I am running an STM32L476RG at 1.8V and have noticed that I2C-3 does not generate a clock or data signal. I have tried isolating the MCU from the rest of the circuit and found that there is still no signal at the MCU pins, so i know the circuit is not loading the chip. I have changed the MCU and still get an identical signal, and I have run the circuit with an external MCU at 3.3V with no problems. Because of all this I a confident the problem is with the MCU, although I2C-1 is working perfectly. When I look at the output of the SCL3 and SDA3 with a scope, i can see the voltae rise to about 0.9V and then slowly drop back down. In comparisson, the SCL1 and SDA1 lines of the working I2C-1 rises to about 2V, and then the clock and data signals start as the voltage level drops back down.
I am wondering if it could be a product of running it at the low voltage threshold? are there any other pins (pulled high or something?) I need to worry about when using the I2C3? I can't find anything in the datasheet about this.
