Should I use pullup or pulldown ?

[milan.rajik]

I am using STM32F407VGT6 for my project. I am new to STM32. I am using it for GSM communication and I want to know whether I have to pullup or pull down UART lines. What value of resistors I have to use. 4.7k ? It operates at 3.3V.

 

[Nasser750gx]

If you're making a PCB, in general, always put a footprint for both. Also if I were you, I would also put a series footprint as well. If not needed, you can leave the footprints un-populated (the series one should be populated with a 0Ohm resistor). However, if you encountered noise, you could make a low-pass filter using the series footprint and one of the pull-up or pull-down footprints.

Based on Datasheet, I see the STM has both internal pull-up and pull-down. For external pull-up (or down), 4.7K would be ok in most of the cases.


Good Luck,