MIDI BLE Tutorial

Contributors: MTaylor

BLE to DIN

In the previous example of converting serial messages to BLE messages, the shortcut of "only send single, full MIDI message packets" was used. Going the other way, it's more difficult because the central can't be configured to only send one type of packet, they will be packetized as defined by the MIDI BLE standard. The program must separate the data which comes in many forms.

To do so, a routine is created (called processPacket) that has the following logical flow.

To implement, the program uses array indeces, left and right. The left one starts at array position 2 because it is always a status byte. Next, the right is moved to seek out the next status byte by looking at the MSB positions. Afterwards, the size of the data following the left status byte is known, and can be dealt with. The left index is then moved to the next status byte and the process is repeated until the indeces exceed the bounds.

Inside, it calls another routine (transmitMIDIonDIN) that interfaces the FortySevenEffects library. This is to isolate the functions from each other and provide a method of testing out the routines without having a full system running.

This code is the MIDI BLE Starter program with expanded processPacket function that translates incoming BLE packets into serial MIDI.

language:c
#include <MIDI.h>
#include "nrf52.h"
#include <BLEPeripheral.h>

#define BLUE_STAT_PIN     7   // LED on pin 7
#define RED_STAT_PIN     11   // LED on pin 11
#define GREEN_STAT_PIN   12   // LED on pin 12
#define BTN_PIN           6   // User button 

unsigned long msOffset = 0;
#define MAX_MS 0x01FFF //13 bits, 8192 dec

// create peripheral instance, see pinouts above
//const char * localName = "nRF52832 MIDI";
BLEPeripheral blePeripheral;
BLEService service("03B80E5A-EDE8-4B33-A751-6CE34EC4C700");
BLECharacteristic characteristic("7772E5DB-3868-4112-A1A9-F2669D106BF3", BLERead | BLEWriteWithoutResponse | BLENotify, 20 );
BLEDescriptor descriptor = BLEDescriptor("2902", 0);

MIDI_CREATE_INSTANCE(HardwareSerial, Serial, MIDI);

void setup() {
    delay(1000);

    //Setup diag leds
    pinMode(BLUE_STAT_PIN, OUTPUT);
    pinMode(RED_STAT_PIN, OUTPUT);
    pinMode(GREEN_STAT_PIN, OUTPUT);
    digitalWrite(BLUE_STAT_PIN, 1);
    digitalWrite(RED_STAT_PIN, 1);
    digitalWrite(GREEN_STAT_PIN, 1);

    //Setup nRF52832 user button
    pinMode(BTN_PIN, INPUT_PULLUP);

    setupBLE();

    // Initiate MIDI communications, listen to all channels
    MIDI.begin(MIDI_CHANNEL_OMNI);
    MIDI.turnThruOff();

    // The nRF52832 converts baud settings to the discrete standard rates.
    // Use the nrf52.h names to write a custom value, 0x7FFC80 after beginning midi
    NRF_UARTE_Type * myUart;
    myUart = (NRF_UARTE_Type *)NRF_UART0_BASE;
    myUart->BAUDRATE = 0x7FFC80;

    //Write data to the serial output pin to make sure the serial output is working.
    //Sometimes serial output only allows 1 byte out then hangs.  Resetting the
    //nRF52832 resolves the issue
    digitalWrite(RED_STAT_PIN, 0);
    MIDI.sendNoteOn(42, 66, 1);
    delay(500);
    MIDI.sendNoteOff(42, 66, 1); 
    digitalWrite(RED_STAT_PIN, 1);

}

void loop()
{
    BLECentral central = blePeripheral.central();
    if(digitalRead(BTN_PIN) == 0){
        digitalWrite(GREEN_STAT_PIN, 0);
        MIDI.sendNoteOff(0x45, 80, 1);
        delay(100);
        digitalWrite(GREEN_STAT_PIN, 1);
    }
    if (central) {
        //Prep the timestamp
        msOffset = millis();

        digitalWrite(BLUE_STAT_PIN, 0);
        // central connected to peripheral

        while (central.connected()) {
            digitalWrite(GREEN_STAT_PIN, 0);
            //If connected, send midi data by the button here
            if(digitalRead(BTN_PIN) == 0){
                digitalWrite(GREEN_STAT_PIN, 0);
                MIDI.sendNoteOn(0x45, 80, 1);
                delay(100);
                MIDI.sendNoteOff(0x45, 80, 1);
                digitalWrite(GREEN_STAT_PIN, 1);
            }
            //Check if data exists coming in from BLE
            if (characteristic.written()) {
                digitalWrite(RED_STAT_PIN, 0);
                processPacket();
                digitalWrite(RED_STAT_PIN, 1); 
            }
        }
    }
    //No longer connected.  Turn off the LEDs.
    digitalWrite(BLUE_STAT_PIN, 1);
    digitalWrite(GREEN_STAT_PIN, 1);
    //Delay to show off state for a bit
    delay(100);
}

//This function decodes the BLE characteristics and calls transmitMIDIonDIN
//if the packet contains sendable MIDI data.
void processPacket()
{
    //Receive the written packet and parse it out here.
    uint8_t * buffer = (uint8_t*)characteristic.value();
    uint8_t bufferSize = characteristic.valueLength();

