Pull-up versus pull-down resistors

To date, all of my PCB designs have included pull-down resistors on the inputs (and outputs). I normally use a 10K 1% resistor in order to provide consistency for things like thermistors when applied. I’ve seen a lot of designs that call for pull-up resistors instead and now I’m wondering if I’ve headed the wrong direction with my designs.

Also, on a previous design with another micro-controller, I always put 0.1uF caps to ground on the inputs, having been advised it’s not a bad idea (helps get rid of noise) and certainly can’t hurt. Thoughts?

I have seen more circuits with pull-up resistors than pull-down. Pull-down will load the circuit if the normal state of the pin in high. As far as the 0.1uF cap, it all depend of the impedance and type of signal on the input.

If you are doing a thermistor, it needs to be biased, ie current needs to flow through it, which is why you have (eg) 3.3v - thermistor - input pin - 10k - ground.

You can also have 3.3v - 10k - input pin - thermistor - ground; this just changes the calculations but is essentially identical from an operational point of view.

Putting a cap on the input pin will definitely reduce noise coupling and also help compensate for charge injection when the sampler samples the pin. The only downside is that it will limit the rate of change of the pin (but that’s not generally an issue if you’re sampling a thermistor and using a 0.1uF cap).