OMG I2C

setup.ino

#include <Wire.h>

#define SDA 4
#define SCL 5
#define I2CADDR 0x20

byte rxByte[2];
bool whichByte = false;
byte selectedChannel = 0;
byte selectedBank = 0;

void handleRx(int numBytes) {
  for (uint8_t i = 0; i < max(numBytes, 2); i++) {
    rxByte[i] = Wire.read();
  }
  if (numBytes > 2) {
    // Eat everything after the first two bytes
    while (Wire.available()) {
      Wire.read();
    }
  }

  // Decode
  if (rxByte[0] != 0x13) {
    // UNKNOWN COMMAND, IGNORING
    return;
  }

  selectedChannel = rxByte[1] & 0x0F;
  selectedBank = selectedChannel / 4;
}

setup() {
  Wire.setSDA(SDA);
  Wire.setSCL(SCL);
  Wire.begin(I2CADDR);
  Wire.onReceive(handleRx);
}

loop() {
  // do stuff
}