Quickest way to get highest set bit on port in to a variable?

[bmoe79]

What PIC device?

It´s a PIC18F45K80

[towlerg]

sorry can't write PIC assembler but something like

WReg= 8
Rotate Left f through Carry, 1
jump on carry, done
dec WReg
Rotate Left f through Carry, 1
jump on carry, done
etc...

or maybe BTFSC - BitTest, skip if clear

George

[Charlie]

To repeat, look up NCD in the manual. I think it does what you want.

[towlerg]

Except he wants the fastest not the easiest.

[HenrikOlsson]

This is what the NCD routine looks like in the pbppic18.lib file when stripping out the non runtime stuff:

;****************************************************************
;* NCD        : Priority encode                                 *
;*                                                              *
;* Input      : R0 = 16 bit value                               *
;* Output     : W = bit number                                  *
;*                                                              *
;* Notes      :                                                 *
;****************************************************************

NCD     clrf    R0 + 1
NCDL    movwf   R0
        movlw   17              ; Set result
ncdloop addlw   -1              ; Count down result - sets C so loop will end
        rlcf    R0, F           ; Shift upper bit to carry
        rlcf    R0 + 1, F
        bnc     ncdloop         ; If carry set then done
        goto    DUNN

I'll say what I usually say which is that I pretty much suck at assembly language stuff but that looks pretty tight to me. Obviously the execution time will vary depending on how many times it has to iterate thru the loop before it finds a bit that is set. Then there are helper macros which puts the byte/Word in question into the correct register (R0) and so on, if interested they are in the pbppic18.mac file.

Is there a faster way? Perhaps but I'd give NCD a try and get some performance data on that first - otherwise you don't know what to beat (or if you even need to beet it) - IMHO of course.

/Henrik.

[rsocor01]

Like Archangel mentioned above, you should also try the LOOKUP command. This command takes only a few cycles to execute.

[towlerg]

Henric, never the less, not as quick as 8 rotates and 8 jump on carry in-line. Worst case, with no bits set in the target thats only 16 cycles.

rscor01, I didn't realize LOOKUP would do that. How would that go?

George