How large/what kind of a display are you multiplexing?  You don't need
to update it very often (30-60 times per second).  You have over 8000
instructions/second to update it and do your other work.  If it's
7-segment LED than each segment shouldn't take more than 5-10
instruction cycles to update, fewer with good design.  Of course this
also depends on how your data to be displayed is stored...

-Adam

Pic Dude wrote:

>Using a 16F872, but not familiar with being able to
>use a second crystal to drive TMR1.  I'll hit the
>datasheet again after this.
>
>Think you're missing a bit of context -- The 32.768 is
>fine and easily divided to 1 sec, but I need something
>higher to multiplex the displays w/o flickering.
>
>131.072khz still has a bit of flicker, 1Mhz crystals
>are waaayyy higher in price, and above 1.705Mhz there
>are FCC regs to be worried about.
>
>Thanks,
>-Neil.
>
>
>
>-----Original Message-----
>From: pic microcontroller discussion list
>[mailto:PICLIST@MITVMA.MIT.EDU]On Behalf Of Michael Rigby-Jones
>Sent: Thursday, June 13, 2002 10:43 AM
>To: PICLIST@MITVMA.MIT.EDU
>Subject: Re: [PIC]: FCC question
>
>
>
>
>>-----Original Message-----
>>From: Pic Dude [SMTP:picdude@AVN-TECH.COM]
>>Sent: Thursday, June 13, 2002 4:12 PM
>>To:   PICLIST@MITVMA.MIT.EDU
>>Subject:      Re: [PIC]: FCC question
>>
>>In my sleep last night I was trying to devise a system
>>running 2 clocks -- the pic would run in a basic RC
>>system setup for about 1 MHz, and that would handle all
>>the display and other perihperal stuff; and I'd use an
>>external oscillator at any low frequency (32.788 or less
>>if available) to generate the 1hz signals for the clock.
>>This would have to be sent to the external interrupt
>>pin.
>>
>>However, AFAIK (w/o looking at docs), I can't assign a
>>pre-scaler to that interrupt, so it seems like I'd get
>>way more interrupts than I need and it seems wasteful,
>>and may cause some confusion during EEPROM writes, etc.
>>
>>Yes, I'm avoiding using external freq dividers.  Need to
>>think and research some more about this one...
>>
>>
>>
>Don't know which PIC you are using but the bigger ones with timer 1 have
>provision for a second crystal to drive  timer 1.  If not you could use an
>external oscillator and drive TMR0 and intterupt on overflow .  32.768Khz
>needs to be divided by 2^15 to get 1 second, so setting TMR0 pre-sclaer to
>1:128 and interrupting on rollover (1:256) would give exactly 1 second
>interrupts.
>
>Regards
>
>Mike
>
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>
>
>
>
>

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