Arduino Due Slave mode

main.cpp

#include <Arduino.h>

// Pinout https://www.theengineeringprojects.com/wp-content/uploads/2018/09/introduction-to-arduino-due-2-1.png
// With arduino macros https://github.com/manitou48/DUEZoo/blob/master/spislave.ino
// Good doc https://microchipdeveloper.com/32arm:sam-bare-metal-c-programming
// another good code about arduino due SPI slave https://github.com/MrScrith/arduino_due/blob/master/spi_slave.ino

// FUNCTIONS
void initSPI();
void enableOnReceiveDataInterrupt();
void disableOnReceiveDataInterrupt();
uint8_t readSPIData();
void writeSPIData(int data);
void waitUntilDataReceived();
void waitUntilDataSent();
void enableSPIPins();

void setup() {
  Serial.begin(115200);
  while (!Serial);
  initSPI();
}

void loop() {
  waitUntilDataReceived();
  uint8_t dataReceived = readSPIData();
  Serial.print(dataReceived);
}

void initSPI() {
  // Sets MISO as OUTPUT
  // NSS0 (PA28) is pin digital 10
  pinMode(MISO, OUTPUT);
  pinMode(MOSI, INPUT);
  pinMode(SCK, INPUT);
  pinMode(MISO, LOW);
  // turn off SPI
  REG_SPI0_CR = SPI_CR_SPIDIS;
  // Write protection is disabled
  REG_SPI0_WPMR &= ~(SPI_WPMR_WPEN);
  // turn on periheral power (page 38) SPI0 and PIOA.
  REG_PMC_PCER0 |= PMC_PCER0_PID24 | PMC_PCER0_PID11;
  enableSPIPins();
  // RESET SPI
  // enable interrupt register
  // Slave mode & mode fault detection is disabled
  REG_SPI0_MR &= ~(SPI_MR_MSTR);
  REG_SPI0_MR |= SPI_MR_MODFDIS;
  // Select SPI MODE (NCPHA, CPOL) and number of bits (8 default)
  REG_SPI0_CSR |= SPI_CSR_BITS_8_BIT | 0x02;
  // turn on SPI
  REG_SPI0_CR = SPI_CR_SPIEN;
}

void enableSPIPins() {
  // 1: Disables the PIO from controlling the corresponding pin (enables peripheral control of the pin).
  REG_PIOA_PDR |= PIO_PDR_P25 | PIO_PDR_P26 | PIO_PDR_P27 | PIO_PDR_P28;
}

void enableOnReceiveDataInterrupt() {
  // Enable interrupt on receive data
  REG_SPI0_IER |= SPI_IER_RDRF;
}

void disableOnReceiveDataInterrupt() {
  // Disable interrupt on receive data
  REG_SPI0_IDR |= SPI_IDR_RDRF;
}

uint8_t readSPIData() {
  return REG_SPI0_RDR & 0xFF;
}

void writeSPIData(int data) {
  REG_SPI0_TDR = data & 0xFF;
}

// We can either use this or an interruption.
// Interruption might be better since it is not blocking!
void waitUntilDataReceived() {
  while ((REG_SPI0_SR & SPI_SR_RDRF) == 0);
}

void waitUntilDataSent() {
  while ((REG_SPI0_SR & SPI_SR_TDRE) == 0);
}