5150/arduino/kbd/kbd.ino

// keyboard wiring diagram: https://www.minuszerodegrees.net/5150/misc/5150_keyboard_reset.jpg

constexpr int kClockPin = PC6;
constexpr int kDataPin = PC4;

constexpr int kBitDelayMicros = 100;
constexpr int kCodeDelayMicros = 200;
constexpr int kKeyDelayMicros = 100;

void setup() {
  // put your setup code here, to run once:
  Serial.begin(115200);
  pinMode(LED_BUILTIN, OUTPUT);

  pinMode(kClockPin, INPUT);
  pinMode(kDataPin, INPUT);

  Serial.println("kbd 0.1");
}

// 0. reset
// 0.0. clock line will be held low externally for 20 ms
//      when back up, keyboard needs to clock 0xAA as a response
//
// 1. operation
// 1.0. just directly clock out scan codes of pressed keys

void sendCode(int code) {
  Serial.printf("sending code 0x%02x\n", code);
  // preconditions: clock and data pins are INPUT

  while (digitalRead(kDataPin) == LOW || digitalRead(kClockPin) == LOW) {
    // wait. we're not allowed to send
  }

  digitalWrite(kDataPin, LOW);
  digitalWrite(kClockPin, LOW);

  pinMode(kClockPin, OUTPUT);
  delayMicroseconds(kBitDelayMicros);
  pinMode(kClockPin, INPUT);
 // delayMicroseconds(kDelayMicros);

  for (int i = 0; i < 8; i++) {
    if ((code & (1 << i)) == 0) {
      // send a 0
      pinMode(kDataPin, OUTPUT);
    }  // else do nothing, it's already a 1
    delayMicroseconds(kBitDelayMicros);

    pinMode(kClockPin, OUTPUT);
    delayMicroseconds(kBitDelayMicros);
    pinMode(kDataPin, INPUT);
    pinMode(kClockPin, INPUT);
  }
  delayMicroseconds(kCodeDelayMicros);
}

enum State {
  kReady,
  kMaybeReset,
};

constexpr int kResetDelay = 19;

State state = State::kReady;
int lastclocklow = 0;

constexpr int kLetterCodes[] = {
  0x1E,  // 'a'
  0x30,
  0x2E,
  0x20,
  0x12,
  0x21,
  0x22,
  0x23,
  0x17,
  0x24,
  0x25,
  0x26,
  0x32,
  0x31,
  0x18,
  0x19,
  0x10,
  0x13,
  0x1F,
  0x14,
  0x16,
  0x2F,
  0x11,
  0x2D,
  0x15,
  0x2c,
};

constexpr int kNumberCodes[] = {
  0x0B,  // '0'
  0x02,
  0x03,
  0x04,
  0x05,
  0x06,
  0x07,
  0x08,
  0x09,
  0x0A,
};

void sendNormalCode(int code) {
  sendCode(code);
  sendCode(code | 0x80);
}

void sendShiftCode(int code) {
  sendCode(42); // left shift
  sendNormalCode(code);
  sendCode(42 | 0x80);
}

void sendAsciiChar(int c) {
  if (c >= 'a' && c <= 'z') {
    return sendNormalCode(kLetterCodes[c - 'a']);
  } else if (c >= '0' && c <= '9') {
    return sendNormalCode(kNumberCodes[c - '0']);
  } else if (c >= 'A' && c <= 'Z') {
    return sendShiftCode(kLetterCodes[c - 'A']);
  }
  switch (c) {
    case '-': return sendNormalCode(12);
    case '=': return sendNormalCode(13);
    case '[': return sendNormalCode(26);
    case ']': return sendNormalCode(27);
    case ';': return sendNormalCode(39);
    case '\'': return sendNormalCode(40);
    case ',': return sendNormalCode(51);
    case '.': return sendNormalCode(52);
    case '/': return sendNormalCode(53);
    case ' ': return sendNormalCode(57);
    case '\n': return sendNormalCode(28);

    case '!': return sendShiftCode(2);
    case '@': return sendShiftCode(3);
    case '#': return sendShiftCode(4);
    case '$': return sendShiftCode(5);
    case '%': return sendShiftCode(6);
    case '^': return sendShiftCode(7);
    case '&': return sendShiftCode(8);
    case '*': return sendShiftCode(9);
    case '(': return sendShiftCode(10);
    case ')': return sendShiftCode(11);
    case '_': return sendShiftCode(12);
    case '+': return sendShiftCode(13);

    case '{': return sendShiftCode(26);
    case '}': return sendShiftCode(27);
    case ':': return sendShiftCode(39);
    case '"': return sendShiftCode(40);
    case '<': return sendShiftCode(51);
    case '>': return sendShiftCode(52);
    case '?': return sendShiftCode(53);
  }
}

void loop() {
  static int led_counter = 0;
  static int led = HIGH;
  if (led_counter > 400000) {
    led_counter = 0;
    led = (led == HIGH) ? LOW : HIGH;
    digitalWrite(LED_BUILTIN, led);
  }
  led_counter += 1;

  int clockp = digitalRead(kClockPin);
  if (state == State::kReady && clockp == LOW) {
    state = State::kMaybeReset;
    lastclocklow = millis();
  } else if (state == State::kMaybeReset) {
    if (clockp == HIGH && millis() - lastclocklow > kResetDelay) {
      delay(1);
      state = State::kReady;
      sendCode(0xaa);
      Serial.println("Reset!");
    }
  }

  if (Serial.available() > 0) {
    int c = Serial.read();
    sendAsciiChar(c);
    //delayMicroseconds(kKeyDelayMicros);
  }
}