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This example is for Wiring version 0024+. If you have a previous version, use the examples included with your software. If you see any errors or have comments, please let us know.

/* Copyright (C) 2009 Jeff Hoefs. All rights reserved. Copyright (C) 2009 Shigeru Kobayashi. All rights reserved.
This library is free software; you can redistribute it and/or modify it under the terms of the GNU Lesser General Public License as published by the Free Software Foundation; either version 2.1 of the License, or (at your option) any later version.
See file LICENSE.txt for further informations on licensing terms.

    #include <TwoWire.h>
    #include <Firmata.h>

    #define I2C_WRITE B00000000
    #define I2C_READ B00001000
    #define I2C_READ_CONTINUOUSLY B00010000
    #define I2C_STOP_READING B00011000
    #define I2C_READ_WRITE_MODE_MASK B00011000

    #define MAX_QUERIES 8

    unsigned long currentMillis;     // store the current value from millis()
    unsigned long nextExecuteMillis; // for comparison with currentMillis
    unsigned int samplingInterval = 32;  // default sampling interval is 33ms
    unsigned int i2cReadDelayTime = 0;  // default delay time between i2c read request and Wire.requestFrom()
    unsigned int powerPinsEnabled = 0;  // use as boolean to prevent enablePowerPins from being called more than once

    #define MINIMUM_SAMPLING_INTERVAL 10

    #define REGISTER_NOT_SPECIFIED -1

    struct i2c_device_info {
      byte addr;
      byte reg;
      byte bytes;
    };

    i2c_device_info query[MAX_QUERIES];

    byte i2cRxData[32];
    boolean readingContinuously = false;
    byte queryIndex = 0;

    /* reference: BlinkM_funcs.h by Tod E. Kurt, ThingM, http://thingm.com/ */
    // Enables Pins 2 and 3 to be used as GND and Power
    // so that I2C devices can be plugged directly
    // modified for Wiring to use pins 2 and 3 on port D
    static void enablePowerPins(byte pwrpin, byte gndpin) {
      if(powerPinsEnabled == 0) {
        DDRD |= _BV(pwrpin) | _BV(gndpin);
        PORTD &=~ _BV(gndpin);
        PORTD |=  _BV(pwrpin);
        powerPinsEnabled = 1;
        Firmata.sendString("Power pins enabled");
        delay(100);
      }
    }

    void readAndReportData(byte address, int theRegister, byte numBytes)
    {
      if (theRegister != REGISTER_NOT_SPECIFIED) {
        Wire.beginTransmission(address);
        Wire.send((byte)theRegister);
        Wire.endTransmission();
        delayMicroseconds(i2cReadDelayTime);  // delay is necessary for some devices such as WiiNunchuck
      }
      else {
        theRegister = 0;  // fill the register with a dummy value
      }

      Wire.requestFrom(address, numBytes);

      // check to be sure correct number of bytes were returned by slave
      if(numBytes == Wire.available()) {
        i2cRxData[0] = address;
        i2cRxData[1] = theRegister;
        for (int i = 0; i < numBytes; i++) {
          i2cRxData[2 + i] = Wire.receive();
        }
        // send slave address, register and received bytes
        Firmata.sendSysex(I2C_REPLY, numBytes + 2, i2cRxData);
      }
      else {
        if(numBytes > Wire.available()) {
          Firmata.sendString("I2C Read Error: Too many bytes received");
        } else {
          Firmata.sendString("I2C Read Error: Too few bytes received");
        }
      }

    }

    void sysexCallback(byte command, byte argc, byte *argv) {
      byte mode;
      byte slaveAddress;
      byte slaveRegister;
      byte data;
      int  delayTime;

      if (command == I2C_REQUEST) {
        mode = argv[1] & I2C_READ_WRITE_MODE_MASK;
        slaveAddress = argv[0];

        switch(mode) {
        case I2C_WRITE:
          Wire.beginTransmission(slaveAddress);
          for (byte i = 2; i < argc; i += 2) {
            data = argv[i] + (argv[i + 1] << 7);
            Wire.send(data);
          }
          Wire.endTransmission();
          delayMicroseconds(70); // TODO is this needed?
          break;
        case I2C_READ:
          if (argc == 6) {
            // a slave register is specified
            slaveRegister = argv[2] + (argv[3] << 7);
            data = argv[4] + (argv[5] << 7);  // bytes to read
            readAndReportData(slaveAddress, (int)slaveRegister, data);
          }
          else {
            // a slave register is NOT specified
            data = argv[2] + (argv[3] << 7);  // bytes to read
            readAndReportData(slaveAddress, (int)REGISTER_NOT_SPECIFIED, data);
          }
          break;
        case I2C_READ_CONTINUOUSLY:
          if ((queryIndex + 1) >= MAX_QUERIES) {
            // too many queries, just ignore
            Firmata.sendString("too many queries");
            break;
          }
          query[queryIndex].addr = slaveAddress;
          query[queryIndex].reg = argv[2] + (argv[3] << 7);
          query[queryIndex].bytes = argv[4] + (argv[5] << 7);
          readingContinuously = true;
          queryIndex++;
          break;
        case I2C_STOP_READING:
          readingContinuously = false;
          queryIndex = 0;
          break;
        default:
          break;
        }
      }
      else if (command == SAMPLING_INTERVAL) {
        samplingInterval = argv[0] + (argv[1] << 7);

        if (samplingInterval < MINIMUM_SAMPLING_INTERVAL) {
          samplingInterval = MINIMUM_SAMPLING_INTERVAL;
        }

        samplingInterval -= 1;
        Firmata.sendString("sampling interval");
      }

      else if (command == I2C_CONFIG) {
        delayTime = (argv[4] + (argv[5] << 7));                        // MSB
        delayTime = (delayTime << 8) + (argv[2] + (argv[3] << 7));     // add LSB

        if((argv[0] + (argv[1] << 7)) > 0) {
          enablePowerPins(PORTC3, PORTC2);
        }

        if(delayTime > 0) {
          i2cReadDelayTime = delayTime;
        }

        if(argc > 6) {
          // If you extend I2C_Config, handle your data here
        }

      }
    }

    void systemResetCallback()
    {
      readingContinuously = false;
      queryIndex = 0;
    }

    void setup()
    {
      Firmata.setFirmwareVersion(2, 0);

      Firmata.attach(START_SYSEX, sysexCallback);
      Firmata.attach(SYSTEM_RESET, systemResetCallback);

      for (int i = 0; i < TOTAL_DIGITAL_PINS; ++i) {
        pinMode(i, OUTPUT);
      }

      Firmata.begin(57600);
      Wire.begin();
    }

    void loop()
    {
      while (Firmata.available()) {
        Firmata.processInput();
      }

      currentMillis = millis();
      if (currentMillis > nextExecuteMillis) {
        nextExecuteMillis = currentMillis + samplingInterval;

        for (byte i = 0; i < queryIndex; i++) {
          readAndReportData(query[i].addr, query[i].reg, query[i].bytes);
        }
      }
    }