Arduino-USB-HID-RetroJoystickAdapter/C64_4joy_adapter/stuff.ino

// Protovision 4 player interface / Classical Games adapter (1997)
// Arduino Pro Micro
// https://en.wikipedia.org/wiki/Commodore_64_joystick_adapters
// https://www.protovision.games/hardw/build4player.php?language=en
// http://cloud.cbm8bit.com/penfold42/joytester.zip
// https://arduino.stackexchange.com/questions/8758/arduino-interruption-on-pin-change/8926
// http://www.nongnu.org/avr-libc/user-manual/inline_asm.html
// http://ww1.microchip.com/downloads/en/devicedoc/atmel-7766-8-bit-avr-atmega16u4-32u4_datasheet.pdf
// http://ww1.microchip.com/downloads/en/devicedoc/atmel-0856-avr-instruction-set-manual.pdf
// http://www.pighixxx.net/wp-content/uploads/2016/07/pro_micro_pinout_v1_0_red.png
// https://opencircuit.shop/ProductInfo/1000378/ATmega32U4-Datasheet.pdf
// https://cdn.sparkfun.com/datasheets/Dev/Arduino/Boards/Pro_Micro_v13b.pdf

// TXD (INT3,PD3) and RXD (INT2,PD2) to userport L.
// INT2(RXD) is used for rising edge and INT3(TXD) for falling edge.
// Interrupts takes only less than 1us to change output port state after select-signal.

// Joystick port 3
// GND = GND (8)
#define up1 (~PD & _BV(7)) // 6,PD7 = (1)
#define down1 (~PC & _BV(6)) // 5,PC6 = (2)
#define left1 (~PD & _BV(4)) // 4,PD4 = (3)
#define right1 (~PD & _BV(0)) // 3,PD0 = (4)
#define fire1 (~PD & _BV(1)) // 2,PD1 = (6)

// Joystick port 4
// GND = GND (8)
#define up2 (~PF & _BV(7)) // A0,PF7 = (1)
#define down2 (~PF & _BV(6)) // A1,PF6 = (2)
#define left2 (~PF & _BV(5)) // A2,PF5 = (3)
#define right2 (~PF & _BV(4)) // A3,PF4 = (4)
#define fire2 (~PE & _BV(6)) // 7,PE6 = (6)

// Arduino <-> Userport
// VCC = +5V (2)
// GND = GND (A)
#define upC 1 // 15,PB1 = PB0 (C) 
#define downC 3 // 14,PB3 = PB1 (D)
#define leftC 2 // 16,PB2 = PB2 (E)
#define rightC 6 // 10,PB6 = PB3 (F)
#define fire1C 4 // 8,PB4 = PB4 (H)
#define fire2C 5 // 9,PB5 = PB5 (J)
#define selectC (PIND & _BV(2)) // RXD,PD2(INT2)+TXD,PD3(INT3) = PB7 (L)

// LEDS (inverted):
// RX = D17,PB0
// TX = -,PD5

//volatile uint16_t last_interrupt;
volatile uint8_t *ptr;

ISR(INT2_vect, ISR_NAKED) { // rising edge, output joystick 3
    asm volatile(
    "    push r0                \n"  // save register r0
    "    lds r0, %[gpio]        \n"  // GPIOR1 address is 42 and out-command works only with 0-31
    "    out %[pin], r0         \n"  // so we need lds-command and r0 register
    "    pop r0                 \n"  // restore previous r0
    "    out %[gpio0], %[gpio]  \n"  // restore previous r0
    "    rjmp INT2_vect_part_2  \n"  // go to part 2 for required prologue and epilogue
    :: [pin] "I" (_SFR_IO_ADDR(PORTB)), [gpio0] "I" (_SFR_IO_ADDR(GPIOR0)), [gpio] "I" (_SFR_IO_ADDR(GPIOR1)));
}

