🛠 Boîte à rythmes USB MIDI : classes MIDI, MIDIx16 et port hôte USBHost_t36

Objectives

The Teensy 4.1 is the reference brain for DIY synthesizers: it presents itself as a USB MIDI class-compliant device (no drivers to install) with up to 16 virtual MIDI cables (MIDIx4/MIDIx16), and can simultaneously act as a USB host on its second port — both at 480 Mbit/s. This tutorial builds a trigger pad, then discusses reading an external USB keyboard.

Hardware

• 1 Teensy 4.1 + micro-USB data cable
• 1 push button on pin 24 (to GND)
• Optional for host mode: the special PJRC USB host cable and a USB device (keyboard, MIDI controller)

Key points

• Tools > USB Type > MIDI (or MIDIx4 / MIDIx16 for multiple cables).
• The USB serial port disappears when USB Type = MIDI: to debug, use a second port or change the type.
• Documented requirement: drain inputs with while (usbMIDI.read) {} in every loop, otherwise USB buffers fill up and block.
• In host mode, the USBHost_t36 library reads keyboards, MIDI controllers, and audio DACs on the second port (480/12/1.5 Mbit/s, hubs supported) via the dedicated host cable.

Complete code

#include <Bounce.h>

Bounce padBtn(24, 10); // anti-rebond 10 ms sur une broche du 4.1

void setup {}

void loop {
  padBtn.update;
  if (padBtn.fallingEdge) usbMIDI.sendNoteOn(60, 127, 1);
  if (padBtn.risingEdge)  usbMIDI.sendNoteOff(60, 0, 1);

  // vider les entrées MIDI, sous peine de saturer les tampons USB
  while (usbMIDI.read) { }
  delay(1);
}

Expected verification

In your DAW (or a MIDI monitor like MIDI Monitor / MIDI-OX), the Teensy appears as a class-compliant MIDI device; each press sends a Note On 60 velocity 127 on channel 1. With MIDIx16, sixteen distinct virtual instruments can be addressed via a single USB cable.

Common errors

• Forgetting while (usbMIDI.read): after a few seconds, no output is sent anymore — the classic MIDI issue on Teensy.
• Staying in USB Type = Serial: usbMIDI simply does not exist at compile time.
• Looking for the serial monitor after switching to MIDI: the USB serial interface has been replaced by the MIDI interface, this is normal.
• Plugging a standard USB device into the host port without the special host cable: the connector does not match, the PJRC cable is required.
• For complete controllers (encoders, faders, LED matrices), Josh Nishikawa's MIDIcontroller library relies on these classes and the 8 independent serial ports.