I ran this off a pic16c74 a few years ago, can't remember the details
though. If you like tonight I can find the circuit etc and see how I did it.

Justin

-----Original Message-----
From: Jinx [mailto:joecolquitt@CLEAR.NET.NZ]
Sent: Thursday, 28 November 2002 11:58 AM
To: PICLIST@MITVMA.MIT.EDU
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")

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