ESP32 Simple MIDI Monitor

By Kevin on December 9, 2024

This project started as a bit of a test for my ESP32 WROOM Mozzi Experimenter PCB but ended up a little more functional, so I thought it worth writing up in its own right.

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.

Parts list

ESP32 WROOM-DA module or DevKit (see photos).
ESP32 WROOM Mozzi Experimenter PCB.
Serial MIDI interface.

The Circuit

This is using my ESP32 WROOM Mozzi Experimenter PCB but it would be possible to hook up a 3V3 compatible MIDI module (see Ready-Made MIDI Modules) to the following pins on the ESP32 module:

GPIO 16	MIDI Receive
GPIO 17	MIDI Transmit
3V3	MIDI VCC
GND	MIDI GND

The UART configuration should be set to UART 1 as shown below.

The Code

This code uses the Arduino MIDI Library from https://github.com/FortySevenEffects/arduino_midi_library.

It is very similar to the Simple MIDI Monitor but has a couple of significant changes to support the ESP32 and be a little more full-featured.

First and foremost, the ESP32 has several UART interfaces - three to be exact (later ESP32 models have two, but the original ESP32 I'm using has three). By default, UART0 is connected to the USB port, UART1 is connected to onboard flash memory, and UART2 is available for use.

In Arduino terms, these correspond to Serial, Serial1, and Serial2.

My ESP32 WROOM Mozzi Experimenter PCB supports UART0 and UART2, although I've slightly erroneously labelled it UART1 on the PCB! But these two are mapped as follows:

Board Label	Real Port	TX	RX
UART 0	UART 0	1	3
UART 1	UART 2	17	16

One quirk - it would appear from the ESP32 Arduino core that the default pins for UART2 are RX=4, TX=25 (see HardwareSerial.h), yet in the pinout for the module I'm using (see here), it shows GPIO17 and 16 for UART2, so that is what I wired up in my PCB.

To initialise the serial port using Serial2 thus requires changing the TX/RX pin. Unfortunately this cannot be done (as far as I could see) when initialsing the MIDI library - which always calls Serial.begin() automatically.

So I had to re-initialise the serial port with the new pins as shown below. This works as the ESP32 library will detach and re-attach the serial pins if already configured.

MIDI_CREATE_INSTANCE(HardwareSerial, Serial2, MIDI);

void setup() {
  MIDI.begin(MIDI_CHANNEL_OMNI);
  Serial2.begin(31250, SERIAL_8N1, 16, 17);
}

Once initialised, the main loop runs the following sequence:

loop():
  IF MIDI.read() THEN
    IF MIDI channel message THEN
      Print out details of the MIDI channel message
    ELSE
      Print out just the command from a system message

I've written a number of helper functions to make printing the details easier. It will also light up the on-board LED (on D2) when a NoteOn is received and turn if off on NoteOff.

The code will also support using UART0 (Serial), but in that case all serial port printing to the seral monitor is disabled.

Here is some example output.

1	0x90 NoteOn	0x35	0x60
1	0xE0 PBend	0x00	0x60
1	0xE0 PBend	0x00	0x68
1	0xE0 PBend	0x00	0x70
1	0x80 NoteOff	0x32	0x40
1	0x80 NoteOff	0x35	0x40
1	0x80 NoteOff	0x34	0x40
1	0x80 NoteOff	0x30	0x40
1	0xC0 ProgCh	0x00
1	0xB0 CtrlCh	0x00	0x00
1	0xB0 CtrlCh	0x20	0x00
1	0xC0 ProgCh	0x1E
3	0xB0 CtrlCh	0x0B	0x00
0xFB
0xF8

At the end, I turned on MIDI sync pulses and caught a few.

Find it on GitHub here.

Closing Thoughts

Working out how to use the second serial port wasn't quite as straight forward as I was expecting, but I got there in the end.

Curiously there doesn't seem to be much information about it online that I could easily find, and certainly not very much about how to use it with the MIDI library with custom pins.

I don't know why the core has different pins by default to those suggested in the pinout of my module, but seeing as the ESP32 has quite a comprehensive GPIO multiplexing setup, I'm not surprised.

Kevin