Arduino/ATmega-MiCS6814-I2C/ATmega-MiCS6814-I2C.ino

/*
 * ATmega controller for MiCS-6814 gas sensor - I2C Adapter
 * 
 * Adapted from Firmware for Groove Multichannel Gas Sensor
 * Using ATmega core from:
 * https://raw.githubusercontent.com/carlosefr/atmega/master/package_carlosefr_atmega_index.json
 * 
 * Pin mapping:
 * Heater Enable (out): PB0 / Digital 8 (Inverted logic)
 * NH3 resistance (in): PC0 / Analog 0
 * CO  resistance (in): PC1 / Analog 1
 * NO2 resistance (in): PC2 / Analog 2
 * SDA: PC4
 * SCL: PC5
 */

#include <Wire.h>
#include <EEPROM.h>

// Pin Constants
#define HEATER_EN   8
#define NH3_IN      A0
#define CO_IN       A1
#define NO2_IN      A2

// I2C Communication (Taken from Groove sensor, hoping to use the same library on client side)
#define DEFAULT_I2C_ADDR        0x04
#define CMD_ADC_NH3             1     // NH3 channel
#define CMD_ADC_CO              2     // CO channel
#define CMD_ADC_NO2             3     // NO2 channel
#define CMD_ADC_ALL             4     // All channels
#define CMD_CHANGE_I2C          5     // I2C address change
#define CMD_READ_EEPROM         6     // Read stored data from EEPROM as unsigned int
#define CMD_SET_R0_ADC          7     // Set R0 ADC value
#define CMD_GET_R0_ADC          8     // Get R0 ADC value
#define CMD_GET_R0_ADC_DEFAULT  9     // Get factory R0 ADC value
#define CMD_CONTROL_LED         10    // Control LED (no LED on my board, but Groove supports is, so ... meh!)
#define CMD_CONTROL_PWR         11    // Heater control

// EEPROM Addresses (Match Groove addresses)
#define EEPROM_INIT_DONE        0
#define EEPROM_DEFAULT_ADC_NH3  2
#define EEPROM_DEFAULT_ADC_CO   4
#define EEPROM_DEFAULT_ADC_NO2  6
#define EEPROM_ADC_NH3          8
#define EEPROM_ADC_CO           10
#define EEPROM_ADC_NO2          12
#define EEPROM_I2C_ADDR         20


// Runtime storage for data
uint32_t timer;             // Timer value of last update
uint8_t  adcData[6];        // Storage for the current resistance values
uint8_t  i2CCmd;            // Current command passed from onReceive to onRequest
uint8_t  i2CData;           // Command parameters
uint8_t  i2CResp[6];        // Buffer for I2C response

/**
 * EEPROM helper function to write a single uint16_t value 
 * to two adjacient eeprom cells.
 * The upper half (MSB) will be written to the given address,
 * the lower half (LSB) will be written to the next address.
 * 
 * @param addr
 *        The eeprom addr to store the MSB in. 
 *        The next eeprom address will be used for the LSB.
 * @param value
 *        The uint16_t value to store.
 */
void eeprom_write16(uint16_t addr, uint16_t value) {
  // Store MSB in address, shift value and mask
  EEPROM.write(addr, (value >> 8) & 0xFF);
  // Store LSB in next address
  EEPROM.write(addr + 1, value & 0xFF);
}

/**
 * EEPROM helper function to read a single uint16_t value
 * from two adjacient eeprom cells.
 * 
 * @param addr
 *        The eeprom addr to read the MSB from. 
 *        The the LSB will be read from  the next eeprom address.
 * @return The stored uint16_t value.
 */
uint16_t eeprom_read16(uint16_t addr) {
  // Read two bytes separately
  uint8_t msb = EEPROM.read(addr);
  uint8_t lsb = EEPROM.read(addr + 1);

  // Combine through shifting and bitwise-or
  return (msb << 8) | lsb;
}

/**
 * Initialize EEPROM values if not already written
 */
void eeprom_init() {
  // Check if data has already been stored in EEPROM
  if (eeprom_read16(EEPROM_INIT_DONE)) {
    eeprom_write16(EEPROM_INIT_DONE, 1126);
    eeprom_write16(EEPROM_I2C_ADDR, DEFAULT_I2C_ADDR);
    eeprom_write16(EEPROM_DEFAULT_ADC_NH3, 123);
    eeprom_write16(EEPROM_DEFAULT_ADC_CO, 123);
    eeprom_write16(EEPROM_DEFAULT_ADC_NO2, 123);
    eeprom_write16(EEPROM_ADC_NH3, 123);
    eeprom_write16(EEPROM_ADC_CO, 123);
    eeprom_write16(EEPROM_ADC_NO2, 123);
  }
}

/**
 * Read the analog pin repeatedly and return the average value.
 * 
 * @param pin
 *        The (analog) pin to read from.
 * @return The average value read.
 */
uint16_t analogReadAvg(uint8_t pin) {
  // Analog read returns value between 0 and 1024, so at most 10 bits are needed.
  // If we read 64 times we can just store that in 16 bits of a uint16_t.

