Math help?

[Mike - K8LH]

Hi guys,

Am I doing this simple two step 16 bit math operation correctly? I'll simulate it as soon as I wake up...

;
;  CCP1CONbits.CCP1M0 = 0;
;  CCPR1L += (20000-Pulse)%256;
;  CCPR1H += (20000-Pulse)/256;
;
;  pulse is lo so setup CCP1 to go hi on pulse "off-time" match
;
Pulse_Lo
        bcf     CCP1CON,CCP1M0  ; setup CCP1 to go hi next int    |B0
        movf    PulseLo,W       ; W = Pulse%256                   |B0
        sublw   low  d'20000'   ; W = 20000%256 - Pulse%256       |B0
        skpc                    ; borrow? no, skip, else          |B0
        decf    CCPR1H,f        ; borrow from CCPR1H              |B0
        addwf   CCPR1L,f        ; CCPR1L += (20000-Pulse)%256     |B0
        skpnc                   ; carry? no, skip, else           |B0
        incf    CCPR1H,f        ; carry into CCPR1H               |B0
        movf    PulseHi,W       ; W = Pulse/256                   |B0
        sublw   high d'20000'   ; W = 20000/256 - Pulse/256       |B0
        addwf   CCPR1H,f        ; CCPR1H += (20000-Pulse)/256     |B0
;

 

[Mike - K8LH]

Also should have mentioned that if you've got an easier and/or better way to do it (fewer instructions, more intuitive, etc.), please pass it along. I suck at math (grin).

 

[blueroomelectronics]

Just wanted to note you've used skpc & skpnc awesome.

 

[Nigel Goodwin]

Just wanted to note you've used skpc & skpnc awesome.

Personally I tend to disapprove of using those macros, it only confuses beginners.

 

[Mike - K8LH]

Personally I tend to disapprove of using those macros, it only confuses beginners.

You're certainly entitled to your opinion but I believe promoting the use of meaningless _config lines in example "tutorial" programs and providing example keypad code that could short PIC output pins when more than one key is pressed, all for the sake of not confusing beginners, is not cool.

The Skip Carry (skpc) and Skip No Carry (skpnc) pseudo instructions are much more intuitive than the equivalent btfss STATUS,C and btfsc STATUS,C instructions, respectively. And the 12/14 bit "branch" pseudo instructions bz, bnz, bc, bnc, etc., are real instructions on the 16 bit core devices.

 

[blueroomelectronics]

I think the pseudo instructions make the core more readable.

 

[Nigel Goodwin]

I think the pseudo instructions make the core more readable.

That's because you come from the Parallax assembler which was basically a collection of such macros.

Although I would agree that they do make it more readable.

 

[Oznog]

I wonder if it doesn't make sense to denote macros in the name- like "skpc_m" -to make the code clearer?

 

[blueroomelectronics]

Actually they're part of MPASM. They are what they are. You don't need to define them; just use them.

I've attached the Firefly 16F88 version of both the main instruction & pseudo commands (omitting the silly b (branch)) Don't have the cycle times worked out though.

 

[atferrari]

Nowadays

For the 18F family their use seems to be discouraged. Nowhere I saw them mentioned in connection with the 18F.

Never used them, thanks God.

 

[blueroomelectronics]

For the 18F family their use seems to be discouraged. Nowhere I saw them mentioned in connection with the 18F.

Never used them, thanks God.

The 18F are teriffic chips, a little intimidating at first but in fact eaiser to program than the 16F & 12F chips.