-to-Zero board that is provided as open source, so I’ve had some made.

This post details how to get that running with my MiniDexed Zero Eurorack module.

Warning! I strongly recommend using old or second hand equipment for your experiments.  I am not responsible for any damage to expensive instruments!

If you are new to microcontrollers and single board computers, see the Getting Started pages.

Pi Zero RP2040s

As mentioned last time I found a number of options for a RP2040 based dev board in a Raspberry Pi Zero form factor. But this post is about this one:

• https://github.com/bablokb/pcb-pico-pi-base

It is fairly simple to build. It just requires a Pico and GPIO header pins.

There is an option for pull-ups on the I2C bus, but I’ve not bothered with them here. There is also a breakout header for a reset switch if required (it will support two sizes of switch by the looks of things).

Mapping over to MiniDexed/RPi Zero

The pinout is slightly different to the Stacky-pi, so here is an updated table of the GPIO mappings and which are required to be used with my MiniDexed board.

The two key problem areas will be the I2S interface and encoder, which both require consecutive GPIO pins for the PIO code to do its magic.

The encoder should be fine – pins RE A and RE B map onto the Pico’s GP11 and 12.

The I2S interface might be ok – with a BCLK on GP18, it will be expecting LCLK on GP19. Data on GP21 should be ok.

Unlike the previous attempt, I’m hopeful I can just get this running ok with the correct pin mappings…

Changing I2C Bus and UARTs

Unlike the first attempt, I2C is mapped onto GP2 and GP3 which is what I was using in the original picoDexed. So that is all fine, multiplexed onto the I2C bus 1.

There is an issue with the UART however as picoDexed uses the following by default:

• UART 0 – GP 0,1 – Serial debug
• UART 1 – GP 4,5 – MIDI

I can swap these over so that UART0 (GP0,1) is MIDI, but that has to be matched with a change in the debug serial port too. But unfortunately, as far as I can see, that has to be configured in the master CMakeLists.txt file (as I talked about in Part 3).

New picoDexed GPIO Configuration

Given the above, the following new GPIO pins should be defined in config.h:

#define PWM_PIN         10

#define I2S_DATA_PIN    16
#define I2S_BCLK_PIN    18
#define I2S_LRCLK_PIN   19  // Implied by BCLK=12

#define MIDI_UART       0
#define MIDI_TX_PIN     0   // Not used
#define MIDI_RX_PIN     1

#define DEBUG_UART_TX_PIN  8
#define DEBUG_UART_RX_PIN  9

#define DISPLAY_I2C_BUS  1
#define DISPLAY_I2C_SDA  2
#define DISPLAY_I2C_SCL  3
#define DISPLAY_I2C_ADDR 0x3C
#define DISPLAY_W 128
#define DISPLAY_H 32

#define ENCODER_A_PIN  11
#define ENCODER_B_PIN  12 // Not used
#define ENCODER_SW_PIN 10 // Not used

In addition to this, to keep using the debug output requires the following lines adding to ‘target_compile_definitions’ in CMakeLists.txt.

    PICO_DEFAULT_UART=1
    PICO_DEFAULT_UART_TX_PIN=8
    PICO_DEFAULT_UART_RX_PIN=9

I’ve added a separate configuration file (config-ER.h) in the repository to allow this version to be built, but the CMakelists.txt change above has not been included.

I’ve also added a picodexed-v0.03-ER.uf2 file in the build area which can be downloaded and installed directly onto the Pico to provide the above configuration ready to go.

Bringing it all together…

The nice thing about this PCB is that I can map everything nicely over to the pinouts used with my MiniDexed EuroRack PCB meaning that once the Pico has the custom firmware installed, it will just plug in and work and no hardware changes or patching is required at all!

I was slightly concerned that the USB port of the Pico might clash with the two installed electrolytic capacitors on the MiniDexed PCB, but in my case I can just about get away with it!

Here is the final assembled unit.

Closing Thoughts

I finally have my Eurorack picoDexed which is pretty neat. Big thanks to bablokb for putting that PCB up online. That saved me a job. And it was particularly nice that things like assuming consecutive pins for the I2S mapping was included as that made using the PIO I2S code a lot easier.

I’d like to see what the power usage is like now as I’m really after a lower power Dexed engine compared to the full Zero version.

Naturally at some point I might still make my own picoDexed Eurorack PCB, but this is a pretty good solution as far as I’m concerned, so that wouldn’t really add much for me now.

Kevin

#midi #minidexed #pcb #picodexed #raspberryPiPico

picoDexed + MiniDexed EuroRack

Since attempting my picoDexed + StackyPi + MiniDexed EuroRack build and failing, I’ve found another Pico-to-Zero board that is provided as open source, so I’ve had some made.

This post details how to get that running with my MiniDexed Zero Eurorack module.

Warning! I strongly recommend using old or second hand equipment for your experiments.  I am not responsible for any damage to expensive instruments!

If you are new to microcontrollers and single board computers, see the Getting Started pages.

Pi Zero RP2040s

As mentioned last time I found a number of options for a RP2040 based dev board in a Raspberry Pi Zero form factor. But this post is about this one:

• https://github.com/bablokb/pcb-pico-pi-base

It is fairly simple to build. It just requires a Pico and GPIO header pins.

There is an option for pull-ups on the I2C bus, but I’ve not bothered with them here. There is also a breakout header for a reset switch if required (it will support two sizes of switch by the looks of things).

Mapping over to MiniDexed/RPi Zero

The pinout is slightly different to the Stacky-pi, so here is an updated table of the GPIO mappings and which are required to be used with my MiniDexed board.

The two key problem areas will be the I2S interface and encoder, which both require consecutive GPIO pins for the PIO code to do its magic.

The encoder should be fine – pins RE A and RE B map onto the Pico’s GP11 and 12.

The I2S interface might be ok – with a BCLK on GP18, it will be expecting LCLK on GP19. Data on GP21 should be ok.

Unlike the previous attempt, I’m hopeful I can just get this running ok with the correct pin mappings…

Changing I2C Bus and UARTs

Unlike the first attempt, I2C is mapped onto GP2 and GP3 which is what I was using in the original picoDexed. So that is all fine, multiplexed onto the I2C bus 1.

There is an issue with the UART however as picoDexed uses the following by default:

• UART 0 – GP 0,1 – Serial debug
• UART 1 – GP 4,5 – MIDI

I can swap these over so that UART0 (GP0,1) is MIDI, but that has to be matched with a change in the debug serial port too. But unfortunately, as far as I can see, that has to be configured in the master CMakeLists.txt file (as I talked about in Part 3).

New picoDexed GPIO Configuration

Given the above, the following new GPIO pins should be defined in config.h:

#define PWM_PIN         10

#define I2S_DATA_PIN    16
#define I2S_BCLK_PIN    18
#define I2S_LRCLK_PIN   19  // Implied by BCLK=12

#define MIDI_UART       0
#define MIDI_TX_PIN     0   // Not used
#define MIDI_RX_PIN     1

#define DEBUG_UART_TX_PIN  8
#define DEBUG_UART_RX_PIN  9

#define DISPLAY_I2C_BUS  1
#define DISPLAY_I2C_SDA  2
#define DISPLAY_I2C_SCL  3
#define DISPLAY_I2C_ADDR 0x3C
#define DISPLAY_W 128
#define DISPLAY_H 32

#define ENCODER_A_PIN  11
#define ENCODER_B_PIN  12 // Not used
#define ENCODER_SW_PIN 10 // Not used

In addition to this, to keep using the debug output requires the following lines adding to ‘target_compile_definitions’ in CMakeLists.txt.

    PICO_DEFAULT_UART=1
    PICO_DEFAULT_UART_TX_PIN=8
    PICO_DEFAULT_UART_RX_PIN=9

I’ve added a separate configuration file (config-ER.h) in the repository to allow this version to be built, but the CMakelists.txt change above has not been included.

I’ve also added a picodexed-v0.03-ER.uf2 file in the build area which can be downloaded and installed directly onto the Pico to provide the above configuration ready to go.

Bringing it all together…

The nice thing about this PCB is that I can map everything nicely over to the pinouts used with my MiniDexed EuroRack PCB meaning that once the Pico has the custom firmware installed, it will just plug in and work and no hardware changes or patching is required at all!

I was slightly concerned that the USB port of the Pico might clash with the two installed electrolytic capacitors on the MiniDexed PCB, but in my case I can just about get away with it!

Here is the final assembled unit.

Closing Thoughts

I finally have my Eurorack picoDexed which is pretty neat. Big thanks to bablokb for putting that PCB up online. That saved me a job. And it was particularly nice that things like assuming consecutive pins for the I2S mapping was included as that made using the PIO I2S code a lot easier.

