Post by thread:27k too big for a pull up?

I have been looking at some encoders for use in a pic project. I just noticed they spec sheet said 5000 max on resistance . It looks like a 27k pull up might work but I am not sure. I would have take into account the slow rise time that would give I guess. Any one use a pull up that high reliably?

The internal pullups in the PIC are about 12K according to the spec sheet, and these give no problems at pull-up time. 27K will be OK so long as the encoder doesn't have too much capacitance. Use dV/dt = volt change per second = I(amps)/C(farads) as voltage rise rate given current (I) and capacitance (C). Compute I as Vcc/Rpullup e.g. 5/27K = 185 microamps. Estimate low-to-high time based on the voltage thresholds of the logic input you are considering. If this is too slow for your clock speed, just read the pin a couple of times to be sure to be sure... Since your ON resistance is 5K, your low state voltage will be Vcc*5K/(27K+5K) = 0.16Vcc. Check that this will produce a valid logic 0. Regards, SJH Canberra, Australia