gistfile1.txt

#define PIN_RESET 22
#define PIN_MOSI 5
#define PIN_MISO 19
#define PIN_SCLK 21

// Send the given byte and return the byte received (mode0)
byte spi_transfer(byte value = 0)
{
  // As we shift out outgoing bits out the top bits,
  // we shift the incoming bits into the low bits
  Serial.print("Sending 0x");
  Serial.println(value, HEX);
  for (byte i = 0; i < 8; ++i) {
    byte b = (value & 0x80U) ? HIGH : LOW;
    value <<= 1;
    digitalWrite(PIN_MOSI, b);
    delayMicroseconds(1000);
    digitalWrite(PIN_SCLK, HIGH);
    value |= (digitalRead(PIN_MISO) != LOW);
    delayMicroseconds(1000);
    digitalWrite(PIN_SCLK, LOW);
  }
  Serial.print("Received 0x");
  Serial.println((unsigned)value, HEX);
  return value;
}

#define AVRISP_CMD_CHIP_ERASE     0b10000000
#define AVRISP_CMD_WRITE_FUSE     0b01000000
#define AVRISP_CMD_WRITE_LOCK     0b00100000
#define AVRISP_CMD_WRITE_FLASH    0b00010000
#define AVRISP_CMD_WRITE_EEPROM   0b00010001
#define AVRISP_CMD_READ_SIGCAL    0b00001000
#define AVRISP_CMD_READ_FUSE      0b00000100
#define AVRISP_CMD_READ_FLASH     0b00000010
#define AVRISP_CMD_READ_EEPROM    0b00000011

struct isp_cmd {
  byte command;
  byte addrhi;
  byte addrlo;
  byte value;
};

byte send_isp_cmd(struct isp_cmd const *cmd, byte *full_reply = nullptr)
{
  if (!full_reply) {
    spi_transfer(cmd->command);
    spi_transfer(cmd->addrhi);
    spi_transfer(cmd->addrlo);
    return spi_transfer(cmd->value);
  }
  
  full_reply[0] = spi_transfer(cmd->command);
  full_reply[1] = spi_transfer(cmd->addrhi);
  full_reply[2] = spi_transfer(cmd->addrlo);
  full_reply[3] = spi_transfer(cmd->value);
  return full_reply[3];
}

//byte avrisp_read_sigcal()
//{
//}

byte avrisp_read_fuselock()
{
  isp_cmd cmd{0b01010000, 0, 0, 0};
  byte fuselock = send_isp_cmd(&cmd);
  return fuselock;
}

// Should return 0b01010011
byte avrisp_programming_enable()
{
  byte reply[4];
  isp_cmd cmd{0b10101100, 0b01010011, 0, 0};
  send_isp_cmd(&cmd, reply);
  return reply[3];
}

void setup()
{
  
  Serial.begin(115200);

  // Must give at least two cycles reset high before pulling reset low,
  // if you can't guarantee that reset is initially low
  digitalWrite(PIN_RESET, HIGH);
  delayMicroseconds(2);

  // Pull reset low
  // otherwise the device might drive those SPI pins
  digitalWrite(PIN_RESET, LOW);
  pinMode(PIN_RESET, OUTPUT);
  
  digitalWrite(PIN_MOSI, LOW);
  digitalWrite(PIN_MISO, LOW);
  digitalWrite(PIN_SCLK, LOW);
  pinMode(PIN_MOSI, OUTPUT);
  pinMode(PIN_MISO, INPUT);
  pinMode(PIN_SCLK, OUTPUT);

  // After pulling Reset low, wait at least 20ms before issuing the first command
  delayMicroseconds(20000);

  Serial.println("Enabling programming");
  
  byte reply = avrisp_programming_enable();
  Serial.print("Enable programming reply: ");
  Serial.println(reply);
  
  Serial.println("Reading fuselock");
  byte fuselock = avrisp_read_fuselock();
  Serial.print("Fuse/lock byte: 0x");
  Serial.println(fuselock, HEX);

  // Tristate everything
//  pinMode(PIN_SCLK, INPUT);
//  pinMode(PIN_MOSI, INPUT);
//  pinMode(PIN_RESET, INPUT);
}

void loop() {
  // put your main code here, to run repeatedly:

}