I’d like to see what the power usage is like now as I’m really after a lower power Dexed engine compared to the full Zero version.

Naturally at some point I might still make my own picoDexed Eurorack PCB, but this is a pretty good solution as far as I’m concerned, so that wouldn’t really add much for me now.

Kevin

#midi #minidexed #pcb #picodexed #raspberryPiPico

picoDexed + MiniDexed EuroRack

Since attempting my picoDexed + StackyPi + MiniDexed EuroRack build and failing, I’ve found another Pico-to-Zero board that is provided as open source, so I’ve had some made.

This post details how to get that running with my MiniDexed Zero Eurorack module.

Warning! I strongly recommend using old or second hand equipment for your experiments.  I am not responsible for any damage to expensive instruments!

If you are new to microcontrollers and single board computers, see the Getting Started pages.

Pi Zero RP2040s

As mentioned last time I found a number of options for a RP2040 based dev board in a Raspberry Pi Zero form factor. But this post is about this one:

• https://github.com/bablokb/pcb-pico-pi-base

It is fairly simple to build. It just requires a Pico and GPIO header pins.

There is an option for pull-ups on the I2C bus, but I’ve not bothered with them here. There is also a breakout header for a reset switch if required (it will support two sizes of switch by the looks of things).

Mapping over to MiniDexed/RPi Zero

The pinout is slightly different to the Stacky-pi, so here is an updated table of the GPIO mappings and which are required to be used with my MiniDexed board.

The two key problem areas will be the I2S interface and encoder, which both require consecutive GPIO pins for the PIO code to do its magic.

The encoder should be fine – pins RE A and RE B map onto the Pico’s GP11 and 12.

The I2S interface might be ok – with a BCLK on GP18, it will be expecting LCLK on GP19. Data on GP21 should be ok.

Unlike the previous attempt, I’m hopeful I can just get this running ok with the correct pin mappings…

Changing I2C Bus and UARTs

Unlike the first attempt, I2C is mapped onto GP2 and GP3 which is what I was using in the original picoDexed. So that is all fine, multiplexed onto the I2C bus 1.

There is an issue with the UART however as picoDexed uses the following by default:

• UART 0 – GP 0,1 – Serial debug
• UART 1 – GP 4,5 – MIDI

I can swap these over so that UART0 (GP0,1) is MIDI, but that has to be matched with a change in the debug serial port too. But unfortunately, as far as I can see, that has to be configured in the master CMakeLists.txt file (as I talked about in Part 3).

New picoDexed GPIO Configuration

Given the above, the following new GPIO pins should be defined in config.h:

#define PWM_PIN         10

#define I2S_DATA_PIN    16
#define I2S_BCLK_PIN    18
#define I2S_LRCLK_PIN   19  // Implied by BCLK=12

#define MIDI_UART       0
#define MIDI_TX_PIN     0   // Not used
#define MIDI_RX_PIN     1

#define DEBUG_UART_TX_PIN  8
#define DEBUG_UART_RX_PIN  9

#define DISPLAY_I2C_BUS  1
#define DISPLAY_I2C_SDA  2
#define DISPLAY_I2C_SCL  3
#define DISPLAY_I2C_ADDR 0x3C
#define DISPLAY_W 128
#define DISPLAY_H 32

#define ENCODER_A_PIN  11
#define ENCODER_B_PIN  12 // Not used
#define ENCODER_SW_PIN 10 // Not used

In addition to this, to keep using the debug output requires the following lines adding to ‘target_compile_definitions’ in CMakeLists.txt.

    PICO_DEFAULT_UART=1
    PICO_DEFAULT_UART_TX_PIN=8
    PICO_DEFAULT_UART_RX_PIN=9

I’ve added a separate configuration file (config-ER.h) in the repository to allow this version to be built, but the CMakelists.txt change above has not been included.

I’ve also added a picodexed-v0.03-ER.uf2 file in the build area which can be downloaded and installed directly onto the Pico to provide the above configuration ready to go.

Bringing it all together…

The nice thing about this PCB is that I can map everything nicely over to the pinouts used with my MiniDexed EuroRack PCB meaning that once the Pico has the custom firmware installed, it will just plug in and work and no hardware changes or patching is required at all!

I was slightly concerned that the USB port of the Pico might clash with the two installed electrolytic capacitors on the MiniDexed PCB, but in my case I can just about get away with it!

Here is the final assembled unit.

Closing Thoughts

I finally have my Eurorack picoDexed which is pretty neat. Big thanks to bablokb for putting that PCB up online. That saved me a job. And it was particularly nice that things like assuming consecutive pins for the I2S mapping was included as that made using the PIO I2S code a lot easier.

I’d like to see what the power usage is like now as I’m really after a lower power Dexed engine compared to the full Zero version.

Naturally at some point I might still make my own picoDexed Eurorack PCB, but this is a pretty good solution as far as I’m concerned, so that wouldn’t really add much for me now.

Kevin

#midi #minidexed #pcb #picodexed #raspberryPiPico

picoDexed + MiniDexed EuroRack

Since attempting my picoDexed + StackyPi + MiniDexed EuroRack build and failing, I’ve found another Pico-to-Zero board that is provided as open source, so I’ve had some made.

This post details how to get that running with my MiniDexed Zero Eurorack module.

Warning! I strongly recommend using old or second hand equipment for your experiments.  I am not responsible for any damage to expensive instruments!

If you are new to microcontrollers and single board computers, see the Getting Started pages.

Pi Zero RP2040s

As mentioned last time I found a number of options for a RP2040 based dev board in a Raspberry Pi Zero form factor. But this post is about this one:

• https://github.com/bablokb/pcb-pico-pi-base

It is fairly simple to build. It just requires a Pico and GPIO header pins.

There is an option for pull-ups on the I2C bus, but I’ve not bothered with them here. There is also a breakout header for a reset switch if required (it will support two sizes of switch by the looks of things).

Mapping over to MiniDexed/RPi Zero

The pinout is slightly different to the Stacky-pi, so here is an updated table of the GPIO mappings and which are required to be used with my MiniDexed board.

The two key problem areas will be the I2S interface and encoder, which both require consecutive GPIO pins for the PIO code to do its magic.

The encoder should be fine – pins RE A and RE B map onto the Pico’s GP11 and 12.

The I2S interface might be ok – with a BCLK on GP18, it will be expecting LCLK on GP19. Data on GP21 should be ok.

Unlike the previous attempt, I’m hopeful I can just get this running ok with the correct pin mappings…

Changing I2C Bus and UARTs

Unlike the first attempt, I2C is mapped onto GP2 and GP3 which is what I was using in the original picoDexed. So that is all fine, multiplexed onto the I2C bus 1.

There is an issue with the UART however as picoDexed uses the following by default:

• UART 0 – GP 0,1 – Serial debug
• UART 1 – GP 4,5 – MIDI

I can swap these over so that UART0 (GP0,1) is MIDI, but that has to be matched with a change in the debug serial port too. But unfortunately, as far as I can see, that has to be configured in the master CMakeLists.txt file (as I talked about in Part 3).

New picoDexed GPIO Configuration

Given the above, the following new GPIO pins should be defined in config.h:

#define PWM_PIN         10

#define I2S_DATA_PIN    16
#define I2S_BCLK_PIN    18
#define I2S_LRCLK_PIN   19  // Implied by BCLK=12

#define MIDI_UART       0
#define MIDI_TX_PIN     0   // Not used
#define MIDI_RX_PIN     1

#define DEBUG_UART_TX_PIN  8
#define DEBUG_UART_RX_PIN  9

#define DISPLAY_I2C_BUS  1
#define DISPLAY_I2C_SDA  2
#define DISPLAY_I2C_SCL  3
#define DISPLAY_I2C_ADDR 0x3C
#define DISPLAY_W 128
#define DISPLAY_H 32

#define ENCODER_A_PIN  11
#define ENCODER_B_PIN  12 // Not used
#define ENCODER_SW_PIN 10 // Not used

In addition to this, to keep using the debug output requires the following lines adding to ‘target_compile_definitions’ in CMakeLists.txt.

    PICO_DEFAULT_UART=1
    PICO_DEFAULT_UART_TX_PIN=8
    PICO_DEFAULT_UART_RX_PIN=9

I’ve added a separate configuration file (config-ER.h) in the repository to allow this version to be built, but the CMakelists.txt change above has not been included.

I’ve also added a picodexed-v0.03-ER.uf2 file in the build area which can be downloaded and installed directly onto the Pico to provide the above configuration ready to go.

