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:

Wiring on atmega: VALUE-PIN

• VCC 7
• GND 8
• SCL 28
• SDA 27

anyone got an idea whats missing?

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