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Capacitor value for 18F4520's 40Mhz Crystal value

 
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neverlog



Joined: 02 May 2010
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Capacitor value for 18F4520's 40Mhz Crystal value
PostPosted: Sat Jul 10, 2010 7:23 am     Reply with quote
Dear All,

Is the 18F4520 support 40Mhz?

if yes, what is the capacitor value for 40Mhz crystal?

Please help me. Thanks!

I am trying to search the datasheet, but it only give capacitor value for 25Mhz.
bungee-



Joined: 27 Jun 2007
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PostPosted: Sat Jul 10, 2010 7:49 am     Reply with quote
I usually put 12pF capacitors there. But It should be tested for yor application.
On other hand, you could use resonator or oscillator for more precision for your application.
mkuang



Joined: 14 Dec 2007
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PostPosted: Sat Jul 10, 2010 10:35 am     Reply with quote
There were previous posts about how you will have stability issues using a 40Mhz crystal. A better option might be to use a 10Mhz and PLL it up to 40 Mhz.
PCM programmer



Joined: 06 Sep 2003
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PostPosted: Sat Jul 10, 2010 12:08 pm     Reply with quote
Quote:
Is the 18F4520 support 40Mhz?

if yes, what is the capacitor value for 40Mhz crystal?


Look in this section of the 18F4520 data sheet:
Quote:
TABLE 26-6: EXTERNAL CLOCK TIMING REQUIREMENTS

It says the highest frequency for HS mode is 25 MHz. So you can't use
a 40 MHz crystal. You have to use 10 MHz and 4x PLL as explained in a
previous post.

You can use an external 40 MHz oscillator, but it's cheaper to use the
10 MHz crystal and PLL.
neverlog



Joined: 02 May 2010
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PostPosted: Mon Jul 12, 2010 7:56 pm     Reply with quote
Thank you for all the reply and the advice!
neverlog



Joined: 02 May 2010
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PostPosted: Mon Jul 12, 2010 7:59 pm     Reply with quote
Hi,

Can any one have any good reference for how to use 10 MHz and 4x PLL ?

Thanks in advance!
PCM programmer



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PostPosted: Mon Jul 12, 2010 8:23 pm     Reply with quote
Example program:
http://www.ccsinfo.com/forum/viewtopic.php?t=29761&start=1
neverlog



Joined: 02 May 2010
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PostPosted: Mon Jul 12, 2010 10:25 pm     Reply with quote
PCM programmer wrote:
Example program:
http://www.ccsinfo.com/forum/viewtopic.php?t=29761&start=1


Thanks for the help!!
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