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

Orin Eman orin.eman at gmail.com
Fri Nov 25 13:49:09 EST 2011


On Fri, Nov 25, 2011 at 12:17 AM, Attila Kinali <attila at kinali.ch> wrote:

> On Thu, 24 Nov 2011 12:05:13 -0700
> Kevin Rosenberg <kevin at rosenberg.net> wrote:
>
> > 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.
>
> Using the protection diodes as part of the circuit is bad design practice.
> Use instead explicit shottky diodes (like BAT54S) for clamping.
>


I agree entirely.  I had an AtoD converter that if I let the protection
diodes conduct at all, would be several counts out.  In this case, even a
schottky diode wasn't enough!  I ended up clamping to slightly less than
Vdd using a schottky diode and a TL431 to provide about Vdd-0.2V.

Orin.



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