endTransmission() does not return SCL high on NAK

[muadeeb]

• Arduino board: None -- raw ATTiny25

• Arduino IDE version (found in Arduino -> About Arduino menu): 1.8.3 (Windows Store 1.8.6.0)

-I'm using a Tiny25 to scan for an unknown number of I2C devices so I later can only send data to devices that I know exist. In doing this, I'm sending a known request for version info from the slave devices. The problem is if the device doesn't exist then TinyWireM.endTransmission() does not return the SCL line to high.

You can see the clock not returning to high here:

Close up of where the problem starts:

Data 0x14 is the reply from a device, address 0x11 is a non-existent possible device.

Snippet of the scanning code:
for (int a = 0x10; a < 0x18; a++) { TinyWireM.beginTransmission(a); TinyWireM.send(0x7F); TinyWireM.endTransmission(); if (TinyWireM.requestFrom(a, 1) > 0) { Addresses[AirChains++] = a; while(TinyWireM.available()) { TinyWireM.receive(); } } delayMicroseconds (50); }

[muadeeb]

I am able to work around the issue by changing the following code in TinyWireM.cpp

uint8_t USI_TWI::endTransmission(uint8_t stop){ // actually sends the buffer
  bool xferOK = false;
  uint8_t errorCode = 0;
  xferOK = USI_TWI_Start_Read_Write(USI_Buf,USI_BufIdx+1); // core func that does the work
  USI_BufIdx = 0;
  if (xferOK) {
    if (stop) {
      errorCode = USI_TWI_Master_Stop();
      if (errorCode == 0) {
        errorCode = USI_TWI_Get_State_Info();
        return errorCode;
      }
    }
    return 0;
  }
  else {                                  // there was an error
    errorCode = USI_TWI_Get_State_Info(); // this function returns the error number
	USI_TWI_Master_Stop(); //Forces a STOP to be sent -- added by Muadeeb
    return errorCode;
  }
}

[manoloaterol]

Same problem here with a Tiny24a.

Works perfect after the change.

Thanks @muadeeb !

[oomek]

This workaround does not work on Attiny85 unfortunately, The SCL is still low after NACK. Any clues?