Converting an old organ to MIDI

// Sketch to convert an older electric organ (Eminent S20) into a MIDI instrument.
// By Matt Mets, completed in 2009
//
// This code is released into the public domain.  Attribution is appreciated.
//
// MIDI source code from the ITP website:
// http://itp.nyu.edu/physcomp/Labs/MIDIOutput
 
#include <Wire.h>
 
 
#define LEDpin 13                  // Just the standard output LED
#define volumePedal 1              // The analog input for the volume control
 
char velocity;                     // Use the foot pedal to determine this
 
// Define the MIDI channels that each instrument transmits on
#define keyboardChannel 0
 
// Number of key banks to scan.  There are eight 'normal' key banks, and 2 special ones.
#define numKeyBanks 8
#define numKeysPerBank 12
 
// Lookup table, used to determine the select line to turn on to read the keys in that key bank.
static char keyBankSelectLines[] = {2,3,4,5,6,7,8,11};
 
// Storage for the previous value of each bank of keys.  There are 10 access lines, with a maximum of 12 inputs per line.
// The 12 bits of data on each line are split into low and high banks.
unsigned char noteStatesL[numKeyBanks];
unsigned char noteStatesH[numKeyBanks];
 
// Storage for the instrument data associated with each key.  Must be filled in at startup.
unsigned char noteChannels[numKeyBanks][numKeysPerBank];
unsigned char noteValues[numKeyBanks][numKeysPerBank];
 
 
// Initialize IO ports
void setup()
{
  // Setup the I2C bus (analog pins 4&5) in master mode.  This bus is used to talk to the IO expander chips.
  Wire.begin();
 
  //  Set the serial port to the correct baud rate for MIDI
  Serial.begin(31250);
 
  // Set all initial note bank vales to 0
  for (unsigned int i = 0; i < numKeyBanks; i++) {
    noteStatesL[i] = 0;
    noteStatesH[i] = 0;
  }
 
  pinMode(LEDpin, OUTPUT);    // Status LED
 
  // Set all of the bank select lines to outputs
  for (unsigned char bankNo = 0; bankNo < numKeyBanks; bankNo++) {
    pinMode(keyBankSelectLines[bankNo], OUTPUT);
  }
 
  // Set up the instrument data for the keys
 
  // first off, clear everything.
  for (unsigned char bank = 0; bank < numKeyBanks; bank ++) {
    for (unsigned char key = 0; key < numKeysPerBank; key++) {
      noteChannels[bank][key] = 0;
      noteValues[bank][key] = 0;
    }
  }
 
  // The bottom half of the keyboard comprised of banks 4-6, starting at F
  unsigned char noteValue = 24;    // Octave 2, C note
  for (unsigned char bank = 4; bank < 7; bank ++) {
    for (unsigned char key = 0; key < 12; key++) {
      noteChannels[bank][key] = keyboardChannel;
      noteValues[bank][key] = noteValue++;
    }
  }
 
  // The top keyboard row is comprised of banks 0-3, starting at F
  for (unsigned char bank = 0; bank < 4; bank ++) {
    for (unsigned char key = 0; key < 12; key++) {
      noteChannels[bank][key] = keyboardChannel;
      noteValues[bank][key] = noteValue++;
    }
  }
  // And one extra one on bank 9 note 1
  noteChannels[7][1] = keyboardChannel;
  noteValues[7][1] = noteValue++;
}
 
//  Send a MIDI command.
//  cmd is the command and channel (0x90 for channel 1 note on)
//  data1 is the note
//  data2 is the velocity
void midiCommand(char cmd, char data1, char data2) {
  Serial.print(cmd, BYTE);
  Serial.print(data1, BYTE);
  Serial.print(data2, BYTE);
}
 
// Map an integer into a charater given a set range
unsigned char mapInteger(int input, int min, int max) {
  return (((input - min)*255)/(max-min));
}
 
 
// Main loop
unsigned char newNoteStateL;
unsigned char newNoteStateH;
 
void loop()
{ 
  // Read the velocity in
//  velocity = mapInteger(analogRead(volumePedal),400,900);
  velocity = 90;
 
  for (char bankNo = 0; bankNo < numKeyBanks; bankNo++) {
 
    digitalWrite(keyBankSelectLines[bankNo], HIGH);    // Select the key bank
 
    Wire.requestFrom(0x38, 1);                      // Read the low data
    while (Wire.available()) {
      newNoteStateL = Wire.receive();
    }
 
    Wire.requestFrom(0x39, 1);                      // Read the high data
    while (Wire.available()) {
      newNoteStateH = Wire.receive();
    }
 
    digitalWrite(keyBankSelectLines[bankNo], LOW);     // Deselect the key bank
 
    if (newNoteStateL != noteStatesL[bankNo]) {      // Check if any notes in this bank are different
      char stateDelta = newNoteStateL ^ noteStatesL[bankNo];
      for (unsigned char i = 0; i < 8; i++) {
        if ((stateDelta >> i) & 1) {
          if ((newNoteStateL >> i) & 1) {
            // Send a MIDI on message
            midiCommand(0x90 + noteChannels[bankNo][i], noteValues[bankNo][i], velocity);
          }
          else {
            // Send a MIDI off message
            midiCommand(0x80 + noteChannels[bankNo][i], noteValues[bankNo][i], velocity);
          }
        }
      }
    }
 
    if (newNoteStateH != noteStatesH[bankNo]) {      // Check if any notes in this bank are different
      digitalWrite(LEDpin, HIGH);
      char stateDelta = newNoteStateH ^ noteStatesH[bankNo];
      for (unsigned char i = 0; i < 8; i++) {
        if ((stateDelta >> i) & 1) {
          if ((newNoteStateH >> i) & 1) {
            // Send a MIDI on message
            midiCommand(0x90 + noteChannels[bankNo][i+8], noteValues[bankNo][i+8], velocity);
          }
          else {
            // Send a MIDI off message
            midiCommand(0x80 + noteChannels[bankNo][i+8], noteValues[bankNo][i+8], velocity);
          }
        }
      }
    }
 
    noteStatesL[bankNo] = newNoteStateL;
    noteStatesH[bankNo] = newNoteStateH;
  }
}