Whats the difference between a weak pull-up and a strong pull-up? I typically use 10K to pull-up a digital line connected to say something like a push button.

I've also been using 10K resistors to pull up the Rx line into a UART because at power-up the transmit side of this line floats for a little bit and the microcontroller on the receive side gets confused. Adding the 10K solved the problem, but I'm wondering if there are any implications to using a 10K vs the 100K I've seen in some other designs. Is there a formula to calculate the optimal resistance WRT baud rate? Thanks.

 

Take a look at the input currents IIH of some typical TTL gates.
For example,
IIH for 7400 is 40μA @ VI = 2.4V
IIH for 74LS00 is 20μA @ VI = 2.7V

Hence it depends on which family of TTL you are using.
100k would not be sufficient pull-up for 7400 and a very weak pull-up for 74LS00.

1k-5k would be strong pull-up.
5k-20k would be weak pull-up.

You refer to MCU inputs. Which MCU? Most MCUs today are CMOS, not BJT TTL.
Check the datasheet of your MCU.
10k to 50kΩ pull-up would be acceptable as weak pull-up for MCU inputs.

 

Pull-ups draw current so in general, use the largest value resistor you can to make it work. Especially important in low power circuits.

 

Switching speeds will depend on RC charge and discharge times.
If RC is too large, switching speeds will be degraded. Hence don't use large values of R.

 

Whats the difference between a weak pull-up and a strong pull-up? I typically use 10K to pull-up a digital line connected to say something like a push button.

I've also been using 10K resistors to pull up the Rx line into a UART because at power-up the transmit side of this line floats for a little bit and the microcontroller on the receive side gets confused. Adding the 10K solved the problem, but I'm wondering if there are any implications to using a 10K vs the 100K I've seen in some other designs. Is there a formula to calculate the optimal resistance WRT baud rate? Thanks.

Before you could come up with a formula to calculate an optimal value of something, you would need to decide on a metric by which to judge what optimal even means? There are all kinds of competing effects, such as power consumption and noise-immunity, that must be traded off and what is important in one application may be of little concern in another, hence the "optimal" solution in each would be different.

 

Noise susceptibilty is sometimes a consideration as well. As an example, I have one product in which I had to change from 10k pull-up resistors to 1k on the keypad row lines because under some conditions electrical noise interfered with normal operation.