At 02:14 PM 2/19/2006 +0000, you wrote:
>Hi,
>What is this LAT registers, what do they do, something read-modify-write
>I completely don't understand this. Why do I need anything else if I have
>PORT and TRIS ?

Using a RMW instruction on a PORT register (eg. bsf PORTA, 0) will read
all the bits in PORTA, set bit 0, then write the bits back again.

However, when you use the PORT address, the R part of the RMW reads the
levels actually on the port pins, which, for reasons of loading or because
the pins have not settled, or because it's an open-drain output, may not
be at the same logical levels as the port latch itself. This may vary
from part-to-part, with external hardware conditions or asynch timing
conditions (was there just an interrupt?).  The state of the
latch may thus undergo an unintended change (in bits other than 0 in the
above example). You may have to have a shadow latch register in RAM and
put more instructions in there to get it to work consistently (of course
it consumes RAM and runs slower at bit-diddling, which is one thing PICs
are pretty good at).

Using the LAT address means that R part of the RMW reads from the port
*latch*, not the port pins, so RMW instructions work right every time.

When introduced, it was LONG overdue, IMHO. Particularly since the 18F
series can execute instructions faster than the port pins can reliably
change state.

Best regards,

Spehro Pefhany --"it's the network..."            "The Journey is the reward"
speff@interlog.com             Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog  Info for designers:  http://www.speff.com
->> Inexpensive test equipment & parts http://search.ebay.com/_W0QQsassZspeff


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