    //Pointers used to search through payload.
    uint8_t lPtr = 0;
    uint8_t rPtr = 0;
    //lastStatus used to capture runningStatus 
    uint8_t lastStatus;
    //Decode first packet -- SHALL be "Full MIDI message"
    lPtr = 2; //Start at first MIDI status -- SHALL be "MIDI status"
    //While statement contains incrementing pointers and breaks when buffer size exceeded.
    while(1){
        lastStatus = buffer[lPtr];
        if( (buffer[lPtr] < 0x80) ){
            //Status message not present, bail
            return;
        }
        //Point to next non-data byte
        rPtr = lPtr;
        while( (buffer[rPtr + 1] < 0x80)&&(rPtr < (bufferSize - 1)) ){
            rPtr++;
        }
        //look at l and r pointers and decode by size.
        if( rPtr - lPtr < 1 ){
            //Time code or system
            transmitMIDIonDIN( lastStatus, 0, 0 );
        } else if( rPtr - lPtr < 2 ) {
            transmitMIDIonDIN( lastStatus, buffer[lPtr + 1], 0 );
        } else if( rPtr - lPtr < 3 ) {
            transmitMIDIonDIN( lastStatus, buffer[lPtr + 1], buffer[lPtr + 2] );
        } else {
            //Too much data
            //If not System Common or System Real-Time, send it as running status
            switch( buffer[lPtr] & 0xF0 )
            {
            case 0x80:
            case 0x90:
            case 0xA0:
            case 0xB0:
            case 0xE0:
                for(int i = lPtr; i < rPtr; i = i + 2){
                    transmitMIDIonDIN( lastStatus, buffer[i + 1], buffer[i + 2] );
                }
                break;
            case 0xC0:
            case 0xD0:
                for(int i = lPtr; i < rPtr; i = i + 1){
                    transmitMIDIonDIN( lastStatus, buffer[i + 1], 0 );
                }
                break;
            default:
                break;
            }
        }
        //Point to next status
        lPtr = rPtr + 2;
        if(lPtr >= bufferSize){
            //end of packet
            return;
        }
    }
}

//This function takes a midi packet as input and calls the appropriate library
//function to transmit the data.  It's a little redundant because the library
//reforms midi data from the calls and sends it out the serial port.
//
//Ideally, the MIDI BLE object would feed a MIDI library object as a serial
//object removing all of this code.
//
//A benefit of this redundant code is that it's easy to filter messages, and
//exposes how the library works.
void transmitMIDIonDIN( uint8_t status, uint8_t data1, uint8_t data2 )
{
    uint8_t channel = status & 0x0F;
    channel++;
    uint8_t command = (status & 0xF0) >> 4;
    switch(command)
    {
    case 0x08: //Note off
        MIDI.sendNoteOff(data1, data2, channel);
        break;
    case 0x09: //Note on
        MIDI.sendNoteOn(data1, data2, channel);
        break;
    case 0x0A: //Polyphonic Pressure
        MIDI.sendAfterTouch(data1, data2, channel);
        break;
    case 0x0B: //Control Change
        MIDI.sendControlChange(data1, data2, channel);
        break;
    case 0x0C: //Program Change
        MIDI.sendProgramChange(data1, channel);
        break;
    case 0x0D: //Channel Pressure
        MIDI.sendAfterTouch(data2, channel);
        break;
    case 0x0E: //Pitch Bend
        MIDI.send(midi::PitchBend, data1, data2, channel);
        break;
    case 0x0F: //System
        switch(status)
        {
            case 0xF1: //MTC Q frame
                MIDI.sendTimeCodeQuarterFrame( data1 );
                break;
            case 0xF2: //Song position
                MIDI.sendSongPosition(( (uint16_t)(data1 & 0x7F) << 7) | (data2 & 0x7F));
                break;
            case 0xF3: //Song select
                MIDI.sendSongSelect( data1 );
                break;
            case 0xF6: //Tune request
                MIDI.sendTuneRequest();
                break;
            case 0xF8: //Timing Clock
            case 0xFA: //Start
            case 0xFB: //Continue
            case 0xFC: //Stop
            case 0xFE: //Active Sensing
            case 0xFF: //Reset
                MIDI.sendRealTime( (midi::MidiType)status );
                break;
            default:
                break;
        }
        break;
    default:
        break;
    }   
}

void setupBLE()
{
    blePeripheral.setLocalName("BLE to DIN"); //local name sometimes used by central
    blePeripheral.setDeviceName("BLE to DIN"); //device name sometimes used by central
    //blePeripheral.setApperance(0x0000); //default is 0x0000, what should this be?
    blePeripheral.setAdvertisedServiceUuid(service.uuid()); //Advertise MIDI UUID

    // add attributes (services, characteristics, descriptors) to peripheral
    blePeripheral.addAttribute(service);
    blePeripheral.addAttribute(characteristic);
    blePeripheral.addAttribute(descriptor);

    // set initial value
    characteristic.setValue(0);

    blePeripheral.begin();
}