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

Robert Atkinson robert8rpi at yahoo.co.uk
Fri Nov 25 16:21:45 EST 2011


Microchip cerainly condone using input protection diodes of PIC devices as clamps. There are application notes for zero-crossing detection which connect the input to the 115V AC line via a resistor. Note that these are intentional protection diodes, not unavoidable parasitic junctions. Typical Absolute max clamp current (inc. 16F628) is +-20mA. As a side note, when using these diodes for ESD protection, Microchip recommend using 0.01uF supply decoupling capacitors close to the chip rather than 0.1uF. This reduces the peak current. Trace inductance limits the effect of more distant capacitive loading.

Robert G8RPI.

 From: Attila Kinali <attila at kinali.ch>
To: Discussion of precise time and frequency measurement <time-nuts at febo.com> 
Sent: Friday, 25 November 2011, 8:17
Subject: Re: [time-nuts] Clocking a PIC16F628A from a Rubidium Standard
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.
Better would be to use a resisitive divider (probably with a capacitive
divider in parallel), a coupling capacitor to connect it to the clock
input. You can limit the swing of the signal to less than 1V as the clock
input doesnt require a big signal (when using a crystal, the "signal" can
be as low as a few mV, depending on the chip)

            Attila Kinali

Why does it take years to find the answers to
the questions one should have asked long ago?

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