Making a MIDI keyboard from an old children's synthesizer

//    +5V
//   ST_CP  74HC595
#define latchPin      11
//   SH_CP  74HC595
#define clockPin      13
//   DS  74HC595
#define dataPin       12

//     +5V
#define latchPin_IN   9
#define clockPin_IN   10
#define dataPin_IN    8

// ,       1-16    
#define BUT_OUT0      7
#define BUT_OUT1      6
#define BUT_OUT2      5
#define BUT_OUT3      4

byte but_out[4] = {BUT_OUT0, BUT_OUT1, BUT_OUT2, BUT_OUT3};
//       
byte kbd_in[5] = {0, 0, 0, 0, 0};
byte kbd_in_new[5] = {0, 0, 0, 0, 0};

//       
byte btn_in[4] = {0, 0, 0, 0};
byte btn_in_new[4] = {0, 0, 0, 0};

//  3  -   (1 - , 0 - )
byte kbd_out = 0b11100000;

//  
void init_electronics() {
  //  ,   +5V
  //  OUTPUT
  pinMode(latchPin, OUTPUT);
  pinMode(clockPin, OUTPUT);
  pinMode(dataPin, OUTPUT);

  //  ,   +5V
  pinMode(latchPin_IN, OUTPUT);
  pinMode(clockPin_IN, OUTPUT);
  pinMode(dataPin_IN, INPUT);

  //          
  for (byte i = 0; i < 4; i++) pinMode(but_out[i], OUTPUT);
}

//      74HC595
void write_5V(byte a) {
    digitalWrite(latchPin, LOW);
    //    dataPin
    shiftOut(dataPin, clockPin, MSBFIRST, a); 
     //"" ,      
    digitalWrite(latchPin, HIGH); 
}

void read_5V(byte *btn, byte *kbd) {
  digitalWrite(clockPin_IN, HIGH);
  digitalWrite(latchPin_IN, LOW);
  digitalWrite(latchPin_IN, HIGH);
  
  //    -    
  *btn = shiftIn(dataPin_IN, clockPin_IN, LSBFIRST);
  
  //    -    
  *kbd = shiftIn(dataPin_IN, clockPin_IN, LSBFIRST);
}

void setup() {
  init_electronics();
  write_5V(kbd_out);
  Serial.begin(115200);
}

//       ,   
void read_kbd_and_btn_state(byte *btn, byte *kbd) {
 byte tmp;
  for (byte i = 0; i < 5; i++) {
    write_5V(kbd_out | (1 << i));
    read_5V(&tmp, &kbd[i]);
  }
}

void noteOn(int cmd, int pitch, int velocity) {
  Serial.write(cmd);
  Serial.write(pitch);
  Serial.write(velocity);
}

//     num  ,   
// 0 -  0,  0
// 1 -  0,  1
// 2 -  1,  1
// 3 -  1,  0
byte compare_bit(byte old_state, byte new_state, byte num) {
  byte tmp_old, tmp_new;
  tmp_old = (old_state >> num) & 1;
  tmp_new = (new_state >> num) & 1;
  if ((tmp_old == 0) && (tmp_new == 0)) return 0;
  if ((tmp_old == 0) && (tmp_new == 1)) return 1;
  if ((tmp_old == 1) && (tmp_new == 1)) return 2; 
  if ((tmp_old == 1) && (tmp_new == 0)) return 3;  
}

void loop() {
  read_kbd_and_btn_state(btn_in_new, kbd_in_new);
  for (byte i = 0; i < 5; i++) {
    for (byte j = 0; j < 8; j++) {
      switch (compare_bit(kbd_in[i], kbd_in_new[i], 7 - j)) {
        //  
        case 1: noteOn(0x90, 0x1D + (8 * i + j), 0x7F);
                break;
        //  
        case 2: //noteOn(0x00, 0x1D + (8 * i + j), 0x7F);
                break;
        //  
        case 3: noteOn(0x90, 0x1D + (8 * i + j), 0x00);
                break;
      }
    }
    kbd_in[i] = kbd_in_new[i];
  }
}