I2C / TWI: set up atmega328p as slave

Question

I try to run an atmega328p as a i2c slave. even with the bare minimum the slave acts as its not wired at all: NACK is transmitted after request (see logic analyzer).

The master should read a constant value in this example. iam using the wire.h lib. pullup resistors on SDA, SCL 4.7kOhm.

master.ino: esp8266 (working, sends requests)

#include <Wire.h>
#define DEFAULT_SLAVE_ADDRESS 0x0A
void setup()
{
 Wire.begin(); // join i2c bus as master
 Serial.begin(9600);
 Serial.println("\n Hello");
 delay(2000);
}
void loop()
{
 //Wire.beginTransmission(DEFAULT_SLAVE_ADDRESS);
 //Wire.write(0);
 //Wire.endTransmission();
 //delay(10);
 Wire.requestFrom(DEFAULT_SLAVE_ADDRESS, 1);
 Serial.println(Wire.read());
 delay(1000);
} 

slave.ino: atmega328p

#include <Wire.h>
#define F_CPU 8000000UL
volatile uint8_t nr;
void setup()
{
 Wire.begin(0x0A); // join i2c bus with address #4
 //Wire.onReceive(receiveEvent); // register event
 Wire.onRequest(requestEvent);
}
void loop()
{
}
//void receiveEvent(int howMany)
//{
// while(0 < Wire.available()) // loop through all but the last
// {
// int x = Wire.read(); // receive byte as an integer
// }
//}
void requestEvent()
{
 nr = 99; 
 Wire.write(nr); 
}

Serial output from master: (got no val)

-1
-1
...

Logic analizer output: nack

Wiring on atmega: VALUE-PIN

• VCC 7
• GND 8
• SCL 28
• SDA 27

enter image description here

anyone got an idea whats missing?

Edit: removed receiveEvent(), added CPU speed and it worked

• Sim Son: You seem to have a problem only with the read operation at the moment. I would remove the onReceive part to troubleshoot the problem and test if the example code works (arduino.cc/en/Tutorial/LibraryExamples/MasterReader).
• gilhad: esp8266 is usually 3.3V while atmega328p is 5V - do your hardware setup take it in account? And have the voltage on SDA,SCL the right values for HIGH and LOW on BOTH master and slave?
• timemage: Can you make your 328P's loop() do something, like blink an led so that you know the 328P is operational and at the correct frequency?
• micro_: @SimSon thanks for your reply. when i removed the receiveEvent it worked. i figured out the the problem is the Wire.available() function on the atmega.
• Sim Son: @gilhad the ATmega can be operated at 3.3V, I guess that's the case here. The ATmega also NACKs the read header, which means the address has been transmitted successfully. That doesn't make it look like a hardware issue.