  uint16_t sum = 0;
  for (int c = 0; c < 16; ++c) {
    sum += analogRead(pin);
  }

  // Shift 6 bits right to calculate average (div by 64)
  return (sum >> 4);
}

/**
 * Reads the current values from the sensor
 * and stores the values in the global data.
 */
void updateValues() {
  uint16_t adcNH3 = analogReadAvg(NH3_IN);
  uint16_t adcCO = analogReadAvg(CO_IN);
  uint16_t adcNO2 = analogReadAvg(NO2_IN);

  adcData[0] = adcNH3 >> 8;   // MSB of NH3 value
  adcData[1] = adcNH3 & 0xFF; // LSB of NH3 value
  adcData[2] = adcCO >> 8;    // MSB of CO value
  adcData[3] = adcCO & 0xFF;  // LSB of CO value
  adcData[4] = adcNO2 >> 8;   // MSB of NO2 value
  adcData[5] = adcNO2 & 0xFF; // LSB of NO2 value
}

/**
 * Handler for data received from the master.
 */
void receiveCallback(uint16_t availData) {
  // Check possible package sizes
  if (availData == 1) {
    // Store command for the requestCallback
    i2CCmd = Wire.read();
  } else if (availData == 2) {
    // Command and one byte of data
    i2CCmd = Wire.read();
    i2CData = Wire.read();

    // Process direct commands
    if (i2CCmd == CMD_CHANGE_I2C) {
      // Store new address in eeprom and restart Wire library with new address
      eeprom_write16(EEPROM_I2C_ADDR, i2CData);
      Wire.begin(i2CData);
    } else if (i2CCmd == CMD_CONTROL_PWR) {
      if (i2CData) {
        // Turn heater on
        digitalWrite(HEATER_EN, LOW); 
      } else {
        // Turn heater off
        digitalWrite(HEATER_EN, HIGH);
      }
    }
  } else if (availData == 7) {
    // Lots of data, only happens for new calilbration data
    i2CCmd = Wire.read();
    uint16_t newADCData[3];

    // Read all three channels
    for (int i = 0; i < 3; ++i) {
      uint8_t msb = Wire.read();
      uint8_t lsb = Wire.read();
      newADCData[i] = (msb << 8) | lsb;
    }

    // Check if command valid

    if (i2CCmd == CMD_SET_R0_ADC) {
      eeprom_write16(EEPROM_ADC_NH3, newADCData[0]);
      eeprom_write16(EEPROM_ADC_CO, newADCData[1]);
      eeprom_write16(EEPROM_ADC_NO2, newADCData[2]);
    }
  }
}

/**
 * Handler for sending data to the master.
 * Requires command to be set by receiveCallback.
 */
void requestCallback() {
  switch (i2CCmd) {
    // Resistor data
    case CMD_ADC_NH3:
      Wire.write(&adcData[0], 2);
      break;
    case CMD_ADC_CO:
      Wire.write(&adcData[2], 2);
      break;
    case CMD_ADC_NO2:
      Wire.write(&adcData[4], 2);
      break;
    case CMD_ADC_ALL:
      Wire.write(adcData, 6);
      break;

    case CMD_READ_EEPROM:
      i2CResp[0] = EEPROM.read(i2CData);
      i2CResp[1] = EEPROM.read(i2CData + 1);
      Wire.write(i2CResp, 2);
      break;

    case CMD_GET_R0_ADC:
      i2CResp[0] = EEPROM.read(EEPROM_ADC_NH3);
      i2CResp[1] = EEPROM.read(EEPROM_ADC_NH3 + 1);
      i2CResp[2] = EEPROM.read(EEPROM_ADC_CO);
      i2CResp[3] = EEPROM.read(EEPROM_ADC_CO + 1);
      i2CResp[4] = EEPROM.read(EEPROM_ADC_NO2);
      i2CResp[5] = EEPROM.read(EEPROM_ADC_NO2 + 1);
      Wire.write(i2CResp, 6);
      break;

    case CMD_GET_R0_ADC_DEFAULT:
      i2CResp[0] = EEPROM.read(EEPROM_DEFAULT_ADC_NH3);
      i2CResp[1] = EEPROM.read(EEPROM_DEFAULT_ADC_NH3 + 1);
      i2CResp[2] = EEPROM.read(EEPROM_DEFAULT_ADC_CO);
      i2CResp[3] = EEPROM.read(EEPROM_DEFAULT_ADC_CO + 1);
      i2CResp[4] = EEPROM.read(EEPROM_DEFAULT_ADC_NO2);
      i2CResp[5] = EEPROM.read(EEPROM_DEFAULT_ADC_NO2 + 1);
      Wire.write(i2CResp, 6);
      break;

    default:
      break;
  }
}