ISR(INT2_vect_part_2) { GPIOR0 = &GPIOR1; }

ISR(INT3_vect, ISR_NAKED) { // falling edge, output joystick 4
    asm volatile(
    "    push r0                \n"  // save register r0
    "    lds r0, %[gpio]        \n"  // GPIOR2 address is 42 and out-command works only with 0-31
    "    out %[pin], r0         \n"  // so we need lds-command and r0 register
    "    pop r0                 \n"  // restore previous r0
    "    rjmp INT3_vect_part_2  \n"  // go to part 2 for required prologue and epilogue
    :: [pin] "I" (_SFR_IO_ADDR(PORTB)), [gpio] "I" (_SFR_IO_ADDR(GPIOR2)));
}

ISR(INT3_vect_part_2) { GPIOR0 = &GPIOR2; }

void setup() {
  DDRB = 0xff; PORTB = 0xff; //all PB-ports are outputs and high (0xff = zero state, because signals are inverted)
  DDRF = 0; PORTF = 0xff; // all PF-ports are inputs with pullups
  DDRD = B00100000; PORTD = B11110011; // all PD-ports are inputs (except PD5) with pullups (PD2,PD3 without pullup)
  pinMode(5, INPUT_PULLUP); // pin5 (PC6) is input
  pinMode(7, INPUT_PULLUP); // pin7 (PE6) is input

  if (selectC) ptr = &GPIOR0; else ptr = &GPIOR1;
  GPIOR0 = 0xff; GPIOR1 = 0xff; // start from zero state (signals are inverted)
  
  TIMSK0 = 0; // disable timer0 interrupts (Arduino Uno/Pro Micro millis()/micros() update ISR)
  
  EICRA = B10110000;  // INT2 – rising edge on RXD (Bxx11xxxx), INT3 - falling edge on TXD (B10xxxxxx)
  EIMSK = B1100;  // enable INT2 (Bx1xx) and INT3 (B1xxx)

  //Serial.begin(115200);
  //PORTD &= ~_BV(5); // TX-LED on

  // We can't use millis() or micros() because Timer0 interrupts are disabled. We use 16-bit Timer1 with 1024 prescaler as "clock".
  /*TIMSK1 = 0; // disable timer1 interrupts
  TCCR1A = 0;
  TCCR1B = B00000101; // Timer1, normal mode, prescaler 1024. One tick is 64us.
  TCNT1 = 0; // reset Timer1 counter*/
}

void loop() {
  uint8_t PF, PD, PC, PE;
  PF = PINF; PD = PIND; PC = PINC; PE = PINE;
  uint8_t joy1 = 0xff; uint8_t joy2 = 0xff; // all signals are inverted
  if (up1) bitClear(joy1,upC);
  if (down1) bitClear(joy1,downC);
  if (left1) bitClear(joy1,leftC);
  if (right1) bitClear(joy1,rightC);
  if (up2) bitClear(joy2,upC);
  if (down2) bitClear(joy2,downC);
  if (left2) bitClear(joy2,leftC);
  if (right2) bitClear(joy2,rightC);
  if (fire1) { bitClear(joy1,fire1C); bitClear(joy2,fire1C); }
  if (fire2) { bitClear(joy1,fire2C); bitClear(joy2,fire2C); }

  if (GPIOR0 == &GPIOR1) { PORTD &= ~_BV(5); } else { PORTD |= _BV(5); } //TX-LED on, if joystick 3 are activated
  if (GPIOR0 == &GPIOR2) { bitClear(joy2,0); } //RX-LED on, if joystick 4 are activated

  GPIOR1 = joy1; GPIOR2 = joy2;

  //noInterrupts();
  //PORTB = *ptr; // is this atomic? probably, because ptr is 6-bit pointer.
  //interrupts();

  asm volatile(
    "    cli                \n"
    "    clr r31            \n"
    "    mov r30, %[gpio]   \n"
    "    ld r24, Z          \n"
    "    out %[pin], r24    \n"
    "    sei                \n"
    :: [pin] "I" (_SFR_IO_ADDR(PORTB)), [gpio] "I" (_SFR_IO_ADDR(GPIOR0)));
  
  //delayMicroseconds(10);
  //uint16_t koe = ptr;
  //Serial.println(koe, HEX);
  //delayMicroseconds(10000);
}