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

Hal Murray hmurray at megapathdsl.net
Thu Nov 24 15:55:11 EST 2011

> I have an Efratom 101 frequency standard with a 10 MHz TTL otput. I want to
> use this to clock a PIC 16F628A so I can make a frequency counter with a
> resolution of 1 Hz. I am going to use the design by EI9GQ here http://
> homepage.eircom.net/~ei9gq/counter.html . What I need to know is how i
> should go about electronically interfacing the TTL output from the rubidium
> standard so that it can clock the PIC. Has anyone on the list actually done
> this successfully? 

It will probably just work, but you need to read both data sheets to check 
the details.

"TTL" isn't well defined.  It may mean that it's a digital signal rather than 
a sine wave.  It may mean that it's TTL volltage levels rather than CMOS.  
One common output setup is a strong CMOS driver (typically several sections 
of a chip in parallel) running on 5V with a series 50 ohm resistor.  If the 
receiving end is terminated with a 50 ohm resistor you will see a 2.5V 
signal.  Without the terminator you will see 5V and overshoot/reflections if 
the cable is long relative to the rise time.  2.5V is close to the old TTL 
signal levels.

The PIC clock circuitry can run in several modes.  (This from memory and I 
may be forgetting something or confusing it with other chips.)  The data 
sheet or an app note should have the details.

One mode is for crystals.  It takes 2 pins.  Internally, there is an 
amplifier.  There may be a separate mode for slow (low power) crystals such 
as 32 KHz.

There is another mode for an external signal on the clock-in pin.  It's 
intended for things like this.

If I was doing something like this, I'd probably start with a 50 ohm input 
terminator and a 10K resistor over to the clock input pin.  Then I'd go check 
both data sheets to verify that it would work correctly.

Do you have a scope?

These are my opinions, not necessarily my employer's.  I hate spam.

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