I have an MCU (Texas Instruments tm4c123gh6pm) in which I have configured an I2C slave device. I have taken 3.3V from the MCU pin that provides them and connect them to two 4.7 kOhm resistors (to make the pull-ups). I have tried with 2 1.5V batteries in series as well.

When I connect one of the above two lines to the slave SDA, it remains high (fine) but, for some reason, as soon as I connect SCL to the other pull-up res., it goes low (to 0.8 V or so). For some reason, the slave's SCL pulls down the line while the it is doing nothing (I haven't even connected it to the master).

Why this happens?

Here is how I initialize the I2C slave:

static void I2C1_Init(void) {





I'm providing the pull-up resistors from an external power supply, whose ground I connect to the MCU GND pin. The MCU is powered by a laptop through its USB port. I have observed that the problem above doesn't arise if I don't connect these 2 grounds, but as far as I understand I do need to connect them, right?


2 Answers 2


The SCL pin is initialized, when is a master, to be push-pull since in reality the SCL pins are having an internal pull system. Try with GPIOPinTypeI2C(GPIO_PORTA_BASE, GPIO_PIN_6); without the SCL which should call the OD type. It could be that in slave configuration, there are issues. In fact, is not the best documented part, when used as a I2C slave, and I won't exclude presence of bugs in the IC.

  • \$\begingroup\$ I have tried it and get the same result. \$\endgroup\$
    – Martel
    Dec 24, 2021 at 20:08

I have tried with a different I2C module. I was using I2C1 (pins PA6 and PA7) and now I'm using I2C2 (pins PE4 and PE5). All works fine, it seems my MCU is damaged or faulty.

  • 2
    \$\begingroup\$ If this solved your problem then please "accept" your own answer so other people don't waste time reading your question. \$\endgroup\$ Jan 2, 2022 at 12:09

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