Bringing it all together…

The nice thing about this PCB is that I can map everything nicely over to the pinouts used with my MiniDexed EuroRack PCB meaning that once the Pico has the custom firmware installed, it will just plug in and work and no hardware changes or patching is required at all!

I was slightly concerned that the USB port of the Pico might clash with the two installed electrolytic capacitors on the MiniDexed PCB, but in my case I can just about get away with it!

Here is the final assembled unit.

Closing Thoughts

I finally have my Eurorack picoDexed which is pretty neat. Big thanks to bablokb for putting that PCB up online. That saved me a job. And it was particularly nice that things like assuming consecutive pins for the I2S mapping was included as that made using the PIO I2S code a lot easier.

I’d like to see what the power usage is like now as I’m really after a lower power Dexed engine compared to the full Zero version.

Naturally at some point I might still make my own picoDexed Eurorack PCB, but this is a pretty good solution as far as I’m concerned, so that wouldn’t really add much for me now.

Kevin

#midi #minidexed #pcb #picodexed #raspberryPiPico

picoDexed + MiniDexed EuroRack

Since attempting my picoDexed + StackyPi + MiniDexed EuroRack build and failing, I’ve found another Pico-to-Zero board that is provided as open source, so I’ve had some made.

This post details how to get that running with my MiniDexed Zero Eurorack module.

Warning! I strongly recommend using old or second hand equipment for your experiments.  I am not responsible for any damage to expensive instruments!

If you are new to microcontrollers and single board computers, see the Getting Started pages.

Pi Zero RP2040s

As mentioned last time I found a number of options for a RP2040 based dev board in a Raspberry Pi Zero form factor. But this post is about this one:

• https://github.com/bablokb/pcb-pico-pi-base

It is fairly simple to build. It just requires a Pico and GPIO header pins.

There is an option for pull-ups on the I2C bus, but I’ve not bothered with them here. There is also a breakout header for a reset switch if required (it will support two sizes of switch by the looks of things).

Mapping over to MiniDexed/RPi Zero

The pinout is slightly different to the Stacky-pi, so here is an updated table of the GPIO mappings and which are required to be used with my MiniDexed board.

The two key problem areas will be the I2S interface and encoder, which both require consecutive GPIO pins for the PIO code to do its magic.

The encoder should be fine – pins RE A and RE B map onto the Pico’s GP11 and 12.

The I2S interface might be ok – with a BCLK on GP18, it will be expecting LCLK on GP19. Data on GP21 should be ok.

Unlike the previous attempt, I’m hopeful I can just get this running ok with the correct pin mappings…

Changing I2C Bus and UARTs

Unlike the first attempt, I2C is mapped onto GP2 and GP3 which is what I was using in the original picoDexed. So that is all fine, multiplexed onto the I2C bus 1.

There is an issue with the UART however as picoDexed uses the following by default:

• UART 0 – GP 0,1 – Serial debug
• UART 1 – GP 4,5 – MIDI

I can swap these over so that UART0 (GP0,1) is MIDI, but that has to be matched with a change in the debug serial port too. But unfortunately, as far as I can see, that has to be configured in the master CMakeLists.txt file (as I talked about in Part 3).

New picoDexed GPIO Configuration

Given the above, the following new GPIO pins should be defined in config.h:

#define PWM_PIN         10

#define I2S_DATA_PIN    16
#define I2S_BCLK_PIN    18
#define I2S_LRCLK_PIN   19  // Implied by BCLK=12

#define MIDI_UART       0
#define MIDI_TX_PIN     0   // Not used
#define MIDI_RX_PIN     1

#define DEBUG_UART_TX_PIN  8
#define DEBUG_UART_RX_PIN  9

#define DISPLAY_I2C_BUS  1
#define DISPLAY_I2C_SDA  2
#define DISPLAY_I2C_SCL  3
#define DISPLAY_I2C_ADDR 0x3C
#define DISPLAY_W 128
#define DISPLAY_H 32

#define ENCODER_A_PIN  11
#define ENCODER_B_PIN  12 // Not used
#define ENCODER_SW_PIN 10 // Not used

In addition to this, to keep using the debug output requires the following lines adding to ‘target_compile_definitions’ in CMakeLists.txt.

    PICO_DEFAULT_UART=1
    PICO_DEFAULT_UART_TX_PIN=8
    PICO_DEFAULT_UART_RX_PIN=9

I’ve added a separate configuration file (config-ER.h) in the repository to allow this version to be built, but the CMakelists.txt change above has not been included.

I’ve also added a picodexed-v0.03-ER.uf2 file in the build area which can be downloaded and installed directly onto the Pico to provide the above configuration ready to go.

Bringing it all together…

The nice thing about this PCB is that I can map everything nicely over to the pinouts used with my MiniDexed EuroRack PCB meaning that once the Pico has the custom firmware installed, it will just plug in and work and no hardware changes or patching is required at all!

I was slightly concerned that the USB port of the Pico might clash with the two installed electrolytic capacitors on the MiniDexed PCB, but in my case I can just about get away with it!

Here is the final assembled unit.

Closing Thoughts

I finally have my Eurorack picoDexed which is pretty neat. Big thanks to bablokb for putting that PCB up online. That saved me a job. And it was particularly nice that things like assuming consecutive pins for the I2S mapping was included as that made using the PIO I2S code a lot easier.

I’d like to see what the power usage is like now as I’m really after a lower power Dexed engine compared to the full Zero version.

Naturally at some point I might still make my own picoDexed Eurorack PCB, but this is a pretty good solution as far as I’m concerned, so that wouldn’t really add much for me now.

Kevin

#midi #minidexed #pcb #picodexed #raspberryPiPico

picoDexed + StackyPi + MiniDexed EuroRack

Now that I have a picoDexed with a display an encoder it is very tempting to create a version of my MiniDexed EuroRack PCB for it.

But as a short diversion, there is another possibility – can I use a RP2040 that is already in a Pi Zero form factor with my existing MiniDexed EuroRack PCB Design?

Spoilers: The answer, it turns out, is no. Something is not quite letting this work for me – it’s close, but I’m just not there yet. Here is another attempt where it works: picoDexed + MiniDexed EuroRack.

Warning! I strongly recommend using old or second hand equipment for your experiments.  I am not responsible for any damage to expensive instruments!

If you are new to microcontrollers and single board computers, see the Getting Started pages.

Pi Zero RP2040s

I’m aware of a couple of products that were available to put a RP2040 into a Raspberry Pi Zero form factor:

• Red Robots Pico to Zero adaptor.
• Stacky-Pi by SB Components.
• Waveshare RP2040 PiZero.
• Pico to Pi HAT.
• Update: I’ve just found the following open source design: https://github.com/bablokb/pcb-pico-pi-base – details of how I’ve used this can be found here: picoDexed + MiniDexed EuroRack

The first two were available on Tindie and other places, and as far as I can see, both appear to use the same mapping of RP2040 GPIO to RPi 40-pin header. The Red Robot board doesn’t seem to be available anymore, and whilst there are a few Stacky-Pis on Tindie, in most places it seems to be discontinued.

I don’t know anything about the Waveshare or Pico to Pi, but might grab myself one of each.

I’ll give the open source design a try (and possibly save me designing my own…).

As can be seen above, the Red Robots board is designed to take an actual Pico, whereas the Stacky-Pi is its own board with an on-board RP2040.

Mapping over to MiniDexed/RPi Zero

The following table lists the RPi GPIO connections I used in my MiniDexed board and how they map onto the RP2040 using the above boards.

The two key problem areas will be the I2S interface and encoder, which both require consecutive GPIO pins for the PIO code to do its magic.

The encoder should be fine – pins RE A and RE B map onto the Pico’s GP3 and 4.

The I2S interface is going to be tricky, as with a BCLK on GP28, it will be expecting LCLK on GP29 rather than the GP13 it is currently routed to on the board. Quite apart from the fact that GP29 isn’t even broken out on a Pico.

The obvious thing to explore is if the BCLK connection can be routed through to GP13 on the RPi header, or GP12 for the RP2040. That would be handy if so.

Connecting into GP13 should be possible as this pin is currently unconnected on the MiniDexed PCB. It will involve cutting a track however for GP18. The existing track and its new destination is highlighted below in blue.

Changing I2C Bus and UARTs

There is one other complication however. The picoDexed configuration has the ssd1306 connected to GP2 and GP3 which are multiplexed onto the I2C bus 1.

The adaptor configuration maps SDA/SCL onto GP20/21 which are multiplexed onto I2C bus 0. I updated the code to support changing I2C bus in addition to changing IO pins. Arguably, I should probably have supported this in the first place anyway…

A similar issue exists for the UART, but unfortunately that isn’t quite so easy to change.

