[GPS_Standard] GPS_Standard Digest, Vol 30, Issue 5

[Arie Dogterom]

The PIC accepts up to 50 MHz easily but circuits in front of it might 
limit the maximum frequency,
73
Arie

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> Today's Topics:
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>    1. Re: Custom PIC Chip to Regulate 20 MHz VCO (Bert, VE2ZAZ)
> 
> 
> ----------------------------------------------------------------------
> 
> Message: 1
> Date: Sat, 18 Jul 2009 10:26:22 -0400
> From: "Bert, VE2ZAZ" <ve2zaz at sympatico.ca>
> Subject: Re: [GPS_Standard] Custom PIC Chip to Regulate 20 MHz VCO
> To: "P M Gant" <ha5rxz at gmail.com>,	<gps_standard at mailman.qth.net>
> Message-ID: <BLU0-SMTP68DA3A35300BB72691C5B1F11F0 at phx.gbl>
> Content-Type: text/plain;	charset="Windows-1252"
> 
> Hello P M,
> 
> It took me a while to throw in my two cents because I was away on a camping
> trip. Now I am back into real life...
> 
> Theoretically, changing the nominal value in Hex from 0x6800 (decimal 26624)
> to 0xD000 (decimal 53248) would make the firmware work with a 20MHz OCXO.
> 
> BUT
> 
> You are pushing the envelope in terms of input and output pulse width specs.
> 20MHz will supply Vhigh and Vlow square levels shorter than the specified
> minimum. See the CCPx timing table in the PIC18F2220. Moreover, U1, the
> input RS485 driver, is already running beyond its limit at 10MHz. Same to be
> said on U3, the output driver.
> 
> So it may work at 20MHz, but it is not guaranteed to work under any
> voltage/temperature/aging conbination. If I were you, I would simply add a
> divide-by-two flip-flop stage ahead of my design. This will work,
> guaranteed. But in the end, it is our call...
> 
> Cheers,
> 
> Bert, VE2ZAZ
> 
> 
> ----- Original Message -----
> From: "P M Gant" <ha5rxz at gmail.com>
> To: <gps_standard at mailman.qth.net>
> Sent: Tuesday, July 14, 2009 6:01 AM
> Subject: [GPS_Standard] Custom PIC Chip to Regulate 20 MHz VCO
> 
> 
> Is it possible to prepare a custom version of the PIC to regulate a 20
> MHz oscillator or is this frequency too high?
> 
> HA5RXZ
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> End of GPS_Standard Digest, Vol 30, Issue 5
> *******************************************
>