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PICList Thread
'40mhz clock for PIC18FXX2...'
2003\04\30@172403 by Ted Larson

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I just got my hands on an PIC18F252 to fiddle around with, and I want to
clock it at 40mhz.  On the PIC16's I am used to just throwing a 20mhz
ceramic resonator w/caps on there, and being done with it....1
component...nice and simple.  I couldn't find a 40mhz resonator to make this
as simple an operation.

So, what do most people do for a 40mhz, reliable, simple clock for these?

The data sheet seems to suggest using a HS/PLL circuit to 4xstep-up a 10mhz
crystal.  This seems like overkill, and alot of additional
circuitry...although would be nice without lots of EMI to mess things up.  I
have a couple of 20mhz crystals laying around I could double....but, keeping
it simpler is sounding good to me.  I really don't need the whole selectable
clocking feature that using the PLL really buys.

I am still quite the PIC newbie....so any suggestions would be greatly
appreciated.

Thanks,

- Ted

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2003\04\30@173245 by D. Jay Newman

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> have a couple of 20mhz crystals laying around I could double....but, keeping
> it simpler is sounding good to me.  I really don't need the whole selectable
> clocking feature that using the PLL really buys.

Well, you could run it at 20MHz, or you could use the built-in PLL, like
I've always done. To use a crystal all you need are a couple of small
capacitors. I agree that a resonator is easier, and if you have some, use
them.

I've generally used a 10 MHz resonator with the built-in 4xPLL for 40 MHz.

Good luck!
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2003\04\30@174116 by Ted Larson

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Thanks for the quick reply.  I am thinking that is what I am going to
do...it is certainly the simplest method. Just plug it in, and enable the
PLL I just didn't have a 10mhz resonator in my parts box.

Thanks for the advice!

- Ted


{Original Message removed}

2003\04\30@182412 by Dave Dilatush

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Ted Larson wrote...

>So, what do most people do for a 40mhz, reliable, simple clock for these?

Use a 10 MHz resonator and the HS/PLL clock option on the PIC.

>The data sheet seems to suggest using a HS/PLL circuit to 4xstep-up a 10mhz
>crystal.  This seems like overkill, and alot of additional
>circuitry...although would be nice without lots of EMI to mess things up.  I
>have a couple of 20mhz crystals laying around I could double....but, keeping
>it simpler is sounding good to me.  I really don't need the whole selectable
>clocking feature that using the PLL really buys.

What could be simpler?  There's no "whole selectable clocking
feature" involved here- it's a fixed, 4X frequency
multiplication, and it's no harder to use than any of the other
clock modes.  Works great.

Dave D.

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2003\04\30@185837 by Brandon Tarr

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Try these Digi-Key p/n's:

Surface mount 40MHz resonators
PX400CTR-ND
PX400JCT-ND
PX400JTR-ND
PX400CCT-ND

Or, if you don't like surface mount and don't mind spending a little extra,
do what I do and get yourself a 40MHz crystal oscillator.  They are 1 part,
3 connections, can be a clock source for multiple chips (especially when you
include a buffer) and provide a very robust temperature stable clock signal.



{Original Message removed}

2003\04\30@205759 by Dave Dilatush

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Brandon Tarr wrote...

>Try these Digi-Key p/n's:
>
>Surface mount 40MHz resonators
>PX400CTR-ND
>PX400JCT-ND
>PX400JTR-ND
>PX400CCT-ND
>
>Or, if you don't like surface mount and don't mind spending a little extra,
>do what I do and get yourself a 40MHz crystal oscillator.  They are 1 part,
>3 connections, can be a clock source for multiple chips (especially when you
>include a buffer) and provide a very robust temperature stable clock signal.

Note that while the data sheet lists the maximum input clock
frequency from an external oscillator in the EC and ECIO modes as
40 MHz, the maximum frequency for a crystal or resonator in the
HS mode is only 25 MHz.  The PIC's on-chip oscillator may work at
frequencies higher than that, it isn't guaranteed to.

For a hobby project, using a 40 MHz resonator might work; but if
you're trying to make a manufacturable product, it's not a good
idea.

Dave D.

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2003\04\30@222115 by Bob Ammerman

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The PLL is built into the chip, so all you need is a 10MHz XTAL and 2 small
caps.

Bob Ammerman
RAm Systems


{Original Message removed}

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