isr_override.ld

SECTIONS { 
    __vector_20 = __vector_20_override;
}

main.cpp

#include <Arduino.h>

#define BS_B _BV(0)
#define BS_START _BV(1)
#define BS_SELECT _BV(2)
#define BS_A _BV(3)

#define BS_LEFT _BV(0)
#define BS_DOWN _BV(1)
#define BS_UP _BV(2)
#define BS_RIGHT _BV(3)

namespace pin {
const int up = 4;
const int down = 3;
const int left = 2;
const int right = 5;
const int A = 9;
const int B = 8;
const int select = 6;
const int start = 7;

// These are fixed: port manipulation code assumes that they have specific
// values and won't adjust automatically if these constants are chagned.
const int left_b = 17;
const int down_start = 16;
const int up_select = 15;
const int right_a = 14;

// ISR assumes that these are on PORTD.
const int ab_scan = 21;
const int dpad_scan = 20;
}; // namespace pin

volatile uint8_t buttons_state = 0xF;
volatile uint8_t dpad_state = 0xF;

void updateButtonState(int pin, uint8_t button, volatile uint8_t *state) {
  if (digitalRead(pin)) {
    *state |= button;
    Serial.print("Button ");
    Serial.print(pin);
    Serial.println(" released");
  } else {
    *state &= ~button;
    Serial.print("Button ");
    Serial.print(pin);
    Serial.println(" pressed");
  }
}

// Custom linker script replaces PORTD ISR with this.
ISR(__vector_20_override) {
  PORT_t *portStruct = portToPortStruct(PD);
  uint8_t int_flags = portStruct->INTFLAGS;
  if (digitalPinToBitMask(pin::ab_scan) & int_flags) {
    PORTD.OUT = (PORTD.OUT & (~0xF)) | (buttons_state & 0xF);
    portStruct->INTFLAGS = digitalPinToBitMask(pin::ab_scan);
  }
  if (digitalPinToBitMask(pin::dpad_scan) & int_flags) {
    PORTD.OUT = (PORTD.OUT & (~0xF)) | (dpad_state & 0xF);
    portStruct->INTFLAGS = digitalPinToBitMask(pin::dpad_scan);
  }
}

void setup() {
  Serial.begin(9600);

  pinMode(pin::left_b, OUTPUT);
  pinMode(pin::down_start, OUTPUT);
  pinMode(pin::up_select, OUTPUT);
  pinMode(pin::right_a, OUTPUT);
  pinMode(pin::ab_scan, INPUT_PULLUP);
  pinMode(pin::dpad_scan, INPUT_PULLUP);

  digitalWrite(pin::left_b, HIGH);
  digitalWrite(pin::down_start, HIGH);
  digitalWrite(pin::up_select, HIGH);
  digitalWrite(pin::right_a, HIGH);

  // Configure the port to receive interrupts. No handler specified, since we
  // override the handler for the whole port.
  attachInterrupt(digitalPinToInterrupt(pin::ab_scan), nullptr, FALLING);
  attachInterrupt(digitalPinToInterrupt(pin::dpad_scan), nullptr, FALLING);

  // Input button pins
  for (int pin = 2; pin <= 9; pin++) {
    pinMode(pin, INPUT_PULLUP);
  }

  attachInterrupt(
      pin::A, []() { updateButtonState(pin::A, BS_A, &buttons_state); },
      CHANGE);
  attachInterrupt(
      pin::B, []() { updateButtonState(pin::B, BS_B, &buttons_state); },
      CHANGE);
  attachInterrupt(
      pin::select,
      []() { updateButtonState(pin::select, BS_SELECT, &buttons_state); },
      CHANGE);
  attachInterrupt(
      pin::start,
      []() { updateButtonState(pin::start, BS_START, &buttons_state); },
      CHANGE);
  attachInterrupt(
      pin::up, []() { updateButtonState(pin::up, BS_UP, &dpad_state); },
      CHANGE);
  attachInterrupt(
      pin::down, []() { updateButtonState(pin::down, BS_DOWN, &dpad_state); },
      CHANGE);
  attachInterrupt(
      pin::left, []() { updateButtonState(pin::left, BS_LEFT, &dpad_state); },
      CHANGE);
  attachInterrupt(
      pin::right,
      []() { updateButtonState(pin::right, BS_RIGHT, &dpad_state); }, CHANGE);
}

void loop() {}

Makefile

BOARD := arduino:megaavr:nona4809:mode=off
PORT ?= $(wildcard /dev/cu.usbmodem*)

.PHONY: all upload build format

all: build

format:
	clang-format -i $$(find . -name \*.cpp -o -name \*.ino -o -name \*.h)

build: format
	arduino-cli compile --fqbn $(BOARD) --build-property compiler.c.elf.extra_flags=isr_override.ld .

upload: format
	arduino-cli compile --fqbn $(BOARD) --build-property compiler.c.elf.extra_flags=isr_override.ld --upload --port $(PORT) .