Zero Crossing detector

[Philipp]

I'm trying to learn how to use the Pic's comparator. I want to do a zero crossing detector for a sine wave. Isn't that supose to work , I'm setting A0 and A3 as C1- and C1+ , and A1 and A2 as C2+ and C2-, And I connected the pins A0 and A1 to a 2 sines waves , and A3 and A2 to the ground. I though that when the sine cross the zero , pins A4 and A5 will change their states.

#include <18f4520.h>
#Use delay(clock=4000000)
#fuses NOWDT , XT , PUT ,NOPROTECT , NOBROWNOUT , NOLVP , NOCPD ,NOWRT, NODEBUG
main()
{
while(1)
{
setup_comparator(A0_A3_A1_A2);
enable_interrupts(INT_COMP);
enable_interrupts(GLOBAL);
}
}

[Ttelmah]

You need to generate an 'artificial' zero. Basically, you need an analog zero, between the two PIC supply rails. The comparator inputs (just like those on the rest of the PIC), have clamp diodes preventing them going more than a tiny fraction of a volt below the chip's Vss.
You must have the interrupt handler present, before you enable an interrupt (otherwise, where is the code to go...).
Look at ex_comp.c

Best Wishes

[Phill]

setup_comparator(A1_VR_OUT_ON_A2); // sets two comparators(A1 and VR and A2 as the output)
setup_vref(VREF_HIGH|15);

when the ex put setup_comparator(A1_VR_OUT_ON_A2); that means that the A1 it's the input and the VR its the comparison set in the setup_vref(VREF_HIGH|15) , 3,6V?. So to compare with zero I need to put setup_vref(VREF_Low|0)?

[PCM programmer]

Add a DC offset to your sine wave and set the Comparator Vref at
that voltage.

See Figure 6 on page 14 of Microchip appnote AN823:
http://ww1.microchip.com/downloads/en/AppNotes/00823a.pdf
It shows how to add a positive DC offset to a signal, so that you
can shift the midpoint of the signal into a suitable voltage for
the comparator (such as +2.5v for a PIC running at +5v).
The diagram shows that you can do this with just two resistors.


You didn't give any specs on your input sine wave (amplitude, frequency,
existing DC offset), so I don't know if the above advice will work.
If you need more help, then give that information.