I can take a similar approach to the above for the Serial MIDI link – allowing it to use either UART0 or UART1.

But that has to be matched with a change in the debug serial port too. But unfortunately, as far as I can see, that has to be configured in the master CMakeLists.txt file (as I talked about in Part 3).

New picoDexed GPIO Configuration

Given the above, the following new GPIO pins should be defined in config.h:

#define PWM_PIN         10

#define I2S_DATA_PIN    15
#define I2S_BCLK_PIN    12
#define I2S_LRCLK_PIN   13  // Implied by BCLK=12

#define MIDI_UART       0
#define MIDI_TX_PIN     0   // Not used
#define MIDI_RX_PIN     1

#define DEBUG_UART_TX_PIN  8
#define DEBUG_UART_RX_PIN  9

#define DISPLAY_I2C_BUS  0
#define DISPLAY_I2C_SDA  20
#define DISPLAY_I2C_SCL  21
#define DISPLAY_I2C_ADDR 0x3C
#define DISPLAY_W 128
#define DISPLAY_H 32

#define ENCODER_A_PIN  3
#define ENCODER_B_PIN  4 // Not used
#define ENCODER_SW_PIN 2 // Not used

In addition to this, to keep using the debug output requires the following lines adding to ‘target_compile_definitions’ in CMakeLists.txt.

    PICO_DEFAULT_UART=1
    PICO_DEFAULT_UART_TX_PIN=8
    PICO_DEFAULT_UART_RX_PIN=9

Bringing it all together…

I decided to hack on my (already pretty hacked) prototype MiniDexed EuroRack board and cut and patch the trace to the DAC as described above.

The complication being the GPIO header pin I need to get to is under the OLED display, but I could just about do it – see below.

It doesn’t work. Unfortunately.

First of, I have to say, I’ve not found the Stacky-Pi particularly reliable. It took me ages to get it to successfully boot up into accepting a uf2 file, and once installed, I was struggling to get it to reliably boot and run.

Weirdly it seems to work best when I have a finger on the flash chip which seems to imply some board layout/grounding/stability issues to me…

From what I can see, the display, MIDI, serial debug and encoder are working fine though once it does get up and running.

But I just can’t get any reliable sound out of the thing at all. I managed sound once, but that was it. It is all quiet unreliable for me – far too unreliable to be useful.

Shame, as it actually looks really cool!

Closing Thoughts

It has been a frustrating afternoon. I’m having to leave this one here for now as there are just too many unknowns at the moment to really get to the bottom of what is going on.

I thought the Stacky-Pi would be a quick and easy fix, but haven’t ever used them before, and the fact that they are discontinued and there is very little information that I can find online about them makes me think perhaps they aren’t worth persevering with.

So I have a number of options now:

• Try a Waveshare RP2040 PiZero. As there are a lot more peripherals, I’m not sure how much what I’m doing will translate across, to be honest, it cost wise, it’s essentially the same as a Zero itself, which I know “just works”.
• Do I design my own Pico to RPI GPIO converter board to let me use that with my existing MiniDexed EuroRack design? Tempting and probably not that hard.
  • Update: I’ve since found (and ordered) https://github.com/bablokb/pcb-pico-pi-base
    • Update 2: And it works! picoDexed + MiniDexed EuroRack
• Do I attempt to do something with the Pico version of my EuroRack 6HP MCU Experimenter Module? Sounds initially easy but I suspect forcing a MiniDexed into this module eventually will hit other at the moment unforeseen issues.
• Or do I just go for it and put together a special picoDexed EuroRack module itself.

I might have one more go with the Stacky-Pi. I haven’t quite given up. I’ll have to do some research – maybe sprinkling a few capacitors around the board or some updated GND connections might help. Answers on a postcard (or in the comments) if you have any ideas.

I’ll get a Waveshare RP2040 PiZero on order, as I’ve had quite a bit of success with their own “Zero” miniature boards so far, and now I’d like it know if it would work 🙂

To be continued…

Kevin

#define #midi #minidexed #picodexed #raspberryPiPico #StackyPi

picoDexed + StackyPi + MiniDexed EuroRack

Now that I have a picoDexed with a display an encoder it is very tempting to create a version of my MiniDexed EuroRack PCB for it.

But as a short diversion, there is another possibility – can I use a RP2040 that is already in a Pi Zero form factor with my existing MiniDexed EuroRack PCB Design?

Spoilers: The answer, it turns out, is no. Something is not quite letting this work for me – it’s close, but I’m just not there yet. Here is another attempt where it works: picoDexed + MiniDexed EuroRack.

Warning! I strongly recommend using old or second hand equipment for your experiments.  I am not responsible for any damage to expensive instruments!

If you are new to microcontrollers and single board computers, see the Getting Started pages.

Pi Zero RP2040s

I’m aware of a couple of products that were available to put a RP2040 into a Raspberry Pi Zero form factor:

• Red Robots Pico to Zero adaptor.
• Stacky-Pi by SB Components.
• Waveshare RP2040 PiZero.
• Pico to Pi HAT.
• Update: I’ve just found the following open source design: https://github.com/bablokb/pcb-pico-pi-base – details of how I’ve used this can be found here: picoDexed + MiniDexed EuroRack

The first two were available on Tindie and other places, and as far as I can see, both appear to use the same mapping of RP2040 GPIO to RPi 40-pin header. The Red Robot board doesn’t seem to be available anymore, and whilst there are a few Stacky-Pis on Tindie, in most places it seems to be discontinued.

I don’t know anything about the Waveshare or Pico to Pi, but might grab myself one of each.

I’ll give the open source design a try (and possibly save me designing my own…).

As can be seen above, the Red Robots board is designed to take an actual Pico, whereas the Stacky-Pi is its own board with an on-board RP2040.

Mapping over to MiniDexed/RPi Zero

The following table lists the RPi GPIO connections I used in my MiniDexed board and how they map onto the RP2040 using the above boards.

The two key problem areas will be the I2S interface and encoder, which both require consecutive GPIO pins for the PIO code to do its magic.

The encoder should be fine – pins RE A and RE B map onto the Pico’s GP3 and 4.

The I2S interface is going to be tricky, as with a BCLK on GP28, it will be expecting LCLK on GP29 rather than the GP13 it is currently routed to on the board. Quite apart from the fact that GP29 isn’t even broken out on a Pico.

The obvious thing to explore is if the BCLK connection can be routed through to GP13 on the RPi header, or GP12 for the RP2040. That would be handy if so.

Connecting into GP13 should be possible as this pin is currently unconnected on the MiniDexed PCB. It will involve cutting a track however for GP18. The existing track and its new destination is highlighted below in blue.

Changing I2C Bus and UARTs

There is one other complication however. The picoDexed configuration has the ssd1306 connected to GP2 and GP3 which are multiplexed onto the I2C bus 1.

The adaptor configuration maps SDA/SCL onto GP20/21 which are multiplexed onto I2C bus 0. I updated the code to support changing I2C bus in addition to changing IO pins. Arguably, I should probably have supported this in the first place anyway…

A similar issue exists for the UART, but unfortunately that isn’t quite so easy to change.

I can take a similar approach to the above for the Serial MIDI link – allowing it to use either UART0 or UART1.

But that has to be matched with a change in the debug serial port too. But unfortunately, as far as I can see, that has to be configured in the master CMakeLists.txt file (as I talked about in Part 3).

New picoDexed GPIO Configuration

Given the above, the following new GPIO pins should be defined in config.h:

#define PWM_PIN         10

#define I2S_DATA_PIN    15
#define I2S_BCLK_PIN    12
#define I2S_LRCLK_PIN   13  // Implied by BCLK=12

#define MIDI_UART       0
#define MIDI_TX_PIN     0   // Not used
#define MIDI_RX_PIN     1

#define DEBUG_UART_TX_PIN  8
#define DEBUG_UART_RX_PIN  9

#define DISPLAY_I2C_BUS  0
#define DISPLAY_I2C_SDA  20
#define DISPLAY_I2C_SCL  21
#define DISPLAY_I2C_ADDR 0x3C
#define DISPLAY_W 128
#define DISPLAY_H 32

#define ENCODER_A_PIN  3
#define ENCODER_B_PIN  4 // Not used
#define ENCODER_SW_PIN 2 // Not used

In addition to this, to keep using the debug output requires the following lines adding to ‘target_compile_definitions’ in CMakeLists.txt.

    PICO_DEFAULT_UART=1
    PICO_DEFAULT_UART_TX_PIN=8
    PICO_DEFAULT_UART_RX_PIN=9

Bringing it all together…

I decided to hack on my (already pretty hacked) prototype MiniDexed EuroRack board and cut and patch the trace to the DAC as described above.

