What is it?

The Adafruit MacroPad RP2040 is really fun and it could be a great musical instrument, with its 12 keys, rotary encoder, OLED display, RP2040 chip, and 8MB of flash storage.

If only it had a way to get Audio out of it and MIDI into it! Well now you can, with MacroPadSynthPlug!

MacroPadSynthPlug is a small fully-assembled board that plugs into the StemmaQT port (but is not I2C!) and gives you Audio line out (even works w/ headphones) and TRS-A MIDI In for 5-pin MIDI.

The MacroPad RP2040 can now be a stand-alone MIDI synth!

Examples provided for both CircuitPython and Arduino:

• drum_machine - CircuitPython-based drum sequencer (demo1,demo2 )
• dronesynth - Arduino Mozzi 12-oscillator drone synth (demo)
• remixer - CircuitPython-based song remixer (source)
• monosynth1 - Arduino Mozzi 2-oscillator bass monosynth w/ resonant low-pass filter (source)

But you can use any audio output technique that uses PWM.

Batch1 Notes!

Note: The "Batch1" first run of boards required a small bodge wire on the Audio output. This has been performed and tested on all boards. I'll be discounting this run of boards because of it. The Eagle CAD PCB & schematic files have been updated with the fix.

Why did you make it?

Seems like fun? I have built many RP2040-based little hardware synths, like:

• PicoStepSeq
• picotouchsynth
• plinkykeeb
• seeknobs

and wanted the MacroPad RP2040 to join in the fun.

What makes it special?

MacroPadSynthPlug comes fully assembled, using all SMD parts. It can be screwed directly to to the MacroPad RP2040 for an nice compact platform to experiment with different synthesis techniques using the RP2040 chip.

How does it work?

The Raspberry Pi RP2040 chip on the MacroPad RP2040 is amazingly configurable.

Normally a port like the StemmaQT I2C port would only be usable as an I2C bus, or maybe as GPIO pins.

But with the RP2040, those two StemmaQT pins, can be:

• SDA - GP20 - UART1 TX - PWM2B
• SCL - GP21 - UART1 RX - PWM2A

This means we could use the StemmaQT port for all sorts of musical things, like:

• MIDI In / Out!
• Stereo PWM audio out!
• MIDI In / Audio Out (Hey this is what MacroPadSynthPlug does)

To get audio out, all we need is a small PWM filtering circuit. To get MIDI in, a small optoisolator circuit is needed. MacroPadSynthPlug is just this.

This is sort of an "abuse" of the StemmaQT port, as it's meant only for I2C devices. But it's safe, will not damage other StemmaQT devices, but it does mean you "lose" the StemmaQT port for it's normal use.