'27k too big for a pull up?'
David E. Queen
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
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.
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