Is there a faster way to compare?

[RussMartin]

I'm doing something like this (pseudocode):

FOR INDEX=1 TO SOMETHING
IF INDEX>A THEN PIN1=1
IF INDEX>B THEN PIN2=1
IF INDEX>C THEN PIN3=1
IF INDEX>D THEN PIN4=1
IF INDEX>E THEN PIN5=1
NEXT INDEX

Each of those IF . . . THENs appears to chew up a lot of time, each reducing the upper limit on the number of iterations (SOMETHING).

I'm having to do this in/as an interrupt handler and I've got about 7ms (at 8MHz) to do it in.

Is there a faster/better/simpler way? (Someone <i><u>please</u></i> tell me there is!)

Fingers crossed here . . .

[Darrel Taylor]

Are they all BYTE's?
Is it a PWM output?

7ms is a LONG TIME.
<br>

[RussMartin]

INDEX and A through E are bytes. All five pins are on the same port.

Not sure if it's PWM or not. But at the end of the interrupt handler, all pins are reset to 0. Values A through E each represent a proportion of SOMETHING. If A through E were equal, all five pins would need to turn on as nearly as possible at the same instant.

The interrupt (INT_INT) is being driven by a zero-crossing detector at 60 Hz (120 interrupts/second).

Guess it <i>is</i> PWM, period 120 Hz, and I'm varying the duty cycle on each pin.

[Darrel Taylor]

This should be a bit faster ... (untested)

INDEX      VAR BYTE BANK0
SOMETHING  VAR BYTE BANK0
A          VAR BYTE BANK0
B          VAR BYTE BANK0
C          VAR BYTE BANK0
D          VAR BYTE BANK0

PIN1       VAR PORTB.0
PIN2       VAR PORTB.1
PIN3       VAR PORTB.2
PIN4       VAR PORTB.3


ASM
TestStart
      movlw   0              ;FOR INDEX=1 TO SOMETHING
      movwf   _INDEX
      
ForLoop    
      incf    _INDEX,F       
      
      movf    _A,W           ; IF INDEX>A THEN PIN1=1
      subwf   _INDEX,W
      btfsc   STATUS,C
      bsf     _PIN1
      
      movf    _B,W           ; IF INDEX>B THEN PIN2=1
      subwf   _INDEX,W
      btfsc   STATUS,C
      bsf     _PIN2

      movf    _C,W           ; IF INDEX>C THEN PIN3=1
      subwf   _INDEX,W
      btfsc   STATUS,C
      bsf     _PIN3

      movf    _D,W           ; IF INDEX>D THEN PIN4=1
      subwf   _INDEX,W
      btfsc   STATUS,C
      bsf     _PIN4
      
      movf    _INDEX,W       ; Next INDEX
      subwf   _SOMETHING,W
      btfss   STATUS,Z
    goto    ForLoop
ENDASM

[tenaja]

Russ,
I am sure there are other ways to get the pwm you need (such as interrupts), since this method takes up a HUGE amount of cpu time. However, if that doesn't matter and you just need a bit more speed, consider asm.

I compiled your snippet with PBP, and it used up 57 words of program space on a 16F. Proton+ did the same code in 36 words; almost 2/3 the size. PBP's asm file is nearly unreadable, so I can't compare them, but here is the Proton output from a single basic line for both 16F and 18F:

16F...
[TEST.BAS] IF INDEX>A THEN PORTA.1=1
MOVF INDEX,W
SUBWF _A,W
BTFSC STATUS,0
GOTO BC@LL4 'go to the next line...
BSF PORTA,1

18F...
[TEST.BAS] IF INDEX>B THEN PORTA.1=1
MOVF INDEX,W,0
CPFSLT _B,0
F@JUMP BC@LL6 'go to the next line...
BSF PORTA,1,0

[RussMartin]

Thanks for the suggestion, Darrel; I'll try it, expanding to add PIN5 and value E.

Thanks for the comparison, tenaja.

Although now I wonder if I'm looking for the wrong problem. Using the o'scope, I see that each of those IF . . . THENs actually takes only about 22us; that's only 110us for all 5. Maybe it's the FOR . . . NEXT. Each time I added an IF . . . THEN, I had to reduce SOMETHING and rescale each of however many variables I was comparing.

[Darrel Taylor]

... Using the o'scope, I see that each of those IF . . . THENs actually takes only about 22us; that's only 110us for all 5. ...

That's 110us * SOMETHING (the loop count).

If SOMETHING is ever higher than 63, you will have used up the 7ms and more.

With the ASM code I gave, each IF takes only 2us @8mhz.
SOMETHING can be 255 and it will still only take ~2.6ms.

hth,