The complication being the GPIO header pin I need to get to is under the OLED display, but I could just about do it – see below.

It doesn’t work. Unfortunately.

First of, I have to say, I’ve not found the Stacky-Pi particularly reliable. It took me ages to get it to successfully boot up into accepting a uf2 file, and once installed, I was struggling to get it to reliably boot and run.

Weirdly it seems to work best when I have a finger on the flash chip which seems to imply some board layout/grounding/stability issues to me…

From what I can see, the display, MIDI, serial debug and encoder are working fine though once it does get up and running.

But I just can’t get any reliable sound out of the thing at all. I managed sound once, but that was it. It is all quiet unreliable for me – far too unreliable to be useful.

Shame, as it actually looks really cool!

Closing Thoughts

It has been a frustrating afternoon. I’m having to leave this one here for now as there are just too many unknowns at the moment to really get to the bottom of what is going on.

I thought the Stacky-Pi would be a quick and easy fix, but haven’t ever used them before, and the fact that they are discontinued and there is very little information that I can find online about them makes me think perhaps they aren’t worth persevering with.

So I have a number of options now:

• Try a Waveshare RP2040 PiZero. As there are a lot more peripherals, I’m not sure how much what I’m doing will translate across, to be honest, it cost wise, it’s essentially the same as a Zero itself, which I know “just works”.
• Do I design my own Pico to RPI GPIO converter board to let me use that with my existing MiniDexed EuroRack design? Tempting and probably not that hard.
  • Update: I’ve since found (and ordered) https://github.com/bablokb/pcb-pico-pi-base
    • Update 2: And it works! picoDexed + MiniDexed EuroRack
• Do I attempt to do something with the Pico version of my EuroRack 6HP MCU Experimenter Module? Sounds initially easy but I suspect forcing a MiniDexed into this module eventually will hit other at the moment unforeseen issues.
• Or do I just go for it and put together a special picoDexed EuroRack module itself.

I might have one more go with the Stacky-Pi. I haven’t quite given up. I’ll have to do some research – maybe sprinkling a few capacitors around the board or some updated GND connections might help. Answers on a postcard (or in the comments) if you have any ideas.

I’ll get a Waveshare RP2040 PiZero on order, as I’ve had quite a bit of success with their own “Zero” miniature boards so far, and now I’d like it know if it would work 🙂

To be continued…

Kevin

#define #midi #minidexed #picodexed #raspberryPiPico #StackyPi

I've updated my #picoDexed to include a basic encoder/display UI and updated to the latest Pico SDK.

This is a port of Synth_Dexed to the Raspberry Pi PIco.

I also thought it was probably about time I actually documented the basic architecture :)

https://diyelectromusic.com/2025/04/12/raspberry-pi-pico-synth_dexed-revisited/

#MIDI #SynthDIY #DX7 #Synth_Dexed

One last thing I definitely wanted for my PicoDexed was the option for PWM output. This post is by way of a short coda detailing how to do PWM on a Raspberry Pi Pico.

• Part 1 where I work out how to build Synth_Dexed using the Pico SDK and get some sounds coming out.
• Part 2 where I take a detailed look at the performance with a diversion into the workings of the pico_audio library and floating point maths on the pico, on the way.
• Part 3 where I managed to get up to 16-note polyphony, by overclocking, and some basic serial MIDI support.
• Part 4 for the full MIDI implementation, voice loading, SysEx control and USB-MIDI.
• Part 5 details different options for building hardware to run PicoDexed.

The latest code can be found on GitHub here: https://github.com/diyelectromusic/picodexed

Warning! I strongly recommend using old or second hand equipment for your experiments.  I am not responsible for any damage to expensive instruments!

If you are new to microcontrollers, see the Getting Started pages.

PWM Output Circuit

The official “Hardware Design with the RP2040” guide includes a PWM circuit as follows (see section 3.4.1).

Several others have used a slightly simplified version of this circuit, essentially omitting the buffer stage (U3) and perhaps only implementing a single channel. See for example – Raspberry Pi Pico PWM Audio Output Circuit (Jeremy S. Cook) and “pico-pwm-audio” (Robin Grosset).

An alternative is this circuit used by Tod Kurt for his Circuitpython-tricks – “The output circuitry to get line-out levels is a simple 10:1 voltage-divider and a 1uF capacitor to recenter the signal around 0 volts”:

I’m going with Jeremy S. Cook and Robin Grosset and using the following (diagram from here):

This is my solderless breadboard version of the above:

Raspberry Pi Pico Audio PWM

There appears to be two ways of getting audio style PWM signals out of a Pico:

• Use the PWM peripherals.
• Use the PIO.

It would appear that Raspberry Pi’s pico_audio library in the pico-extras repository uses PIO. I haven’t found a clear explanation as to why the built-in PWM peripherals haven’t been used.

Here are some other resources that show alternative descriptions of PWM for audion on a Pico:

• Greg Chadwick’s “Playing with the Pico” series: Part 3 – “PWM Audio” – walks through the basics of using PWM and DMA.
• Ben Everard’s PIO tutorial in Hackspace magazine – a simplified PIO PWM driver.
• chipfire’s rppico-pwm-sound synthesizer – uses the Pico SDK and DMA to the PWM peripheral.
• Pico Playground sine wave example – uses the Pico Extra’s pico_audio library and PIO PWM.
• Robin Grosset’s Raspberry Pi Pico PWM Audio Project – uses the Pico SDK and the PWM peripheral.

As I’m using the pico_audio library for I2S audio, I’m going to use the default PWM pico_audio library too.

To initialise the pico_audio PWM library requires the following:

const audio_pwm_channel_config_t config =
{
  .core = {
    .base_pin = base_pin,
    .dma_channel = dma_ch,
    .pio_sm = pio_sm
  },
  .pattern = 3,
};

const struct audio_format *output_format;
output_format = audio_pwm_setup(pAudioFormat, -1, &config);    
bool status = audio_pwm_default_connect(pBufferPool, false);
audio_pwm_set_enabled(true);

Everything else is the same as for I2S.

At present, overclocking the Pico causes the PWM frequencies to be messed up, so for now the recommended configuration is 8-note polyphony, 24000 sample rate and no overclocking.

The build uses GPIO 20 for PWM.

Closing Thoughts

I’ve uploaded a prototype PWM UF2 file to GitHub too now in case anyone wants to give that a go too: https://github.com/diyelectromusic/picodexed

Hopefully there is enough information in the above to get something up and running.

Kevin

https://diyelectromusic.wordpress.com/2024/02/19/raspberry-pi-pico-synth_dexed-part-6/

#dx7 #midi #picodexed #pwm #raspberryPiPico #usbMidi

Ok, ignore my last comment about Part 4 being my last post on this topic. I thought I ought to pull together all the hardware notes on how to build one as I haven’t really written that down anywhere.

So this shows the connections required between the Pico, I2S DAC and MIDI IN.

• Part 1 where I work out how to build Synth_Dexed using the Pico SDK and get some sounds coming out.
• Part 2 where I take a detailed look at the performance with a diversion into the workings of the pico_audio library and floating point maths on the pico, on the way.
• Part 3 where I managed to get up to 16-note polyphony, by overclocking, and some basic serial MIDI support.
• Part 4 for the full MIDI implementation, voice loading, SysEx control and USB-MIDI.
• Part 6 includes details of how to use PWM output.

The latest code can be found on GitHub here: https://github.com/diyelectromusic/picodexed

Warning! I strongly recommend using old or second hand equipment for your experiments.  I am not responsible for any damage to expensive instruments!

If you are new to microcontrollers, see the Getting Started pages.

The Circuit

The easiest way to get a PicoDexed up and running for me, is to use one of my MIDI Proto PCBs and the Pimoroni I2S Audio Pack. This does not allow for any debug output.

Note the jumpers are set to use UART 1 on GP4 and GP5.

If only USB-MIDI is required, then the Audio Pack can be plugged directly onto the back of the Pico, in which case simply plugging into the Pico via USB to a computer and connected up the audio out will work.

It is also possible to use any 3V3 compatible MIDI module. The following shows the use of a Pimoroni “dual expander” with a MIDI module connected to GND, 3V3_OUT and GP5 (RX1).

This has the advantage that all other GPIO pins are available, so it is relatively straight forward to include a debug link via GP0/GP1 for UART 0.

A Solderless Breadboard Circuit

Of course it is also possible to build everything on a solderless breadboard.

