ATinyMaster_Wire.ino

#include "Wire.h"

#define I2C_SLAVE_ADDR 0x10

#define RESET_PIN 2

void setup() {
  
  Wire.begin();

  pinMode(RESET_PIN,OUTPUT);
  // reset the slave
  digitalWrite(RESET_PIN, LOW);
  delay(10);
  digitalWrite(RESET_PIN, HIGH);
  
  // wait for slave to finish any init sequence
  delay(2000);
}

void loop() {
  uint8_t i;
  uint8_t inByte = 0;
  uint8_t outByte;
  uint8_t reqBytes;
  
  // generate, save, and send N random byte values
  Wire.beginTransmission(I2C_SLAVE_ADDR);

  outByte = random(10);
  Wire.write(outByte);
  Wire.endTransmission();
  
  //delay (500);  // optional delay if required by slave (like sample ADC)
  
  // read N bytes from slave
  reqBytes = Wire.requestFrom(I2C_SLAVE_ADDR, (int)1);      // Request N bytes from slave

  if(Wire.available()){
    Serial.print("Sent out byte: ");
    Serial.print(outByte);
    Serial.println("");

    Serial.print("returned # bytes: ");
    Serial.print(reqBytes);
    Serial.println("");

    for(i = 0;i<reqBytes;i++){
      Serial.print("Got Bytes back: ");
      inByte = Wire.read();
      Serial.print(inByte);
      Serial.println("");
    }
  }
  else{
    Serial.println("Wire request info empty");
  }
  // delay 1 second so user can watch results
  delay(5000);

}

ATinySlave.ino

#include <TinyWireS.h>

#define I2C_SLAVE_ADDR 0x10


//HERE IS THE PIN OUTS OF THE ATTINY85-20PU:
//https://arduinotech.dk/wp-content/uploads/2013/11/ATTiny85-pin.jpg
//Best labelling i could find.

int led = 3;
int but = 4;

const int DEBOUNCE_TIME = 5;
// the setup routine runs once when you press reset:

// The default buffer size, Can't recall the scope of defines right now
#ifndef TWI_RX_BUFFER_SIZE
#define TWI_RX_BUFFER_SIZE ( 16 )
#endif

uint8_t master_data[16];
// global variable to number of bytes sent from the master.
uint8_t master_bytes;

uint8_t master_byte;

void setup() {                
  // initialize the digital pin as an output.
  pinMode(led, OUTPUT);
  pinMode(but,INPUT);
  
  TinyWireS.begin(I2C_SLAVE_ADDR);
  TinyWireS.onRequest(requestEvent);
  TinyWireS.onReceive(receiveEvent);
  
//LED turns on to indicate program is running and waiting.
  digitalWrite(led,HIGH);
}
/*
 * This is called for each read request we receive, never put more than one byte of data (with TinyWireS.send) to the 
 * send-buffer when using this callback
 */
void requestEvent()
{
  uint8_t blinkCount;
    //Depending on the amount of bytes, lets do a blink code.
  digitalWrite(led,HIGH);

//  for(blinkCount = 0;blinkCount < 3;blinkCount++){
//  
//      digitalWrite(led,HIGH);
//      delay(400);
//      digitalWrite(led,LOW);
//      delay(400);
//  }
  TinyWireS.send(master_byte);
}

/**
 * The I2C data received -handler
 *
 * This needs to complete before the next incoming transaction (start, data, restart/stop) on the bus does
 * so be quick, set flags for long running tasks to be called from the mainloop instead of running them directly,
 */
void receiveEvent(uint8_t numBytes)
{
  digitalWrite(led,LOW);
    master_byte = 0;
    //digitalWrite(led,HIGH);
    if (numBytes < 1)
    {
        // Sanity-check
        return;
    }
    if (numBytes > TWI_RX_BUFFER_SIZE)
    {
        // Also insane number
        return;
    }
    //digitalWrite(led,LOW);

    master_byte = TinyWireS.receive();

//    uint8_t blinkCount;
//    for(blinkCount = 0;blinkCount < master_byte;blinkCount++){
//
//    digitalWrite(led,HIGH);
//    delay(200);
//    digitalWrite(led,LOW);
//    delay(200);
//  }
}



int butState = 0;

// the loop routine runs over and over again forever:
void loop() {

  /**
     * This is the only way we can detect stop condition (http://www.avrfreaks.net/index.php?name=PNphpBB2&file=viewtopic&p=984716&sid=82e9dc7299a8243b86cf7969dd41b5b5#984716)
     * it needs to be called in a very tight loop in order not to miss any (REMINDER: Do *not* use delay() anywhere, use tws_delay() instead).
     * It will call the function registered via TinyWireS.onReceive(); if there is data in the buffer on stop.
  */
  TinyWireS_stop_check();
  //The Megahertz you assign to this, will need to divide the regular delay.
  //If you are on 1 mhz, you don't divide, 300 milliseconds, is 300.
  //If you are on 8 mhz, you need to divide, 300 / 8 = 38 to get the same frequency.
//  butState = digitalRead(but);
//  if(butState == HIGH){
//    digitalWrite(led,HIGH);
//  }
//  else{
//    digitalWrite(led,LOW);
//  }
}