I am simulating a master-slave scenario. The master is the PIC18F4620, the slave is the I2C debugger.

I am sending a slave address of 0x04. R/W = 0 to perform a write operation on the slave.

This is my frame format.

        //Slave address is 0x04.
        //Send data = 0x77
        //send repeated start
        //send data = 0x00 , then send stop condition.

The transmitter which is the PIC18F is transmitting the frame correctly. The problem is that the I2C debugger won`t send ACK, it only sends NOT ACK.

Does anybody know why I am receiving NACK from the I2C debugger?

enter image description here

  • 1
    \$\begingroup\$ A debugger is not intended to be an active participant on the bus, it is only there to monitor the interaction between the controller and the appication device(s). \$\endgroup\$
    – Dave Tweed
    Sep 25, 2022 at 19:38
  • \$\begingroup\$ I see video tutorials on youtube that the debugger responds with ACK \$\endgroup\$ Sep 25, 2022 at 20:35
  • \$\begingroup\$ Maybe some debuggers can be put into a "device emulation" mode. Relatively straightforward to do for generic devices like memories; it'd be much more difficult to do the broader range of I2C sensors and other devices. \$\endgroup\$
    – Dave Tweed
    Sep 25, 2022 at 21:04

2 Answers 2


The debugger just monitors bus activity and logs it. It doesn't transmit anything on the bus itself, it just records what other devices transmit.

So the debugger's not transmitting a NACK or transmitting anything. A NACK is just SDA left pulled high during that bit period and detecting no bus activity looks the same as detecting a NACK.


The waveform sends start and 0x00, so if that is not a valid device address then no device sends ACK.

Note that there is no ACK for the byte 0x77 either.

  • \$\begingroup\$ The waveform shown is just a screenshot. \$\endgroup\$ Sep 25, 2022 at 19:56
  • \$\begingroup\$ Yes, I am aware of that. Screenshot or not, the I2C protocol is still wrong. There is no address sent after start. \$\endgroup\$
    – Justme
    Sep 25, 2022 at 20:00
  • 1
    \$\begingroup\$ In the code snippet i sent 0x04 as the slave address, it is sent but i didn't take a screenshot of it on the waveform \$\endgroup\$ Sep 25, 2022 at 20:02
  • \$\begingroup\$ It is although shown on the i2c debugger widnow \$\endgroup\$ Sep 25, 2022 at 20:03
  • \$\begingroup\$ @MuhammedElmaghraby The "Sr 00" confirms it's wrong. Anyway, that's a separate problem, if you don't have anything on bus that would ACK an address. \$\endgroup\$
    – Justme
    Sep 25, 2022 at 20:27

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