Pull Up Resistors everywhere

Thread Starter


Joined Nov 11, 2016
I just got a PCB in the mail. I designed it last week and here it is. The PCB piggy backs onto an FPGA demo board. I had a few GPIO pins left so I grouped three pins to be used as an I2C or similar protocol if needed. On the CLK pin, I added a place for a pull-up resistor to 3.3v. My thinking is that GPIO pins can sink easier than source and something as light of a load as a clk would still benefit with a ~10K pull-up. Is this good practice?


Joined Oct 2, 2009
Many MCUs today have built-in pull-up and pull-down resistors already on-chip. You need to select and enable them in software if the pins are to be left as inputs.

If you configure unused pins as outputs you do not need pull-up resistors.


Joined Feb 24, 2006
Well it depends on how much resistance is in parallel with the external 10K. The 10K can source a maximum of 500 μA. Most CMOS outputs can source a great deal more than that, so at least it won't hurt. On the other hand, a weak pullup of 40K inside the chip will be aided by the external 10K pullup.


Joined Mar 14, 2008
Normally you don't add a pull-up unless it's needed to keep a node from floating, or the driver is an open-collector or open-source output.


Joined Mar 10, 2018
Check the part datasheet for the range of the pullup.

I know of one ARM family where it ranges internal from 3.5K to 8.5k

Regards, Dana.