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PICList Thread
'Clocking speed'
1998\12\17@053105 by Scott Shidel...

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is there any drawback to running a PIC at full steam ahead?  For instance,
if I have a 20mHz capable processor, is it more prone to erronious
readings at 20mHz? Or is the main reason for going slower for timing
delays, as in doing a/d conversions?



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1998\12\17@065526 by Michael J. Ghormley

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Scott Shidel... wrote:
>
> is there any drawback to running a PIC at full steam ahead?  For instance,
> if I have a 20mHz capable processor, is it more prone to erronious
> readings at 20mHz? Or is the main reason for going slower for timing
> delays, as in doing a/d conversions?

The faster it goes, the more current it draws.  In battery-operated projects tha
t don't
need the speed, I use 32kHz Xtals to keep current to a minimum.

Michael

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Men must learn to pretend to be wise and good.  --  Lao Tzu
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1998\12\17@120812 by David W. Duley

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In a message dated 12/17/98 2:32:09 AM Pacific Standard Time,
spam_OUTshidelTakeThisOuTspamENG.USF.EDU writes:

<< is there any drawback to running a PIC at full steam ahead?  For instance,
if I have a 20mHz capable processor, is it more prone to erronious
readings at 20mHz? Or is the main reason for going slower for timing
delays, as in doing a/d conversions?
 >>
Hi Scott!
I run all of our processors at full speed.  There are several reasons to use
other clock speeds. The first is cost and space.  4.00 mhz crystals are about
$1.50 to $2.00 bucks depending on size.  A TV color burst crystal (3.579mhz)
is about $0.69.  If my aplication is not timing critical then whay pay the
extra.
The second would be to facilitate an easy division to some desired frequency.
Some of the PIcs hav a built in UART for serial transmition.  The baud rates
are directly derived from the CPU clock frequency.  Picking values like
1.8432mhz makes baudrate generation simple and accurate.  Other frequencies
might get you close but with slight errors in the baudrate frequency.
If a part is rated at a specific frequency it should run at that frequency
without a problem.

Dave Duley
http://www.dreitek.com

1998\12\17@122919 by Peter Schultz

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Hi,
Also the lower speed has a big advantage to generate less RFI. You have to
consider that if your design must meet some FCC spec.
Cheers,
PeterS

>is there any drawback to running a PIC at full steam ahead?  For instance,
>if I have a 20mHz capable processor, is it more prone to erronious
>readings at 20mHz? Or is the main reason for going slower for timing
>delays, as in doing a/d conversions?

1998\12\17@130225 by Eric Borcherding

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Scott,

No problem up to a certian limit.    Good grounding paths will be needed,
good bypassing, and ALSO run the PIC at higher voltages within its range.
CMOS technology will perform better a the top end of thier Vcc ranges
in general becuase these is more energy to switch faster.   Ie 5V will switch
twice as fast as 3.3.  This also consumes a little more power but dirt power
vs a little more dirt power is what you pay for 20-33MHz with the PIC series.

Eric

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