The diagram below shows how a cheap GY-PCM5102 module can be used as the I2S DAC and a serial MIDI interface (optional) can be built using my standard 3V3 MIDI IN circuit.

This also allows for some debug output using UART 0 (GP0/GP1).

Things to note:

• The PCM5102 board will have to have solder jumpers on the rear set as follows: 1=L, 2=L, 3=H, 4=L. Sometimes these come preconfigured with solder bridges, sometimes with zero-ohm SMT resistors, and sometimes with no connection made at all. More details here.
• I always get pins 4 and 5 mixed up on MIDI DIN sockets. Here is my MIDI Connections Cheat Sheet which may help.

Pico GPIO Usage

The following GPIO pins are in use or allocated:

PWM Audio Output

It is possible to use PWM audio output on the Pico. Full details can be found here: Raspberry Pi Pico Synth_Dexed? – Part 6.

But this is a lot more limited than I2S and the quality is a lot poorer too. As a DAC can be obtained quite cheaply, the use of an I2S DAC is strongly recommended.

Closing Thoughts

I’ve uploaded a prototype UF2 file to GitHub in case anyone wants to give it a go: everything can be found here: https://github.com/diyelectromusic/picodexed

Hopefully there is enough information in the above to get something up and running.

Kevin

https://diyelectromusic.wordpress.com/2024/02/18/raspberry-pi-pico-synth_dexed-part-5/

#dx7 #midi #pcm5102 #picodexed #raspberryPiPico #usbMidi

I was going to leave things at Part 3 blog-wise, and just get on with filling in the gaps in code now, but I’ve come back to add a few more notes. But this is likely to be the final part now.

Recall so far, I have:

• Part 1 where I work out how to build Synth_Dexed using the Pico SDK and get some sounds coming out.
• Part 2 where I take a detailed look at the performance with a diversion into the workings of the pico_audio library and floating point maths on the pico, on the way.
• Part 3 where I managed to get up to 16-note polyphony, by overclocking, and some basic serial MIDI support.

This is building on the last part and includes notes on how I’ve implemented the following:

• Fuller MIDI support, including control change, program change and pitch bend messages.
• Voice and voice banks, selectable over MIDI.
• MIDI SysEx messages for voice parameters.
• USB MIDI device support.

The latest code can be found on GitHub here: https://github.com/diyelectromusic/picodexed

Warning! I strongly recommend using old or second hand equipment for your experiments.  I am not responsible for any damage to expensive instruments!

If you are new to microcontrollers, see the Getting Started pages.

MIDI Support

I’m not going to walk through all the details of how I’ve added MIDI but suffice to say that once again the implementation owes a lot to MiniDexed and the Arduino MIDI Library.

At the time of writing the following are all supported as they were already supported in Synth_Dexed, so I just needed to glue the bits together.

Channel Voice Messages (only channel 1 at present)

Channel Control Change Messages

* There is a bug with the master tuning. It ought to accept -99 to 99 I believe, but only 0..99 will actually register and there is no way to send -99 via MIDI at the moment. I need to read up on what is going on here and what it ought to do!

** The Mono Mode parameter has the option for specifying how many of the playable voices can be dedicated to mono mode (at least I think that is what it is saying). I only support a value of 0 which I believe is meant to mean “all available voices”.

System Messages

System Exclusive Messages

Any valid Yamaha (DX) system exclusive messages are passed straight into Synth_Dexed. A Yamaha (DX) message has the following format (see the “DX7IIFD/D Supplemental Booklet: Advanced MIDI Data and Charts”):

F0 - start SysEx message
43 - Yamaha manufacturer ID
sd - s=substatus (command class:0,1,2); d=device ID (0..F)
.. data ..
F7 - end SysEx message

The device ID can be set using the UI on a real DX7 to a value between 1 and 16, which becomes a value between 0 and 15 (0..F) as part of the SysEx message (see “DX7IIFD/D Supplemental Booklet: Advanced MIDI Applications, Section 8”). It is a Systems Exclusive value analogous to the MIDI channel for regular channel messages.

There are a range of Sys Ex parameter settings that have been passed onto Synth_Dexed as follows:

At this stage, all of the MIDI support is on a “it’s probably something like this” basis, so it will evolve as I find out what it is meant to be doing!

Voice and Bank Loading

Banks of voices are programmed directly into the code. There is a python script from Synth_Dexed that will take a .syx format voice bank and generate a block of C code. I’ve included a script to download the main 8 banks of standard DX voices and run the script:

#!/bin/sh

# Get voices from
# https://yamahablackboxes.com/collection/yamaha-dx7-synthesizer/patches/

mkdir -p voices

DIR="https://yamahablackboxes.com/patches/dx7/factory"

wget -c "${DIR}"/rom1a.syx -O voices/rom1a.syx
wget -c "${DIR}"/rom1b.syx -O voices/rom1b.syx
wget -c "${DIR}"/rom2a.syx -O voices/rom2a.syx
wget -c "${DIR}"/rom2b.syx -O voices/rom2b.syx
wget -c "${DIR}"/rom3a.syx -O voices/rom3a.syx
wget -c "${DIR}"/rom3b.syx -O voices/rom3b.syx
wget -c "${DIR}"/rom4a.syx -O voices/rom4a.syx
wget -c "${DIR}"/rom4b.syx -O voices/rom4b.syx

./synth_dexed/Synth_Dexed/tools/sysex2c.py voices/* > src/voices.h

This only needs to be run once to create the src/voices.h file which is then included in the build.

Voices have the following format:

uint8_t progmem_bank[8][32][128] PROGMEM =
{
    {      // Bank 1
        {<--128 bytes of packed voice data-->} // Voice 1
        ...
        {<--128 bytes of packed voice data-->} // Voice 32
    }
    {      // Bank 2
        ...
    }
    ...
    {      // Bank 8
        {<--128 bytes of packed voice data-->} // Voice 1
        ...
        {<--128 bytes of packed voice data-->} // Voice 32
    }
}

The system assumes 8 banks of 32 voices each, in the “packed” SYX header format, meaning each voice consists of 128 bytes.

MIDI Bank and Voice Selection

As there are only 8 banks, only BANKSEL (LSB) values 0..7 are valid. Program Change will work in two ways however:

• 0..31 will select voices 1 to 32 in the current bank.
• 31..127 will select voices from the following three adjacent banks.

To select any voice in all 8 banks thus requires the following sequence:

BANKSEL MSB = 0
BANKSEL LSB = 0..7
PROG CHANGE = 0..31

But if bank selection is skipped, then Program Change messages can still be used to select one of the first 128 voices across four consecutive banks.

USB MIDI

The Raspberry Pi Pico SDK uses the TinyUSB protocol stack to implement USB device or host modes and there is an additional option to implement a second USB host port using the Pico’s PIO.

However, USB MIDI appears to only be supported for USB devices at the time of writing, so I’m just using the built-in USB port as a USB device, based on the code provided as part of the TinyUSB examples (more details of how to get basic USB MIDI running here).

TinyUSB MIDI supports two interfaces for reading data, and this wasn’t immediately obvious from the example as that is only sending data and ignores anything coming in.

• USB MIDI Stream mode: this will fill a provided buffer with MIDI data received over USB.
• USB MIDI Packet mode: this will return each 4-byte USB packet individually.

From what I can see of the USB MIDI Spec, all MIDI messages are turned into 4-byte packets for transferring over USB. All normal MIDI messages will consist of 1, 2 or 3 byte messages, and so will fit in a packet each – any unused bytes are padded with 0.

However SysEx messages are a little more complicated and have to be split across multiple packets.

This is the format for a USB MIDI Event Packet (see the “Universal Serial Bus Device Class Definition for MIDI Devices”, Release 1.0):

The code index number is an indication of the contents of each packet. For channel messages, this is basically a repeat of the MIDI command, so a MIDI Note On message might look something like the following:

09 92 3C 64
   Cable 0
   Code Index Number 9
   MIDI Cmd 0x90 (Note On)
   MIDI Channel 3 (0x0=1; 0x1=2; 0x2=3; ... 0xF=16)
   Note 0x3C (60 = C4)
   Velocity 0x64 (100)

But things get a little more complex with System Common or System Exclusive messages which have their own set of codes, depending on the chunking of the packets required.

The critical ones for SysEx are CIN=4,5,6,7 which correspond to SysEx start and then various versions of continuation or end packets. So a larger SysEx message might look something like the following

04 F0 43 10 -- SysEx Start or Continuation
04 34 44 4D -- SysEx Start or Continuation
06 3E F7 00 -- SysEx End after two bytes

Complete message: F0 43 10 34 44 4D 3E F7

So, if I opt to use the packet interface to TinyUSB MIDI then all this has to be sorted out in user code myself. However, the streaming interface will take care of all this for me and just return a buffer full of “traditional” MIDI messages.

