You're probably getting bit by the "read-modify-write"
gotcha because you are operating directly on the PORTA output
register.If your LED circuit is loading the output pins so that
the logic level read by the bsf PORTA instruction reads a logic
zero on that pin -- well that's your answer.

Operating with bsf and bcf instructions directly on port pins is
always a potential problem (or at least the port loading and
timing has to be carefully considered). A better solution is to
keep a shadow register of the port contents, operate on them,
then write the whole shadow register to the port.

C compilers can sometimes obscure the fact that you are operating
directly on the port output register. This is another good reason
for using a shadow register. It's a tad slower (you have to read,
then write the shadow to the port register), but it's always
safe.

My guess is that in your case, the logic levels of the output
port pins are right at the transition level from logic '1' and
logic '0'. Of course this will vary from PIC to PIC and with
temperature, exact load conditions, etc. which makes the
result quite unpredictable!

If you try something like:

TRISA = 0b00000000;
PORTA_Shadow = 0;   // setup PORTA_Shadow as a struct of bits
PORTA = PORTA_Shadow;

while (1)
{
PORTA_Shadow.2 = 0;
PORTA_Shadow.3 = 0;
PORTA_Shadow.4 = 0;
PORTA_Shadow.5 = 0;
PORTA = PORTA_Shadow;
DELAY_MS(250);
PORTA_Shadow.2 = 1;
PORTA_Shadow.3 = 1;
PORTA_Shadow.4 = 1;
PORTA_Shadow.5 = 1;
PORTA = PORTA_Shadow;
DELAY_MS(250);
}

It should work.

Matt Pobursky
Maximum Performance Systems

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