decoding quadrature encoders

[picster]

Thank you Richard. I will test this and let you know the results.

Also, though the encoder is optical, I am going through a 74C14 hex Schmitt trigger with rc filtering to remove any stray noise. all cabling from the encoder is shielded. Monitoring the output results in clean square wave with fast rise and fall times.

Chris

Out of curiosity, what R/C values did you find to be optimal on your schmitt triggers?

Picster

[picster]

Out of curiosity, what R/C values did you find to be optimal on your schmitt triggers?

Picster

Ok, I did some trials myself and ended up with R1=100k, R2=33k, C=0.1uF working well with Schmitt trigger inputs on the PIC I'm using (see circuit attached). That's with a detented encoder, using the same values for the two rotary outputs AND the built-in pushbutton.

[Ioannis]

I would use 10th of this values.

Ioannis

[picster]

I would use 10th of this values.

Ioannis

You certainly could. Yet this works great in my application, for which I am using switch polling, not interrupts. I would imagine it also depends on the characteristics of the encoder.

[Ioannis]

Of course it does. But I am worried about the input impedance of the PIC and leakage currents.

Ioannis

[picster]

Hey Ioannis,

I'm hoping you can shed some additional light on this for me.

I know the "design standard" is to use a 10k pullup for PIC inputs that switch to ground - which from my understanding is really not so much about leakage, but more about noise on the "antenna" that an input wire or trace can become. The internal weak pullups tend to work on a breadboard or on a PC board, as long as the system isn't subject to much electrical noise, but "guaranteeing" that environment is far from possible in real world applications. The internal "weak pullups" have equivalent impedance ranges from 200k to 16k, per PIC specs. Meanwhile, input impedance for ports works out to around 50Mohms per specs. I agree, it is always good to have a "solid" and unquestionable pull-up instead of something where spurious glitches can put you into nebulous range, particularly with an input that's NOT configured as a schmitt trigger and you can get all kinds of undefined questionable results. Absolutely makes sense to use "tried and true" 10k in these situations.

However, despite the above, part of the benefit of using a capacitor on the input (again, only with the schmitt trigger config) is that it will help to eliminate spurious noise, effectively shunting spikes to ground and only letting relatively persistent voltage levels through. Hence the reason, I believe, why these values may be demonstrated to work for a schmitt trigger input configuration. I picked these values around the 0.1uF capacitor since I have a ton of them.

Does this make sense, or am I missing something else that I should be considering?

[picster]

Hey Ioannis,

I'm hoping you can shed some additional light on this for me.

I know the "design standard" is to use a 10k pullup for PIC inputs that switch to ground - which from my understanding is really not so much about leakage, but more about noise on the "antenna" that an input wire or trace can become. The internal weak pullups tend to work on a breadboard or on a PC board, as long as the system isn't subject to much electrical noise, but "guaranteeing" that environment is far from possible in real world applications. The internal "weak pullups" have equivalent impedance ranges from 200k to 16k, per PIC specs. Meanwhile, input impedance for ports works out to around 50Mohms per specs. I agree, it is always good to have a "solid" and unquestionable pull-up instead of something where spurious glitches can put you into nebulous range, particularly with an input that's NOT configured as a schmitt trigger and you can get all kinds of undefined questionable results. Absolutely makes sense to use "tried and true" 10k in these situations.

However, despite the above, part of the benefit of using a capacitor on the input (again, only with the schmitt trigger config) is that it will help to eliminate spurious noise, effectively shunting spikes to ground and only letting relatively persistent voltage levels through. Hence the reason, I believe, why these values may be demonstrated to work for a schmitt trigger input configuration. I picked these values around the 0.1uF capacitor since I have a ton of them.

Does this make sense, or am I missing something else that I should be considering?

Note: 10k/3.3k/0.1uF also works with the switch I'm using, so I've changed to this configuration just to stay "standard", and I need not bother with larger caps.