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Re: [avr-chat] (no subject)

From: David VanHorn
Subject: Re: [avr-chat] (no subject)
Date: Thu, 29 Nov 2007 16:26:13 -0500

> Research your specific uC but when I've looked in datasheets every AVR I
> checked would source or sink 30 mA from most every pin I wondered about.

For loose enough values of "source/sink" :)
That's what those curves are about, basically expressing the output
stage's impedance at given current levels and VCC levels.
It's not strictly resistive, nor exactly linear.
In general, if you keep to low currents and buffer between the uC and
the outside world, you'll be happier.

> Another data sheet mentions, and makes use of, the ability of the
> protection diodes of handling 1 mA. Was being used to handle very high
> voltages. I never felt safe using that but might be practical to drop 5V
> signals down to 1.8V inputs using only a series resistor if the current
> is limited to less than 1 mA.

Yeah, I'm queasy about that as well, the VCC source needs to deal with
that issue too.
What happens if the current in through the protection diodes is more
than the system is taking from VCC?
Does VCC go up and cause problems elsewhere?  Does the regulator fail
(78XX series is famous for this mode)
What happens at the moment that this diode current goes away? Can the
reg turn back on fast enough?

These are all problems I'd rather not have to answer.

> --
> David Kelly N4HHE, address@hidden
> ========================================================================
> Whom computers would destroy, they must first drive mad.
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