On Thu, Jul 3, 2014, at 03:25 PM, Richard R. Pope wrote:
>      True and true. You can also smoke a PIC. I take it that the Atmel=20
> uP is not current limited at the pin? The PIC is but most PICs can only=20
> handle 150mA total on all pins. This is very easy to forget especially=20
> late at night and when in a hurry. I'll have to look at the datasheets.

There is no current limiting on PIC pins. The internal resistance is
such that driving something like an LED won't kill it right away, if the
LED is tied to ground (it's using the P-channel output MOSFET). It is a
bad idea, though.

With a five volt supply on something like a 16F877 the internal
resistance of the N-channel output is 30 to 50 ohms and the P-channel is
almost twice that.

So if you tie an output pin to +5 and write a zero to that port it will
try to pull 100mA through that one pin and it will die soon.

This is just from memory, but I did test it about 10 years ago after a
Microchip field engineer said it was OK to drive LEDs without resistors
from a PIC. Smart guy, he didn't put it in writing :)

Best regards, Bob

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