>I am using the BSF instruction to set bits
>on PORTD on a 16c74A. The whole port is
>configured as output (CLRF TRISD). The
>problem is, when I set a bit on the port, any
>other bits that were set, clear and then the
>bit the instruction specifies, sets.
>Why don't the other bits remain set?


Gavin,

Sounds like youre being caught by the read-modify-write problem.

When you use a bit level instruction on a port, the PIC reads the port,
changes the bit and writes the previously read levels back
to the output latches. But when it reads the pins, it reads
in the actual voltage level at the pin (even if not configured
as an input.)

So suppose you set an output pin high, and then drain enough current
from it to pull the voltage at the pin below the logic-low threshold.
(Example: driving an LED without a series resistor) Then when you BSF
or BCF another pin of the same port, that first pin is read in,
and re-written as LOW.

Result: your specified bit sets correctly, but other bits mysteriously
clear themselves.

One solution is to use a software buffer for the port. Keep a file
register that mirrors your desired output for the port. Then change
the value of a bit in the file register, and write the whole file
register to the port each time you change a bit.


regards

Alec

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