MIDI BLE Tutorial
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.
unsigned long msOffset = 0; // 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(); }