How to assign this variable?

[circuitpro]

In an application, I have wired the lower 4 bits of PORTF to control the inputs of a multiplexer. Right now I have assigned the variable like this:

MUX VAR PORTF

but really, it's only the lower 4 bits. How can I specify that in my declaration?

[mackrackit]

Look at section 4.10 in the manual.

[mister_e]

There's no specific variable type for that. Usually a Shadow register & some BYTEWise operation solve many issue. Something around ....


MUX VAR PORTF
S VAR BYTE

S = (MUX & $F0) | ValueToWrite
MUX = SWhen dealing with port, you may want to avoid successive bit modification to avoid RMW effect.

To read a PORT it's easier

MUX VAR PORTF
S VAR BYTE

S = MUX & $0F

[circuitpro]

I see how I can isolate the lower 4 bits of a port and READ from them

anyvar = PORTF & $0f

but don't know how that pertains to a WRITE. Is that what you mean?

[circuitpro]

thank you mister_e.

[mister_e]

Sorry I'd edit the post while you replied. Seems you got the idea though ;)