[GPS_Standard] Custom PIC Chip to Regulate 20 MHz VCO

[Bert, VE2ZAZ]

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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