Note that there is no concept of Running Status in USB MIDI. Even the oldest USB standard protocol speeds are an order of magnitude, or more, higher than serial MIDI so it isn’t necessary. Every MIDI message will either be a complete 1,2,3 byte message in a single USB packet, or a SysEx multi-packet message as described above.

The basic structure of the USB MIDI handler is as follows:

Init:
   Initialise TinyUSB MIDI stack

Process:
   Run the TinyUSB MIDI task
   IF TinyUSB says MIDI data available:
      Call the stream API to fill our RX buffer
      WHILE data in the RX buffer:
         Call the MIDIParser which reads from the RX buffer
         IF MIDI messages found:
            Call the MIDI Message Handler

Read:
   Grab the next byte from the RX buffer

I’ve actually split this over two files: usbmidi.cpp is the companion to serialmidi.cpp and provides the class that inherits from MIDIDevice (which provides the parser and message handler); usbtask.c provides the interface into the TinyUSB C driver code.

I haven’t done anything special with a USB manufacturer/vendor and device ID yet – so at some point I should see what TinyUSB is using by default and find something unique to PicoDexed (assuming I take it forward in any useful way).

Closing Thoughts

I have a fairly complete implementation now, which is quite nice. I do need to find some way to properly exercise the voice loading over SysEx and it would be good to get some idea of the performance when I throw a MIDI file at it over USB!

I’ve tested some of the parameter changes using the PC version of Dexed. When configured correctly, this can be used to send voice parameter changes to PicoDexed, but I haven’t found a way to download the entire voice as yet.

It’s a shame I can’t just plug in a USB MIDI controller and play it now, but I’ll work on some kind of interface board that should allow me to do it. It will need to be independently powered to act as a USB host anyway.

This is probably going to be my last blog post on PicoDexed for now, but I plan to keep tinkering away at the GitHub repository to see how things go. There are still a couple of limitations, the main one being that everything has to be hard-coded in at present. It would be nice to be able to have some kind of system configuration facility for the MIDI channel if nothing else.

At some point it would also be nice to have a build on the GitHub so others can try it too. And I still need to decide how best to manage the changes I needed to make to Synth_Dexed.

Kevin

https://diyelectromusic.wordpress.com/2024/02/16/raspberry-pi-pico-synth_dexed-part-4/

#dx7 #midi #picodexed #raspberryPiPico #usbMidi

For a while I’ve wanted to get Synth_Dexed up and running on a Raspberry Pi Pico. This is the library written by Holger Wirtz for use with the Teensy microcontroller, and the core synth engine used for MiniDexed.

This is how I got everything up and running although it is too early to know if this is a worthwhile activity or not!

Note: this is just the very first set of tests to see if anything is even feasible, so don’t expect a playable, working synth! It is just making some hardcoded sounds for the time being.

And the performance isn’t what anyone would call stellar… I can currently manage 4-note polyphony if the sample rate is dropped. That is about it right now!

• Part 1: Building Synth_Dexed for the Pico.
• Part 2: Assessing the performance and analysis of the Pico audio library.
• Part 3: MIDI and some basic usability (finally).
• Part 4: More MIDI; Bank and Voice loading; SysEx support; and USB-MIDI.
• Part 5: Details of how to build the hardware.

This is based on ideas and information found from examining the following:

• Synth_Dexed library by Holger Wirtz.
• MiniDexed initial port and code by Rene Stange and Holger Wirtz based on an idea by probonopd discussed here: https://github.com/rsta2/circle/issues/274.
• Earle Philhower’s Raspberry Pi Pico Arduino core for all RP2040 boards.
• Chris Hockuba’s Raspberry Pi Pico version of CMSIS 5.

Reference material:

• Getting Started with Raspberry Pi Pico
• Raspberry Pi Pico C/C++ SDK
• cmake documentation – reference manual and learning resources
• CMSIS 5

Warning! I strongly recommend using old or second hand equipment for your experiments.  I am not responsible for any damage to expensive instruments!

If you are new to microcontrollers, see the Getting Started pages.

Intro and Hardware Requirements

The core aim is to be able to run a instance of Synth_Dexed on one of the cores of a Raspberry Pi Pico and feed it information that controls how it generates sound and then play the samples back out over some kind of audio interface.

Eventually it should receive this information from MIDI or some kind of built-in interface. Sound output ideally would support an I2S audio DAC – I’ve started with the Pimoroni Pico Audio Pack – but I’d like to add PWM and possibly others.

It remains to be seen if the performance maps a single instance to a core or allows for several instances to be running concurrently and at what level of polyphony.

Other requirements that I’m chewing over:

• Polyphonic to some level.
• MIDI.
• Should allow for flexible separation of interface and synth engine.
• Should allow for several Picos to act together to provide multiple synth engines.
• Supports voice bank loading from the Pico’s onboard flash memory “disk”.
• Use the Synth_Dexed library with the minimum necessary changes – ideally none!

To get started and start experimenting will eventually requires the following hardware:

• Raspberry Pi Pico.
• Means of getting audio out – initially its only doing I2S, but PWM could be added.
• Eventually some means of getting control signals in (USB or serial MIDI or hard-coded).

The following GPIO pins are being used:

• I2S: I2S_DATA = GP9; I2S_CLOCK = GP10
• Debug UART: TX = GP0; RX = GP1
• MIDI (eventually): TX = GP4; RX = GP5

Note that the Pimoroni Audio Pack also makes use of GP22 as a mute switch, but I’m not using that.

The Environment

I set up the Raspberry Pi C/C++ SDK according to my previous set of instructions from here: Getting Started with the Raspberry Pi Pico C/C++ SDK and TinyUSB MIDI.

Once again I’m doing all this in the Ubuntu virtual machine.

The created a picodexed project with the following structure:

picodexed
    +--- build
    +--- cmsis
    +--- src
    +--- synth_dexed 

I’m not going through the whole discovery process of how I got to this point, but here are a few notes of things I learned on the way in case I need to refer back to them!

CMSIS is required to support some of the ARM DSP functions used by Synth_Dexed, but not all of it is necessary. After hunting around for how to build this into the Raspberry Pi Pico (the SDK only has some very basic interface definitions, not the whole proper thing) I eventually stumbled across Chris Hockuba’s Raspberry Pi Pico CMSIS – cmsis-pi-pico – on Gitlab so I cloned that into my cmsis area and used that.

A really useful clue as to what is required to get Synth_Dexed running on a different architecture to the original Teensy can be found from examining the following files created as part of the MiniDexed discussion:

• Synth_Dexed/src/synth.h – contains the following useful functions/macros required by the library, but re-implemented for circle: constrain, signed_saturate_rshift, millis.
• MiniDexed/src/Synth_Dexed.mk – contains a list of what has to be built to create the synth engine, including which bits of CMSIS are required.
• MiniDexed/src/dexedadaptor.h – a shallow wrapper around Dexed to control access to its key functions.
• Synth_Dexed/examples/SimplePlay – a sample application of Dexed built for the Teensy using its Audio libraries.

In terms of approach, I basically created the most basic main.cpp I could and created CMakeLists.txt files for the main application and the Synth_Dexed library and then just kept trying to build it to see what was still broken.

I had a frustrating diversion for a while, not noticing that I’d created a main.c and not a main.cpp and that when it included dexed.h from Synth_Dexed it couldn’t find <cstdlib> – the C++ standard library… I spent ages trying to work out why my compiler couldn’t find its own libraries…

One thing that appeared to be missing totally though, was a definition of boolean. Now I could have gone through Synth_Dexed changing boolean to bool, but in the end implemented a “filler” or “wrapper” header file with everything extra that Synth_Dexed needed but was missing – dexed_if_common.h – and stuck a typedef bool boolean; in there.

So far, the only change I’ve had to make to the library itself is to add the following line into Synth_Dexed/src/dexed.h and then make sure the path for include files can find it during compilation:

#include "dexed_if_common.h"

I still haven’t quite got to grips with the whole cmake infrastructure required for building Raspberry Pi Pico projects, so the CMakeLists.txt files I’ve created are almost certainly sub-optimal at present. I still need to get my head around PRIVATE, PUBLIC, INTERFACE qualifiers for example and probably have more directories defined for INCLUDE paths than is strictly necessary.

