> =

> =

> slippyr4 wrote:
> >> use LATB to write.
> >> =

> >> =

> > That's a good point. To clarify, what is the impact of me using PORTB
> > not LATB?
> >
> =

> Higher risk for read-modify-write related problems.
> =


Put in another way - Using LATX instead of PORTX gives you no risk for the =
RMW =

problem. =


The RMW problem is caused by a combination of two things:

1. An instruction that is changing just one bit in a register (bsf, bcf) =

actually first reads the whole register, changes the bit and then writes ba=
ck =

the whole register (thus the name Read - Modify - Write).

2. When the register is a PORT register, the value read is actually the log=
ic =

levels of the physical pins and not necessarily the value that has previous=
ly =

been written to the port.

This can become a problem if the capacitive load on a port pin is relativel=
y =

high (like the gate on a MOSFET for example) and there are two (or more) bi=
t =

changes on the same port right after each other. For example:

BSF RB,0
BSF RB,1

If RB0 initially was 0, BSF RB,0 now starts to change it to a 1. If the =

capacitive load on that pin is high it will take some time for it to rise t=
he =

voltage to a logic high level. If this time is longer than the time until =

another bit in RB is changed, the value that is read back for RB0 will stil=
l be =

a 0 (as seen by the port logic) and when the whole byte is written back RB0=
 has =

unintentionally been reset back to a 0 again.

Newer PICs actually have two registers for one port - The PORT register and=
 the =

LAT register. The LAT register is a latch (or buffer) between the CPU and t=
he =

PORT register. So if you write a value to the LAT register it is also writt=
en =

to the PORT register but it is always read back as the same value that was =

written independent of how long the PORT register actually takes to change.

/Ruben
=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=
=3D=3D=3D=3D=3D
Ruben J=F6nsson
AB Liros Electronic
Box 9124, 200 39 Malm=F6, Sweden
TEL INT +46 40142078
FAX INT +46 40947388
ruben@pp.sbbs.se
=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=
=3D=3D=3D=3D=3D

-- =

http://www.piclist.com PIC/SX FAQ & list archive
View/change your membership options at
http://mailman.mit.edu/mailman/listinfo/piclist