SPI Arduino Interface with Absolute Encoder - arduino

Question

I am trying to retrieve data from an RLS absolute rotary encoder via SPI through Arduino Uno. I have managed to learn the basics of SPI, but I can't seem to get this working. I keep printing the transfer data and keep getting 255.
I also use the recommended pins for Arduino Uno here: https://www.arduino.cc/en/reference/SPI
Here is the link to the Absolute Encoder DataSheet: https://resources.renishaw.com/en/details/data-sheet-orbis-true-absolute-rotary-encoder--97180
Any help is appreciated.
Here is my code I have been trying:
#include <SPI.h>
unsigned int data;
int CS = 10; //Slave Select Pin
// The SS pin starts communication when pulled low and stops when high
void setup() {
Serial.begin(9600);
RTC_init();
}
int RTC_init() {
pinMode(CS, OUTPUT);
SPI.begin();
SPI.setBitOrder(LSBFIRST); // Sets Bit order according to data sheet (LSBFIRST)
SPI.setDataMode(SPI_MODE2); // 2 or 3, Not sure (I have tried both)
digitalWrite(CS, LOW); // Start communications
/*
Commands List:
Command "1" (0x31) – position request (total: 3 bytes) + 4 for multi-turn
Command "3" (0x33) – short position request (total: 2 bytes) + 4 for multi-turn
Command "d" (0x64) – position request + detailed status (total: 4 bytes) + 4 for multi-turn
Command "t" (0x74) – position request + temperature (total: 5 bytes) + 4 for multi-turn
Command "v" (0x76) – serial number (total: 7 bytes)
*/
unsigned int data = SPI.transfer(0x33); // Data
digitalWrite(CS, HIGH); // End communications
return(data);
}
void loop() {
Serial.println(RTC_init());
}

Answer 1

First, you forgot to set your CS pin to output. That won't help.
void setup()
{
Serial.begin(9600);
digitalWrite(CS, HIGH); // set CS pin HIGH
pinMode(CS, OUTPUT); // then enable output.
}
SPI is a two way master/slave synchronous link, with the clock being driven by bytes being sent out by the master. This means that when transacting on SPI, you are expected to send out as many bytes as you want to receive.
As shown on the timing diagram on page 14 of the datasheet, you are expected to send one byte with the command, then as many null bytes as the response requires minus 1.
The diagram shows the clock being low at idle, and input bits being stable when the clock goes low, that's SPI mode 1.
To read the position, for a non-multiturn encoder.
unsigned int readEncoderPos()
{
// this initialization could be moved to setup(), if you do not use
// the SPI to communicate with another peripheral.
// Setting speed to 1MHz, but the encoder can probably do better
// than that. When having communication issues, start slow, then set
// final speed when everything works.
SPI.beginTransaction(SPISettings(1'000'000, MSBFIRST, SPI_MODE1));
digitalWrite(CS, LOW);
// wait at least 2.5us, as per datasheet.
delayMicroseconds(3);
unsigned int reading = SPI.transfer16(0x3300); // 0x3300, since we're sending MSB first.
digitalWrite(CS, HIGH);
// not needed if you moved SPI.beginTransaction() to setup.
SPI.endTransaction();
// shift out the 2 status bits
return reading >> 2;
}