Arduino Uno (master) and ATTiny85 (slave) I2C connection not working

Question

I am trying to send and receive data using I2C between an Arduino Uno and an ATTiny85. The Arduino Uno is using the Wire library. The ATTiny85 is using the TinyWire library and is programmed (and powered) using the Sparkfun Tiny AVR Programmer.

I am not receiving any data.

Arduino Uno I2C master code

#include <Wire.h>
#define SLAVE_ADDR 0x50
void setup()
{
 Wire.begin();
 Serial.begin(9600);
}
void loop()
{
 Wire.requestFrom(SLAVE_ADDR, 1);
 while(Wire.available())
 { 
 byte byteReceived = Wire.read();
 Serial.println(byteReceived, DEC);
 }
}

ATTiny85 I2C slave code

#include "TinyWireS.h"
#define SLAVE_ADDR 0x50
void setup()
{
 TinyWireS.begin(SLAVE_ADDR);
 TinyWireS.onRequest(requestEvent);
}
void loop()
{
}
void requestEvent()
{
 byte byteToSend = 23;
 TinyWireS.send(byteToSend);
}

Answer 1

I modified your Uno sketch slightly to add a delay:

#include <Wire.h>
#define SLAVE_ADDR 0x50
void setup()
{
 Wire.begin();
 Serial.begin(9600);
}
void loop()
{
 Wire.requestFrom(SLAVE_ADDR, 1);
 if (Wire.available())
 { 
 byte byteReceived = Wire.read();
 Serial.println(byteReceived, DEC);
 }
 delay (1000);
}

Upon testing, it worked perfectly!

23
23
23
23
23
23
23
23
23
23
23
23

I suggest a wiring error. Check you have:

Uno ATtiny85
--------------------
A4 pin 5 (SDA)
A5 pin 7 (SCL)
+5V pin 8
Gnd pin 4

Answer 2

I think the problem is (assuming you've got the pins straight) is that there's no time for the byte to get to the Uno. Try while(!Wire.available()) between requestFrom... and the while(Wire.available())

• Nick Gammon: Wire.requestFrom does not return until data is available, or if the slave device cannot be found. It returns the numbers of bytes available.