At 01:40 PM 8/30/01 +0100, you wrote:
>>Yeah, one of those useless paragraphs that intends to communicate
>>some important piece of information but doesn't. Maybe the cap can
>>discharge into MCLR when Vdd drops and cause some damage?
>>Who knows.
>
>Consider what happens when you have a large capacitor (say 10uF for
>arguments sake) and you turn off the power to the PIC system. What is the
>lowest resistance for the capacitor to discharge through? The ESD protection
>diode of course. Now what happens if you happen to have a 5V relay on the
>same supply pulled in at power off time. Capacitor on MCLR tries to supply
>any number of milliamps to relay coil for unknown number of milliseconds.
>
>In any other scenario name your load, and see if the current out of the
>capacitor exceeds the maximum current through the ESD diode :)

Yes, this scenario applies to any capacitor connected to any pin but Vdd.

With small capacitors, the dVdd/dt *may* be small enough to protect
the diodes but it's worth considering if you have high current loads on
the Vdd bus.

Consider also the scenario that has power "blip" down by a volt or two
and immediately return- if there is any load on the Vdd bus of more than
(typically) about 80mA, the current injected into the protection diode
can trigger the suicide SCR (tm) and the chip will rapidly self-destruct
(melting of bonding wires or whatever). I believe this is the situation
that Microchip is alluding to.

Best regards,
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Spehro Pefhany --"it's the network..."            "The Journey is the reward"
speff@interlog.com             Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog  Info for designers:  http://www.speff.com
Contributions invited->The AVR-gcc FAQ is at: http://www.bluecollarlinux.com
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