[time-nuts] Clocking a PIC16F628A from a Rubidium Standard

Kit Scally kScally at BYTECAN.com.au
Thu Nov 24 18:17:56 EST 2011


Stephen,

As well as limiting the input voltage swing into the PIC with a ~2k2 resistor, add an additional 4k7 or 10k resistor (AOT) across the PIC's Vcc rail to ground.  This swamps the rectified DC produced by the input drive signal when the PIC DC supply is removed.
I've used /tvb's  PIC divider in a few projects and experienced the same "it's still working" phenomenon.  Took a while before twigging the input protection diodes were responsible for these "unintended consequences".

Kit
VK2LL


On Nov 24, 2011, at 7:50 AM, Attila Kinali wrote:
> Have a look at the PIC 16F628 data sheet, 
>>snip
Since frequency reference sine wave can exceed Vdd, you want to current 
limit the external clock
<<snip
To limit to 1 ma for an AVR with a TBolt, you'd want to use a 2K
series resistor to drop that extra 2V (7V Pk vs. 5V Vdd) to a 
milliamp of current.

Kevin






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