ESP32 Interrupt Based SPI Slave (receive only)

spi-interrupt-test.ino

// Using ESP32 HSPI (SPI2)
// CS is SS --> 
// WR is SCK --> 
// DATA is MOSI --> 
#define SO    12
#define SI    13
#define SCLK  14
#define SS    15

volatile uint16_t bitcount;
volatile uint8_t bytearray[20];
volatile uint8_t messageFlag;
volatile uint8_t curbyte;
volatile uint8_t bytefiller;

void setup() {
  // put your setup code here, to run once:
  Serial.begin(115200);
  Serial.println("I'm alive!!!");

  messageFlag = 0;
  clearByteArray();
  curbyte = 0;
  bytefiller = 0;

  pinMode(SS, INPUT);
  pinMode(SCLK, INPUT);
  pinMode(SI, INPUT);
  
  //Falling SS enables falling clock interrupt
  //Rising SS disabled falling clock and flags serial output
  attachInterrupt(digitalPinToInterrupt(SS), changeSS, CHANGE);
}

void loop() {
  // put your main code here, to run repeatedly:
  if (messageFlag) {
    messageFlag = 0;
    for (uint8_t i=0; i<20; i++) { Serial.print(bytearray[i]); Serial.print(" "); }
    Serial.println();
  }
}

void clearByteArray(void) {
  for (uint8_t i=0; i<20; i++) {
    bytearray[i] = 0;
  }
}
void changeSS(void) {
  if(digitalRead(SS)) {
    //Rising Edge
    //Serial.println("Rising Edge!");
    //Packet is done, stop listening to clock and flag for a completed message
    detachInterrupt(digitalPinToInterrupt(SCLK));
    //Shift in the remaining bits to the nearest byte
    if (bitcount) {
      bytearray[bytefiller] = (curbyte << (8-bitcount));   
    }

    //Tell main to print the message
    messageFlag = 1;
  }
  else {
    //Serial.println("Falling Edge!");
    //Falling Edge
    //Listen for falling clock and keep track of bit count
    bitcount = 0;
    bytefiller = 0;
    attachInterrupt(digitalPinToInterrupt(SCLK), fallingSCLK, FALLING);
  }
}

void fallingSCLK(void) {
  curbyte = (curbyte << 1) + digitalRead(SI);
  if (++bitcount > 7) {
    bitcount = 0;
    bytearray[bytefiller++] = curbyte;
  }
}