I think in AN521 they assume that 1.8V is close enough to 0v for a 120V
sine wave.  For a low level zero crossing you will probably need a
comparator.  If you just have 0V & 5V supplies you can AC couple the
signal to a level within your range.  Some PICs have built in
comparators but the 877 is not one of them.

How much accuracy do you need? (time & voltage)

Sherpa Doug

> -----Original Message-----
> From: David Dunn [mailto:ddunn@TCAINTERNET.COM]
> Sent: Tuesday, November 06, 2001 11:35 AM
> To: PICLIST@MITVMA.MIT.EDU
> Subject: Re: [PIC]: Zero Crossing Detector
>
>
> this helps a bit, but i already have the hardware side
> figured out, it's not AC LINE that i need zero crossing
> detection, i have a signal that
> i've filtered down to under 5v pp i need to detect zero crossing on.
>
> what i'm not quite figuring out is if a pic pin shows high at
> around 1.8v (i think that's what it is) how is is that you
> can detect a zero
> crossing ?
>
> thanks !
>
> dld
>
> On Tue, 6 Nov 2001 11:20:30 -0500, Douglas Butler wrote:
>
> >I would start with Microchip's application note AN521.  It
> mostly covers
> >hardware issues.
> >
> >Sherpa Doug
> >
> >> -----Original Message-----
> >> From: David Dunn [mailto:ddunn@TCAINTERNET.COM]
> >> Sent: Tuesday, November 06, 2001 11:15 AM
> >> To: PICLIST@MITVMA.MIT.EDU
> >> Subject: [PIC]: Zero Crossing Detector
> >>
> >>
> >> I'm sure in my surfing i've seen that you can do a zero
> >> crossing detector on a pic, possibly with PB0 ? I'm using a 16F877.
> >>
> >> Anybody have links on how to set this up ?
> >>
> >>
> >> Thanks,
> >>
> >> DLD
> >>
> >> --
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> >>
> >>
> >
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