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PICList Thread
'Zero Cross'
1997\03\27@110105 by Peter J (GEA, 068974)

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Hey Pic'ers (pretty close to Packers, eh?),

I've been zero crossing using a lot of goto loops.  That makes me feel a
bit uneasy, but it works so far.  I can imagine loops like that are
dangerous due to the chance an event may be missed and you're stuck or
thrown off count.  Example:

------code start--------
ZEROPLUS        btfsc   PORTA,1 ;this ensures that the beginning of a setting
starts with the                                         ;positive cycle of the a
c line
               goto    ZEROPLUS
Z2              btfss   PORTA,1
               goto    Z2
T_ON            bsf     PORTB,0
A1              btfsc   PORTA,1
               goto    A1
T_OFF           bcf     PORTB,0
               goto    Z2
-----code end--------

You'll note that I'm just verifying the output by having portb bit 0
'follow' the input signal.  Like I said...it works fine, but has anyone
has trouble with relying on such goto loops?

Thanks...Pete
spam_OUTpeter.cesarzTakeThisOuTspamappl.ge.com

1997\03\27@172152 by Philippe TECHER

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At 10:50 27/03/1997 -0500, you wrote:
>Hey Pic'ers (pretty close to Packers, eh?),
>
>I've been zero crossing using a lot of goto loops.  That makes me feel a
>bit uneasy, but it works so far.  I can imagine loops like that are
>dangerous due to the chance an event may be missed and you're stuck or
>thrown off count.  Example:
>------code start--------
>ZEROPLUS        btfsc   PORTA,1 ;this ensures that the beginning of a setting
>starts with the                                         ;positive cycle of
the ac line
{Quote hidden}

# Did you disable the watchdog first ?

# You doesn't talk about the input signal frequency, if signal is slow
there some possibility it stay long enought just at the input triggering
level. Means if Port input trigger level is for example 2V, then you apply a
2V signal at this pin port: There is many chance that while you test
the pin the state will change due to noise input. Sometime you will get
a zero, and sometime you will get a one.
Then if the PIC is running fast enought, you have chance it copy sometime
a zero instead a one (or the inverse) and will obtain something like that:
       ______-_-----------_______-------------_-_-_______________

Don't forget at 4MHz, your test loop will take a maximum
of  3 micro-second. It's fast enought to handle noise.

Best way is to put a comparator at the input with two limitation diodes.
I also see some circuit with an opto-coupler.

Best regards,
       Philippe.

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