Attiny88 as SPI slave

Question

I have programmed Attiny88 with Arduino IDE 1.6.5 using https://github.com/SpenceKonde/ATTinyCore . I chose Attiny88 because it's the cheapest Attiny (as low as $0.5) and at the same time has 28 pins with dedicated SPI. I wrote a code for arduino to work as SPI master, and a code for Attiny88 as SPI slave but I get only 0 as answer from Attiny.

SPI master code for ARduino UNO

#include <SPI.h>

// set up the speed, data order and data mode
SPISettings settingsA(2000000, MSBFIRST, SPI_MODE0); 
const int slaveAPin = 10;

void setup() {
  // set the Slave Select Pins as outputs:
  Serial.begin(9600);
  Serial.println("SPI ON"); 
  pinMode (slaveAPin, OUTPUT);
  SPI.begin(); 
}

uint8_t stat, val1, result;

void loop() {
   // read two bytes from device A
   SPI.beginTransaction(settingsA);
   digitalWrite (slaveAPin, LOW);
   // reading only, so data sent does not matter
  stat = SPI.transfer(0);
  val1 = SPI.transfer(0);
  digitalWrite (slaveAPin, HIGH);
  SPI.endTransaction();
  Serial.println("data");
  Serial.println(stat); 
  Serial.println(val1);
  Serial.println("end");    
  SPI.endTransaction();
  delay(300);
}

SPI slave code for Attiny88:

#include <SPI.h>

// set up the speed, data order and data mode
SPISettings settingsA(2000000, MSBFIRST, SPI_MODE0); 
const int slavePin = 14;
const int ledPin = 9;

uint8_t stat, val1, val2, result;

void setup() {
  // set the Slave Select Pins as outputs:
  pinMode (slavePin, INPUT);
  pinMode (ledPin, OUTPUT);
  // initialize SPI:
  digitalWrite(ledPin, HIGH);
  delay(1000);
  SPI.begin();
  stat =128;
  val1= 1;
}

void loop() {

  if (slavePin==LOW) {
    SPI.beginTransaction(settingsA);    
    SPI.transfer(stat);
    SPI.transfer(val1);
    result = SPI.transfer(0);
    digitalWrite(ledPin, result);
    SPI.endTransaction();
  }

}

Answer 1

You do not appear to be configuring the slave as an SPI slave. Merely changing the pin modes won't achieve this. Example code at SPI - Serial Peripheral Interface - for Arduino - however that does not specifically mention the Attiny88.

In my example in reply #1 on that page I have this code:

  // turn on SPI in slave mode
  SPCR |= bit (SPE);

  // have to send on master in, *slave out*
  pinMode(MISO, OUTPUT);

This sets SPCR bit MSTR to zero (by not mentioning it). If you call SPI.begin() it will be activated in master mode.

It looks like the Attiny88 has the same register.

So in my opinion your code merely has two SPI masters.

Also, you seem to be doing 3 transfers in the master, but only 2 transfers in the slave.