Strange behaviour from PIC16F877 on TMR0

[Archangel]

I thought of one more thing, if your encoder wires are really long the capacitance of the cable may be rounding off the tops of the signal and the PIC just might not like that, in which case and amplifier may be required.
Good Luck
JS

[mister_e]

Maybe better if using opto-coupler instead... just too keep power and logic away. Harder to light a led than introducing a noise wich will be amplified later. Well that's what i did as now.

I had a look to the link you posted. Sound interesting, but exactly in which way you perform the points:

- i'll ensure the PIC Vdd line is correctly filtered

those usual 0.1uF and 10-47uF tantalum as close as possible of the PIC and as many 0.1uF as you have IC... one on each.. still as close as possible.

Bigger cap (47-220uF) + 0.1 uF close to the driver ics...

- i'll screen the PIC and controller section properly to avoid incoming noise and spikes

Proper ground plane design, ground routing, Power lines routing and size against regular signals. Maybe use a grounded metal cabinet to fit it.

- i'll insert a bead on the PIC gnd line

When i deal with multiple Gnds (analog, power, digital) i often add bead core on most of them, usually 1 on analog, 1 on digital.

Do a search for bead core, ground loop, ground plane, ground boucing and i'm sure you may learn a lot of it. It's long to explain and i'm sure i'll mess and miss something. There's pro and cons on each methods... same for ground plane. Sometimes they're usefull, sometimes not. If they're done correctly they're usefull, case not... it will cause you more bugs than benefits.

EDIT: oh i forgot... never leave unused pins floating... tie them somewhere, VDD, VSS via resistor or not. Leaving them floating but setting them to output do nothing good. At least never pass my tests in cars applications.

[mikebar]

Thank you very much Steve.
Except the suggestion about the bead core and the unused pin set as input and never let them floating, I'm already using all your other suggestions.
This mean that I'm in the correct road.

I will try the your other two suggestions.

Thank again you for your time.

Best regards,
Mike.

[Archangel]

I have one LAST Idea as to the source of this problem:
As I understand their operation, a hall Effect switch outputs 1/2 of it's supply voltage in the on condition, so 5v in 2.5 v out, That's dangerously close to the logic low threshold of TTL, so any voltage drop in supply or signal and POOF no encodeer operation. Check the logic high voltage out of the Hall effect sensor and see what it is.
JS

[mikebar]

I have one LAST Idea as to the source of this problem:
As I understand their operation, a hall Effect switch outputs 1/2 of it's supply voltage in the on condition, so 5v in 2.5 v out, That's dangerously close to the logic low threshold of TTL, so any voltage drop in supply or signal and POOF no encodeer operation. Check the logic high voltage out of the Hall effect sensor and see what it is.
JS

Yes, this could be. I've attached the schematics to this thread. As you can see around T2 there is a network made by a transistor. I think that also 2.5V are enough to switch the transistor on/off (really are needed only 0.7V) and is this transistor that drive (by another filter made by zener) the PIC pin RA4. I'm right or I'm missing/misunderstunding something?

Thank you.