A 20Mhz PIC can generate a 500KHz clock via its PWM output.

Set period to 10 (ie set PR2 to 9)

Set duty cycle to 5

And you've got 500KHz, symmetrical.

Bob Ammerman
RAm Systems


----- Original Message -----
From: "Jinx" <joecolquitt@CLEAR.NET.NZ>
To: <PICLIST@MITVMA.MIT.EDU>
Sent: Wednesday, November 27, 2002 7:57 PM
Subject: Re: [OT]: game boy camera


> > On (10) it says the MAX Xck rise time is 0.2 uS
> > (that's MAX)
> >
> > my stamp cannot do this,
> > (I don't think??)
>
> At the pin itself the rise/fall time probably is that fast, but
> generating a 500kHz clock signal is a different matter
>
> > I guess I better get out the pic (877) or whatever is needed
> > (I always have trouble getting the higher Mhz oscillators to
> > function - at all)
>
> You have several options for getting Xck
>
> 1) 500kHz ceramic resonator + a few gates
>
> 2) use a 20MHz crystal for the PIC and divide it down in hardware
> to get 500kHz. 20MHz for a PIC is pretty routine (for a properly
> rated part of course), you shouldn't have any trouble
>
> Tap off the 20MHz from OSC1 or OSC2 with a high impedance
> gate and run it through division h/w as discussed the other day
> ( "[OT]:Need to divide frequency by 163" ). The aim is produce
> a 500kHz Xck with > 800ns hi/lo times as per (10).
>
> 3) A 20MHz PIC may just be able to generate a 500kHz clock via
> TMR0 IRQ at 50% duty cycle. However, the ISR would be coming
> every 5 clock cycles (2 ISRs per Hz - one for high, one for low) so
>
> 4) use one (small/cheap) PIC dedicated to making the 500kHz at
> whatever mark/space ratio is passable by simple instruction cycle
> usage between pin toggles and another PIC for everything else. I
> haven't tried to work it out but you should be able to write a toggling
> routine that takes 10 cycles at 20MHz = 500kHz output
>
> 5) 555 timer
>
> > Let me know if I'm screwing the pooch here....thanks
>
> Please, my reputation for salacious comments is already well
> established ;-) I have no further need to enhance it
>
> (BTW, if some of you guys are in the "doghouse", what does
> that make the house where your missus is ? Starts with "b")
>
> --
> http://www.piclist.com hint: To leave the PICList
> mailto:piclist-unsubscribe-request@mitvma.mit.edu
>
>

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