Eurorack PMOD

Eurorack PMOD is a certified open hardware Eurorack module that plugs directly into many FPGA boards, which makes it easy to combine the world of FPGAs and hardware electronic music synthesis. The latest (R3.3) hardware looks like this and can be ordered here.

How does it work? (tutorial video)

• Plug eurorack-pmod into an FPGA development board of your choice. Here is a list of boards already supported by the examples.
• Get started with some example DSP cores. Examples include calibration, sampling, effects, synthesis sources and so on. The design files can be synthesized to a bitstream using Yosys' oss-cad-suite.

:notes: Project Showcase :sparkles:

• FPGA-based neural-net waveshaper - (from matpalm@)
• Teensy 4.x I2S TDM w/ Eurorack PMOD - (from lytrix@)
• 8-channel USB2 soundcard - mostly in Amaranth.
• Daisy Seed TDM for Eurorack PMOD - (from heartwerker@)
• apu_chord core - (from proppy@)
• Real-time DSP with Rust on LiteX - mostly in Rust/Migen.

R3.3 hardware details

• 3HP module compatible with modular synthesizer systems.
  • Module depth is 35mm with both ribbon cables attached.
• PMOD connector compatible with many FPGA development boards.
• 8 (4 in + 4 out) DC-coupled audio channels, 192KHz / 32bit sampling supported.
• PWM-controlled, user-programmable red/green LEDs on each output channel.
• Jack insertion detection on input & output jacks.
• Calibration EEPROM for unique ID and storing calibration data.
• [new!] Touch and proximity sensing on all unused jacks as an extra input method.
  • See the gateware/cores/touch_cv.sv core for an example of how to use this.

Compared to R3.1, the changes across R3.2 and R3.3 are summarized here

Want one?. More photos can be found below.

Included examples

This repository contains a bunch of example DSP cores which are continuously being updated:

• gateware/cores/pitch_shift.sv - Pitch shifter
• gateware/cores/filter.sv - Filter (low pass + resonance)
• gateware/cores/touch_cv.sv - Touch sensing to CV converter
• gateware/cores/clkdiv.sv - Clock divider
• gateware/cores/sampler.sv - .wav sampler
• gateware/cores/seqswitch.sv - Sequential routing switch
• gateware/cores/digital_echo.sv - Echo / delay effect
• gateware/cores/vca.sv - VCA (voltage controlled amplifier)
• gateware/cores/vco.sv - VCO (voltage controlled oscillator)
• gateware/cores/bitcrush.sv - Bitcrusher

These examples can all run out of the box on the development boards listed below.

Choosing an FPGA development board

An FPGA development board itself is NOT included! Essentially anything iCE40 or ECP5 based that has a PMOD connector will support the open-source tools and the examples in this project. Just make sure you have enough LUTS, >3K is enough to do interesting things.

The following development boards have been tested with eurorack-pmod and are supported by the examples in the github repository

• iCEbreaker (iCE40 based)
• iCESugar (iCE40 based, iCE40UP5K version)
• ECPIX-5 (ECP5 based)
• Colorlight i5 (ECP5 based)
• Colorlight i9 (ECP5 based)
• pico-ice from TinyVision (iCE40 based)
• CCGM1A1 Gatemate EVB from CologneChip - experimental! see here

PMOD Pinout

The PMOD pinout is on the silkscreen on the back side of the board. Details are below. Note that Pin 1 is the SQUARE pad.

1) SDI (AK4619VN SDIN1) 2) SCL (I2C SCL for AK4619VN CODEC, EEPROM, LED + JACK IO expanders) 3) SDO (AK4619VN SDOUT1) 4) SDA (I2C SDA) 5) LRCK (CODEC clock line) 6) PDN (CODEC power down, also connected to LED output enable and JACK reset line -- HIGH means everything is on) 7) BICK (CODEC clock line) 8) MCLK (CODEC clock line) 9) GND 10) GND 11) 3V3 IN 12) 3V3 IN

Getting Started

I have tested builds on Linux, Mac and Windows (under MSYS2). All are tested in CI.

