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

Kevin Rosenberg kevin at rosenberg.net
Thu Nov 24 14:05:13 EST 2011

On Nov 24, 2011, at 7:50 AM, Attila Kinali wrote:
> Have a look at the PIC 16F628 data sheet, it will tell you the requirements
> for the clock input and there should be circuit for external clock signal
> listed. If it's not in the datasheet, it should be in an appnote.
> Usually, chips accept sinusoidal and rectangular inputs up to 0V-VDD swing.
> If i'm reading the schematic correctly and the PIC runs with 5V, then
> you should be able to just feed the Efratom output directly to the PIC.
> Maybe, a ceramic 100n capacitor in this line should be used to do
> a DC isolation (1u cermaic should do as well).

Tom's picPET page http://www.leapsecond.com/pic/picpet.htm describes
using a TBolt sine directly as a clock source. I'm going something
similar with AVRs for an external clock.

Since frequency reference sine wave can exceed Vdd, you want to current 
limit the external clock. For example, an unterminated TBolt puts out 0-7V
Pk-Pk. Atmel, in an app note where they hook up the pins of an AVR
to 220V mains, states the over/under voltage protection diodes should
not carry more than 1 milliamp of current. But, you should both read
the datasheet for the output voltage of the Efratom and measure Pk-Pk
voltage output at the point of your PIC.

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.


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