Lpc2000

At 05:46 AM 4/2/04 +0000, you wrote:
>That would save some instructions, although the Philips Set-Clear
>GPIO operation already costs more instructions to implement data I/O.
>( anybody know that why Philips uses this kind of GPIO operation
>instead of direct setting 0 or 1 ? )

The obvious reason for doing this is to avoid conflicts from multiple 
process (or interrupts).  Other wise you have to maintain shadow copies of 
the outputs and (in an interrupt protected piece of code) update the shadow 
and then the output.  In psuedo code something like:

disable_interrupts()
shadow_output = shadow_output & ~PERIPH_MASK    -- zero area to be updated
shadow_output = shadow_output | (new_value & PERIPH_MASK)       -- update 
with new value
actual_output = shadow_output;
enable_interrupts()

Some other micros provide similar facilities.  The ST10 uses a construct 
where a single register affects the I/O bit but maps two bit in the 
register to each output, one bit patter set the output, a second clears the 
output and the other two cause no effect.  It's also used on internal 
registers to avoid race conditions on bits that may be affected by both the 
peripheral HW and SW.

Finally note the writeup on the IOPIN register in the user manual "Note: 
for test purposes, writing to this register stores the value in the output 
register, bypassing the need to use both the IOSET
and IOCLR registers. This feature is of little or no use in an application 
because it is not possible to write to individual bytes in
this register."


Robert


" 'Freedom' has no meaning of itself.  There are always restrictions,
be they legal, genetic, or physical.  If you don't believe me, try to
chew a radio signal. "

                         Kelvin Throop, III