# I2C slave pulls down SCL while doing nothing

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) {
SysCtlPeripheralEnable(SYSCTL_PERIPH_I2C1);
SysCtlPeripheralEnable(SYSCTL_PERIPH_GPIOA);

GPIOPinConfigure(GPIO_PA6_I2C1SCL);
GPIOPinConfigure(GPIO_PA7_I2C1SDA);

GPIOPinTypeI2CSCL(GPIO_PORTA_BASE, GPIO_PIN_6);
GPIOPinTypeI2C(GPIO_PORTA_BASE, GPIO_PIN_7);

I2CSlaveEnable(I2C1_BASE);
}


## IMPORTANT EDIT

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?