> 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?

Jason,

RA4 is acting as a low-side switch

You have to look at what's happening inside the PIC. Simply speaking,
each PIC pin has two output FETs, one from the pin to ground (low-side),
the other from pin to Vcc (high-side). When you send a 0, the low-side
FET turns on, connecting the pin to ground. When you send a 1, it turns
the high-side FET on, connecting the pin to Vcc (not direct connections,
there's current limiting)

RA4 often doesn't have a high-side FET. So when you send a 1 to the
pin, there's no FET to turn on. On the other hand, there is still the low-
side FET so sending a 0 turns that on, connecting the pin to ground

Look at the I/O section of the datasheet and you'll see diagrams of
what's behind the pins. A 0 sent doesn't mean that 0 is what happens,
if you get my drift

wbr
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