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'more on protection diodes'
1999\10\06@123848 by Jim Hartmann

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Dear list,

A while back I reported that very small currents (<10uA) "into" the upper
protection diode on an input caused the PIC to draw more current into VDD.
Recently I have found that current "out of" the lower protection diode
causes more severe problems.  Currents of about 2mA on the 16C622 can cause
registers within the chip to change or can cause a chip reset (possibly
watchdog).  When pulling current out of a PORT B input, I observed
spontaneous changes to TRISB and OPTION registers as well as reset.

So I guess the moral is to be *very* careful about using the input
protection diodes as clamps for inputs.

-Jim

1999\10\07@101356 by Barry King

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> A while back I reported that very small currents (<10uA) "into" the upper
> protection diode on an input caused the PIC to draw more current into VDD.
> Recently I have found that current "out of" the lower protection diode
> causes more severe problems.  Currents of about 2mA on the 16C622 can cause
> registers within the chip to change or can cause a chip reset (possibly
> watchdog).  When pulling current out of a PORT B input, I observed
> spontaneous changes to TRISB and OPTION registers as well as reset.

Well now, that's very interesting, becuase (at least for the 16C76
that I use) there is a SPECIFICATION that says up to 20 mA in the
clamp diodes is OK.  So you are saying the parts don't meet spec.
Is the spec for the 16C622 different than the 16C76?

Can you tell us more about what the power supply is to this PIC?

Since the clamp diodes are connected to the supply pins, is the
effect you are seeing feedback into the supply?

For example, if you parallel an external diode from the input to the
supply pin, does the same odd behavior occur?

I wonder if you taking the Vss pin negative by clamping a negative
voltage to an insufficient ground path to Vss.  In other words, what's
the rest of the current loop, and where are the voltage drops?

1999\10\07@115025 by Mike Keitz

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On Thu, 7 Oct 1999 09:59:03 -0500 Barry King <spam_OUTbarryTakeThisOuTspamNRGSYSTEMS.COM>
writes:

> Well now, that's very interesting, becuase (at least for the 16C76
> that I use) there is a SPECIFICATION that says up to 20 mA in the
> clamp diodes is OK.  So you are saying the parts don't meet spec.
> Is the spec for the 16C622 different than the 16C76?

The "specification" is an Absolute Maximum Rating.  The only thing these
ratings are good for is to ensure the PIC won't burn out, i.e. suffer
permanent damage.  There is no specification for a current through the
protection diodes durring operation that won't disturb proper operation.
The closest they come to specifying that is specifying the input voltage
ratings.  We'd have to assume that the maximum "normal operation" current
allowed through a protection diode is zero.

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1999\10\08@151346 by Barry King

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> The "specification" is an Absolute Maximum Rating.  The only thing these
> ratings are good for is to ensure the PIC won't burn out, i.e. suffer
> permanent damage.

> There is no specification for a current through the
> protection diodes durring operation that won't disturb proper operation.
> The closest they come to specifying that is specifying the input voltage
> ratings.  We'd have to assume that the maximum "normal operation" current
> allowed through a protection diode is zero.

Mike is absolutely right.

EXTERNAL clamps are needed to prevent the reverse protection
diodes being biased on, for normal operation.

Thanks for clarifying.

-Barry.
------------
Barry King, KA1NLH
Engineering Manager
NRG Systems "Measuring the Wind's Energy"
Hinesburg, Vermont, USA
http://www.nrgsystems.com

1999\10\08@194221 by Dave VanHorn

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> So I guess the moral is to be *very* careful about using the input
> protection diodes as clamps for inputs.
>
> -Jim

Look up "SCR latchup"..
I posted some comments on this WRT using Piezo sounders. It's easy to switch
the sounder such that you effectively create a charge-pump voltage doubler,
and the piezo is a pretty low impedance source.

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