In PIC16F872, Port A pin 4 is the open drain pin and is used to
control an LED.

As you know, the conventional arrangement (connected to ground through
the LED and a series 330 ohm resistor) does not work.

Instead, I have this pin connected to 5V through the LED and series
330 ohm resistor.  I set TRISA4=0 (output mode, low impedance, high
current) to turn on the LED and TRISA4=1 (input, high impedance, low
current) to turn off the LED.

I have found that the LED sometimes won't light even with TRIS4=0.
Fortunately, (after much frustration) I have been able to resolve this
problem by setting RA4=0.

Why is setting RA4=0 necessary for turning on the LED?  I see on the
datasheet that the RA4 pin also doubles as the T0CKI pin and can be a
Schmitt Trigger input as well as an open drain output.  I think this
has something to do with what I have observed, but I'm not sure what.

One strange thing is that this issue never came up when I used the
PIC16F72 microcontroller.  Even stranger is the fact that this issue
never came up when I used the very same microcontroller and circuit to
run a different program.

-- 
Jason Hsu
http://www.jasonhsu.com/swrwatt.html
http://www.jasonhsu.com/swrwatt-c.txt
http://www.jasonhsu.com/swrwatt-asm.txt
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