But it builds and everything I have so far can be found on GitHub. This includes a simple script to take the basic repository and add in CMSIS and Synth_Dexed and hack in that single change to dexed.h. This means that to build what I have so far requires the following:

1. Install the Pico C/C++ SDK, toolchain and libraries (as I described here) including setting PICO_SDK_PATH to the location of the Pico SDK installation.
2. Clone the picodexed repository.
3. Run the getsubmod.sh script (only run this once for a fresh repository).
4. Then build:

kevin@ubuntu:~/src/picodexed$ cd build
kevin@ubuntu:~/src/picodexed/build$ cmake ..
kevin@ubuntu:~/src/picodexed/build$ make
kevin@ubuntu:~/src/picodexed/build$ 

Adding Audio

The Pico has no in-built audio capability, but there is an audio library that can be found in the “pico_extras” repository that supports I2S audio interfaces or PWM audio output on GPIO pins, both implementing using the Pico’s PIO subsystem.

There is an example in the “pico_playground” that shows how to output a sine wave and Pimoroni also have a sample mini-synth application for their I2S Pico Audio Pack.

Links:

• https://github.com/raspberrypi/pico-extras
• https://github.com/raspberrypi/pico-playground
• https://github.com/pimoroni/pimoroni-pico/tree/main/examples/pico_audio

In order to build for use with the pico_audio library, the pico_extras repository needs to be cloned into the source area and the location specified using PICO_EXTRAS_PATH.

There is another cmake file that can then be copied into your own project area to include the library.

cp $PICO_EXTRAS_PATH/external/pico_extras_import.cmake .

And then it can be included in the project’s CMakeLists.txt file, pico_audio_i2s can be added to the list of link libraries, and a compilation definition added to define USE_AUDIO_I2S.

include(pico_extras_import.cmake)

target_link_libraries(picodexed PUBLIC synth_dexed pico_stdlib tinyusb_device tinyusb_board pico_audio_i2s)

target_compile_definitions(picodexed PRIVATE
USE_AUDIO_I2S=1
)

The actual examples for audio output are quite complicated and so far the documentation I’ve found for the pico_audio libraries is pretty minimal.

The Pimoroni build has abstracted most of the functionality out into a separate audio.hpp file with the following interface:

// init_audio initialises the audio library and returns a pointer
// to an object representing the audio buffers.
struct audio_buffer_pool *init_audio(uint32_t sample_rate, uint8_t pin_data, uint8_t pin_bclk, uint8_t pio_sm=0, uint8_t dma_ch=0);

// update_buffer will fill the audio buffer with samples returned
// by the provided callback function.
void update_buffer(struct audio_buffer_pool *ap, buffer_callback cb);

The callback function that is meant to provide samples to the audio subsystem has the following definition:

int16_t getNextSample (void);

Dexed has a getSamples function that can fill a buffer with the next set of samples in either float or int16_t format, but the callback function only returns a single sample at a time.

Rather than attempt to make that fit (I did start off that way) I wrote my own “update_buffer” function to fill the Dexed buffer:

void fillSampleBuffer(struct audio_buffer_pool *ap) {
  struct audio_buffer *buffer = take_audio_buffer(ap, true);
  int16_t *samples = (int16_t *) buffer->buffer->bytes;
  dexed.getSamples(samples, buffer->max_sample_count);
  buffer->sample_count = buffer->max_sample_count;
  give_audio_buffer(ap, buffer);
}

Once slight complication is that the Dexed getSamples function is a protected function. To interface into Dexed I’ve thus created a Dexed Adaptor based on the MiniDexed dexedadaptor. I don’t know how many of these functions will really need to be a wrapper around Dexed, but the capability is there if required.

Typically with embedded audio applications, there is a regular “internal buffer out to audio hardware” layer that has to run at the sample rate and be reliably regular with its timing. This would usually be driven from a timer interrupt or similar. Then there will be a non-timing-critical part of code responsible for filling the buffer itself with samples to be played.

The Pico’s audio library is quite different. Unfortunately, I can’t quite see with the combination of complex C buffer management code, use of the Pico’s DMA and use of the PIO hardware for the I2S protocol itself, quite what the timing expectations of the pico_audio library are and how they should be split up, but from what I can gather, the basic idea is as follows:

• PIO is automatically pulling data from a set of buffers to send over I2S.
• DMA is providing that data somehow at a rate independent of the CPU, but maybe driven by interrupts linked to data requests and the PIO somehow.
• All I have to do is keep the buffer that can be accessed by the DMA fill of audio samples and magic “just happens”.

So for the time being, I’m just calling my update audio equivalent function in a relatively fast loop.

Unfortunately my first attempts all resulting in some kind of horrid growl. But it does turn on and off when the Dexed.keydown and Dexed.keyup calls are made so I was making progress!

Things that eventually fixed that:

• I updated the init_audio function to allow me to pass in the sample buffer size and chose a buffer size of 128 and then ensured this was set to the same in Dexed itself.
• I saw the sine example was using a sample rate of 24000 for I2S so I used that sample rate here too.
• The thing that eventually fixed everything was realising that Dexed is returning a mono stream of data but the default I2S interface is assuming a stereo set of samples. Setting the following in CMakeLists.txt told the I2S library to use mono:

target_compile_definitions(picodexed PRIVATE
    PICO_AUDIO_I2S_MONO_OUTPUT=1
    PICO_AUDIO_I2S_MONO_INPUT=1
    USE_AUDIO_I2S=1
)

The sine library seems to get away with only defining MONO_OUTPUT, but I was getting compilation errors, so I set MONO_INPUT too, even though I’m not using I2S input.

At this point I was getting a nice note playing via Dexed, so I grabbed the voice parameters for Brass 1 out of MiniDexed and dropped them into an array here and with a call to Dexed.loadVoiceParameters, I have a Raspberry Pi Pico playing the Dexed Brass 1 sound!

In terms of sample rates and polyphony, well, things are a little basic! I can currently achieve the following:

• 2-note polyphony at 44100.
• 4-note polyphony at 24000.
• 6-note polyphony, possibly if you squint at it and ask it really nicely, at 12000.

So it isn’t going to win any prizes for quality or quantity, but this is a point that I feel it’s now worth pushing this post out and sticking the progress up on GitHub.

Next, I want some simple MIDI control to act as a real synth. Other things on the “todo” list:

• See what scope there is for optimisation to increase the polyphony ideally with 44.1kHz.
• Specifically look into how it is currently doing the floating point maths.
• Investigate the loading of the core to see how the performance is looking more generally.
• Get some additional voices in there!

Removing Dependency on CMSIS

It turns out that only a few functions are required from the CMSIS/DSP area, so in order to examining them more closely I created my own version of arm_math.h with a corresponding arm_math.c containing the following functions:

void arm_float_to_q15()
void arm_fill_f32()
void arm_sub_f32()
void arm_scale_f32()
void arm_offset_f32()
void arm_mult_f32()
void arm_biquad_cascade_df1_f32()
void arm_biquad_cascade_df1_init_f32()

with any additional definitions they require. Turns out it isn’t very many. The only thing left is to mirror how Synth_Dexed builds for the Teensy and ensure that the compressor isn’t included. This requires some conditional compilation in dexed.h, dexed.cpp and compressor.h.

There doesn’t appear to be a single obvious way to spot a build for a Raspberry Pi Pico though, but there is a definition of RASPBERRYPI_PICO in <board/pico.h> so I used that.

Find it on GitHub

The current state of progress can be found on GitHub here: https://github.com/diyelectromusic/picodexed

Closing Thoughts

I first wanted to do this when I first installed the Pico SDK back in August 2022! Yes it really has taken me that long to even attempt it. Mostly because my C is still quite rusty; my C++ is very much “learn as you go”; I’ve not worked with a CMSIS ARM embedded system in detail before; and the cmake infrastructure for a Raspberry Pi Pico seems so very complicated for what it is.

But you’ve got to start somewhere as they say, and I learn best from having a reason to do something. So even if this doesn’t amount to anything, it is finally making me learn about all these things in a useful way that will hopefully end up with something I’ll have some fun playing with.

The big limitation might be floating point maths. But the Pico does have some built-in fast floating point routines. I don’t know if any of these are enabled and running at the moment. I suspect the CMSIS DSP code is probably doing a “soft” floating point, so that is something to look into. But if it is already using the faster routines, then it may well keep the polyphony so low that it isn’t particularly practical to use. To be continued…

But once again, my renewed respect to all the above-mentioned people who have essentially already done most of the hard work for projects like this that allow someone like me to bumble along and join the various bits together!

By the way, seeing as MiniDexed exists for the Raspberry Pi and MicroDexed exists for the Teensy, PicoDexed seemed the obvious choice!

Kevin

https://diyelectromusic.wordpress.com/2024/01/09/raspberry-pi-pico-synth_dexed/

#CCSDK #dexed #picodexed #raspberryPiPico