----- Forwarded Message -----
From: Robert Atkinson <robert8...@yahoo.co.uk>
To: Orin Eman <orin.e...@gmail.com>
Sent: Saturday, 26 November 2011, 12:21
Subject: Re: [time-nuts] Clocking a PIC16F628A from a Rubidium Standard
Hi,
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.e...@gmail.com>
To: Robert Atkinson <robert8...@yahoo.co.uk>; Discussion of precise time and
frequency measurement <time-nuts@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 <robert8...@yahoo.co.uk> wrote:
>
>Hi,
>
>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.
Orin.
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