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Thread: a single Teensy 4.0 as both USB host and device

  1. #1
    Junior Member
    Join Date
    Oct 2020

    a single Teensy 4.0 as both USB host and device

    I'm trying to host a MIDI device via USB2 (The USB2_DN and USB2_DP pins) on a teensy 4.0, modify the signal, and send the modified signal over USB1 (The main USB port).

    I've wired a USB plug on the backside on the board to 5v, ground, and the D+ and D- solder spots (see the picture link further down) and I'm using the USBHost_t36 library to recieve data.

    With the Example called "InputFunctions" (C:\Program Files (x86)\Arduino\hardware\teensy\avr\libraries\USBHos t_t36\examples\Serial\MIDI\InputFunctions) I am able to read the input from USB2, and print the info to console. But I have not been able to write the input as MIDI over USB1, so that I can detect it in a DAW.

    I've tried multiple other libraries to modify the example code to output both to serial, but also send the signal over USB. I have however not been able to come up with a solution that I can compile.
    The example below uses the library "MIDIUSB" where I get the following errors:

    "In file included from C:\Users\USER\Desktop\MIDItest\MIDItest.ino:3:0:
    C:\Users\USER\Documents\Arduino\libraries\MIDIUSB\ src/MIDIUSB_Defs.h:10:3: error: conflicting declaration 'typedef struct midiEventPacket_t midiEventPacket_t'
    } midiEventPacket_t;
    "In file included from C:\Users\USER\Desktop\MIDItest\MIDItest.ino:2:0:
    C:\Program Files (x86)\Arduino\hardware\teensy\avr\cores\teensy4/MIDIUSB.h:23:3: note: previous declaration as 'typedef struct midiEventPacket_t midiEventPacket_t'
    } midiEventPacket_t;

    I've tried editing the two documents so that the statements don't conflict, but that just made it worse.

    I've also tried the "USBMIDI" library, but that also couldn't compile.

    It seems like the teensy is trying to read and write on the same port, so I need to Host from USB2, and be a device on USB1.

    Image of the wiring for the USB2 connection:

    Included below is the modified example code which produces the errors above

    #include <frequencyToNote.h>
    #include <MIDIUSB.h>
    #include <MIDIUSB_Defs.h>
    #include <pitchToFrequency.h>
    #include <pitchToNote.h>
    /* Receive Incoming USB Host MIDI using functions.  As usbMIDI
       reads incoming messages, handler functions are run.
       See the InputRead example for the non-function alterative.
       This very long example demonstrates all possible handler
       functions.  Most applications need only some of these.
       This example is meant to allow easy copy-and-paste of the
       desired functions.
       Use the Arduino Serial Monitor to view the messages
       as Teensy receives them by USB MIDI
       You must select MIDI from the "Tools > USB Type" menu
       This example code is in the public domain.
    #include <USBHost_t36.h>
    USBHost myusb;
    USBHub hub1(myusb);
    USBHub hub2(myusb);
    MIDIDevice midi1(myusb);
    void setup() {
      // Wait 1.5 seconds before turning on USB Host.  If connected USB devices
      // use too much power, Teensy at least completes USB enumeration, which
      // makes isolating the power issue easier.
      Serial.println("USB Host InputFunctions example");
      // Only one of these System Exclusive handlers will actually be
      // used.  See the comments below for the difference between them.
      // This generic System Real Time handler is only used if the
      // more specific ones are not set.
    void loop() {
      // The handler functions are called when midi1 reads data.  They
      // will not be called automatically.  You must call
      // regularly from loop() for midi1 to actually read incoming
      // data and run the handler functions as messages arrive.
    void myNoteOn(byte channel, byte note, byte velocity) {
      // When a USB device with multiple virtual cables is used,
      // midi1.getCable() can be used to read which of the virtual
      // MIDI cables received this message.
      Serial.print("Note On, ch=");
      Serial.print(channel, DEC);
      Serial.print(", note=");
      Serial.print(note, DEC);
      Serial.print(", velocity=");
      Serial.println(velocity, DEC);
      //addition from the MIDIUSB library
      noteOn(channel, note, velocity);
    void myNoteOff(byte channel, byte note, byte velocity) {
      Serial.print("Note Off, ch=");
      Serial.print(channel, DEC);
      Serial.print(", note=");
      Serial.print(note, DEC);
      Serial.print(", velocity=");
      Serial.println(velocity, DEC);
      //addition from the MIDIUSB library
      noteOff(channel, note, velocity);
    void myAfterTouchPoly(byte channel, byte note, byte velocity) {
      Serial.print("AfterTouch Change, ch=");
      Serial.print(channel, DEC);
      Serial.print(", note=");
      Serial.print(note, DEC);
      Serial.print(", velocity=");
      Serial.println(velocity, DEC);
    void myControlChange(byte channel, byte control, byte value) {
      Serial.print("Control Change, ch=");
      Serial.print(channel, DEC);
      Serial.print(", control=");
      Serial.print(control, DEC);
      Serial.print(", value=");
      Serial.println(value, DEC);
    void myProgramChange(byte channel, byte program) {
      Serial.print("Program Change, ch=");
      Serial.print(channel, DEC);
      Serial.print(", program=");
      Serial.println(program, DEC);
    void myAfterTouchChannel(byte channel, byte pressure) {
      Serial.print("After Touch, ch=");
      Serial.print(channel, DEC);
      Serial.print(", pressure=");
      Serial.println(pressure, DEC);
    void myPitchChange(byte channel, int pitch) {
      Serial.print("Pitch Change, ch=");
      Serial.print(channel, DEC);
      Serial.print(", pitch=");
      Serial.println(pitch, DEC);
    // This 3-input System Exclusive function is more complex, but allows you to
    // process very large messages which do not fully fit within the midi1's
    // internal buffer.  Large messages are given to you in chunks, with the
    // 3rd parameter to tell you which is the last chunk.  This function is
    // a Teensy extension, not available in the Arduino MIDI library.
    void mySystemExclusiveChunk(const byte *data, uint16_t length, bool last) {
      Serial.print("SysEx Message: ");
      printBytes(data, length);
      if (last) {
        Serial.println(" (end)");
      } else {
        Serial.println(" (to be continued)");
    // This simpler 2-input System Exclusive function can only receive messages
    // up to the size of the internal buffer.  Larger messages are truncated, with
    // no way to receive the data which did not fit in the buffer.  If both types
    // of SysEx functions are set, the 3-input version will be called by midi1.
    void mySystemExclusive(byte *data, unsigned int length) {
      Serial.print("SysEx Message: ");
      printBytes(data, length);
    void myTimeCodeQuarterFrame(byte data) {
      static char SMPTE[8]={'0','0','0','0','0','0','0','0'};
      static byte fps=0;
      byte index = data >> 4;
      byte number = data & 15;
      if (index == 7) {
        fps = (number >> 1) & 3;
        number = number & 1;
      if (index < 8 || number < 10) {
        SMPTE[index] = number + '0';
        Serial.print("TimeCode: ");  // perhaps only print when index == 7
        Serial.print(SMPTE[1]);  // perhaps add 2 to compensate for MIDI latency?
        switch (fps) {
          case 0: Serial.println(" 24 fps"); break;
          case 1: Serial.println(" 25 fps"); break;
          case 2: Serial.println(" 29.97 fps"); break;
          case 3: Serial.println(" 30 fps"); break;
      } else {
        Serial.print("TimeCode: invalid data = ");
        Serial.println(data, HEX);
    void mySongPosition(uint16_t beats) {
      Serial.print("Song Position, beat=");
    void mySongSelect(byte songNumber) {
      Serial.print("Song Select, song=");
      Serial.println(songNumber, DEC);
    void myTuneRequest() {
      Serial.println("Tune Request");
    void myClock() {
    void myStart() {
    void myContinue() {
    void myStop() {
    void myActiveSensing() {
      Serial.println("Actvice Sensing");
    void mySystemReset() {
      Serial.println("System Reset");
    void myRealTimeSystem(uint8_t realtimebyte) {
      Serial.print("Real Time Message, code=");
      Serial.println(realtimebyte, HEX);
    void printBytes(const byte *data, unsigned int size) {
      while (size > 0) {
        byte b = *data++;
        if (b < 16) Serial.print('0');
        Serial.print(b, HEX);
        if (size > 1) Serial.print(' ');
        size = size - 1;
    //additions from the MIDIUSB library
    void noteOn(byte channel, byte pitch, byte velocity) {
      midiEventPacket_t noteOn = {0x09, 0x90 | channel, pitch, velocity};
    void noteOff(byte channel, byte pitch, byte velocity) {
      midiEventPacket_t noteOff = {0x08, 0x80 | channel, pitch, velocity};

  2. #2
    Senior Member PaulStoffregen's Avatar
    Join Date
    Nov 2012
    Do not include MIDIUSB. That library is meant for various Arduino boards, but not for Teensy.

    On Teensy, you don't include a library for USB device mode. Instead you use the Tools > USB Type menu. Select MIDI from that menu. Details here:

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