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

Robert Atkinson robert8rpi at yahoo.co.uk
Sat Nov 26 07:31:59 EST 2011

----- Forwarded Message -----
From: Robert Atkinson <robert8rpi at yahoo.co.uk>
To: Orin Eman <orin.eman at gmail.com> 
Sent: Saturday, 26 November 2011, 12:21
Subject: Re: [time-nuts] Clocking a PIC16F628A from a Rubidium Standard

Absolute max I understand, but even at 10% of Abs max you have 2mA, a significant current and more than enough to power a pic at lower clock speeds. A 100k resistor in series with the US mains of 115V would not exceed 10% of abs max. I'd be happy to run a non-critical devices at 75% of Abs max. 

You need to read the voltage spec in conjunction with the current. If there is no current limiting the voltage must be kept 0.3V below / above the supply rails to prevent the diodes conducting. If the current is limited to less than 20mA the voltage will be clamped to within 0.3V of the rails without damage.  

Robert G8RPI

 From: Orin Eman <orin.eman at gmail.com>
To: Robert Atkinson <robert8rpi at yahoo.co.uk>; Discussion of precise time and frequency measurement <time-nuts at febo.com> 
Sent: Saturday, 26 November 2011, 8:43
Subject: Re: [time-nuts] Clocking a PIC16F628A from a Rubidium Standard

On Fri, Nov 25, 2011 at 1:21 PM, Robert Atkinson <robert8rpi at yahoo.co.uk> wrote:

>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.

Yes, absolute max - that below which the part doesn't let the magic smoke out...  but note the comment below the absolute max ratings (I'm taking this from a PIC16F88 datasheet since I happen to be using this part):   "This is a stress rating only and functional operation of the device at those or any other conditions above those indicated in the operation listings of this specification is not implied. Exposure to maximum rating conditions for extended periods may affect device reliability."

Note also the max voltage on any pin is something like Vdd+0.3V.  One wonders how much conducting their diodes do with 0.3V across them.

Personally, I don't let the protection diodes conduct in normal operation.


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