SSI_encoder_test3.ino

#include <Arduino.h>
#include <SPI.h>

#define CLOCK_SPEED         2'000'000       // 2.5 MHz SSI Clock
#define CS_PIN             10
#define SCK_PIN             13              // SSI CLK line

#define SCK_PIN_13_INDEX    0
//#define SCK_PIN_14_INDEX    1
//#define SCK_PIN_27_INDEX    2

SPISettings settingsA(CLOCK_SPEED, MSBFIRST, SPI_MODE1);

uint8_t encoderBuf[4] = {0, 0, 0, 0};

void makeTransfer();
uint16_t decodeEncoderFrame();

void calculateAndPrintPosition(uint16_t& encoderData);

void setup()
{
  SerialUSB.begin(9600);
  SPI.begin();                            // SPI.begin() will initialize the SPI port, as well as CLK pin

  pinMode(CS_PIN, OUTPUT);
  pinMode(SCK_PIN, OUTPUT);               // pinMode() will initialize the CLK pin as output (SPI port can't use it now!)
  digitalWriteFast(SCK_PIN, HIGH);        // Set CLK line HIGH (to meet the requirements of SSI interface)
//  digitalWriteFast(CS_PIN, LOW);

  while (!SerialUSB) {}
  SerialUSB.println("Started!");
}

void loop()
{
  makeTransfer();
  uint16_t encoderData = decodeEncoderFrame();
  calculateAndPrintPosition(encoderData);

  delay(200);
}

void makeTransfer()
{
 
 digitalWrite(CS_PIN, HIGH);
 digitalWrite(CS_PIN, LOW);

 digitalWriteFast(SCK_PIN, HIGH);
  digitalWriteFast(SCK_PIN, LOW);         // Set CLK line LOW (to inform encoder -> latch data)


  delayMicroseconds(1);                   // Running above 500kHz perform Delay First Clock function

  // Before in setup() the pinMode() change the CLK pin function to output,
  // now we have to enable usage of this pin by SPI port with calling SPI.begin():
  // SPI.begin();

  // Or use this one below - a bit faster but SPI port and SCK pin dependent option. I belive there is a more elegant and more
  // scalable solution, maybe even supported by hardware for this purpose? - if anyone can point me it out I would be grateful:

  uint8_t pinIndex = SCK_PIN_13_INDEX;
  volatile uint32_t* reg = portConfigRegister(SPIClass::spi0_hardware.sck_pin[pinIndex]);
  *reg = SPIClass::spi0_hardware.sck_mux[pinIndex];

  SPI.beginTransaction(settingsA);        // We use transactional API

  for (int i = 0; i < 2; i++)
  {
    encoderBuf[i] = SPI.transfer(0xAA); // Transfer anything and read data back
    Serial.print(i);
    Serial.print(" i ");
    Serial.print((int)encoderBuf[i]);
    Serial.println();

  }

  SPI.endTransaction();                   // We use transactional API


  pinMode(SCK_PIN, OUTPUT);               // A while before we set CLK pin to be used by SPI port, now we have to change it manually...
  digitalWriteFast(SCK_PIN, LOW);
  digitalWriteFast(SCK_PIN, HIGH);            // ... back to idle HIGH
}

uint16_t decodeEncoderFrame()
{
  //    uint32_t data = static_cast<uint32_t>(encoderBuf[0] << 24) |
  //                    static_cast<uint32_t>(encoderBuf[1] << 16) |
  //                    static_cast<uint32_t>(encoderBuf[2] << 8)  |
  //                    static_cast<uint32_t>(encoderBuf[3]);           // here transfer8 was made

  uint16_t data =  static_cast<uint16_t>(encoderBuf[0] << 8)  |
                   static_cast<uint16_t>(encoderBuf[1]);           // here transfer8 was made

  Serial.println("data ");
  Serial.print(data);
  Serial.println();
  
  data = data >> 6; // shift right to cut status and parity bits only 'position data' is in data variable now! 

  return data;
}

void calculateAndPrintPosition(uint16_t& encoderData)
{
  // encoderPosition is placed in front (starting from MSB of uint32_t) so shift it for 11 bits to align position data right
  // no shifting needed (we done it in decodeEncoderFrame())
  // int16_t encoderPosition = (encoderData >> 9);

  // you have 10 bits position data so max is 1 << 10 not 1 << 20 as in my case 
  //float angularAbsolutePosition = static_cast<float>(encoderPosition) / static_cast<float>(1 << 20) * 360.0F;
  float angularAbsolutePosition = static_cast<float>(encoderData) / static_cast<float>(1 << 10) * 360.0F;

  SerialUSB.println("Encoder Positon = " + String(encoderData) + " Angle = " + String(angularAbsolutePosition, 4));
}