0. Install the OSS FPGA CAD flow.

   • You may be able to get yosys / verilator from other package managers but I recommend using the releases from YosysHQ so you're using the same binaries that CI is using.
   • On Linux, once the YosysHQ suite is installed and in PATH, you should be able to just use make in the gateware directory.
   • On Windows, CI is using MSYS2 with MINGW64 shell. Install MSYS2, MINGW64, extract the oss-cad-suite from YosysHQ and add it to PATH. Then you should be able to use make in the gateware directory.
   • Note: The gateware is automatically built and tested in CI, so for either platform it may be helpful to look at .github/workflows/main.yml.

1. Build or obtain eurorack-pmod hardware and connect it to your FPGA development board using a ribbon cable or similar. (Double check that the pin mappings are correct, some ribbon cables will swap them on you! Default pinmaps are for the ribbon cables I shipped with hardware, you need to flip the pinmaps for a direct connection PMOD -> FPGA)

2. Try some of the examples. From the gateware directory, type make to see valid commands. By default if you do not select a CORE it will compile a bitstream with the 'mirror' core, which just sends inputs to outputs. Note, you'll need to run git submodule update --init --recursive from the repo root after checkout.

3. Calibrate your hardware using the process described in gateware/cal/cal.py. Use this to create your own gateware/cal/cal_mem.hex to compensate for any DC biases in the ADCs/DACs. (this step is only necessary if you need sub-50mV accuracy on your inputs/outputs, which is the case if you are tuning oscillators, not so much if you are creating rhythm pulses.

Project structure

The project is split into 2 directories, hardware for the PCB/panel and gateware for the FPGA source. Some interesting directories:

• gateware/cores: example user core implementations (i.e sequential switch, bitcrusher, filter, vco, vca, sampler etc).
• gateware/top.sv: top-level gateware with defines for selecting features.
• gateware/cal/cal.py: tool used to calibrate the hardware after assembly, generating calibration memory.
• gateware/drivers: driver for CODEC and I2C devices used on this board.
• hardware/eurorack-pmod-r3: KiCAD design files for PCB and front panel.
• hardware/fab: gerber files and BOM for manufacturing the hardware.

Manufacturing

R3.3 hardware is in stock order here :)

R3.1 Hardware (no longer manufactured)

The above README focuses on R3.3, which is currently being manufactured.

Revision R3.1 was sold out in 2023. It's no longer manufactured, however this repository still supports it if you use the HW_REV=HW_R31 flag when building. I left some of the old photos here in case they are useful.

From the gateware perspective, there is almost no difference between R3.1 and R3.3 and so any cores should be compatible with both (unless they use new features of R3.3 e.g. touch sensitive jacks).

R3.1 boards

R3.1 hardware details

R3.1 technical

• 3HP module compatible with modular synthesizer systems.
  • Module depth is 47mm with both ribbon cables attached
  • This fits nicely in e.g. a 4MS POD 48X (pictured below).
• PMOD connector compatible with most FPGA development boards.
• 8 (4 in + 4 out) DC-coupled audio channels, 192KHz / 32bit sampling supported.
• PWM-controlled, user-programmable red/green LEDs on each output channel.
• Jack insertion detection on input & output jacks.
• Calibration EEPROM for unique ID and storing calibration data.
• I/O is about +/- 8V capable, wider is possible with a resistor change.

Known limitations

• Moved to github issues

Photos

eurorack-pmod R3.1 connected to iCEBreaker

License

Hardware and gateware are released under the CERN Open-Hardware License V2 CERN-OHL-S, mirrored in the LICENSE text in this repository.

If you wish to license parts of this design in a commercial product without a reciprocal open-source license, or you have a ground-breaking idea for a module we could work on together, feel free to contact me directly. See sebholzapfel.com.

Copyright (C) 2022,2023 Sebastian Holzapfel

The above LICENSE and copyright notice does NOT apply to imported artifacts in this repository (i.e datasheets, third-party footprints).

Derivative works

If you create or manufacture your own derivative hardware, the name "apfelaudio", the names of any apfelaudio products and the names of the authors, are not to be used in derivative hardware or marketing materials, except where obligated for attribution and for retaining the above copyright notice.

For example, your modified version of "apfelaudio SoldierCrab" could be called "Sandy Modular - Clawcruncher".