Comparitive speed of comparing IF

[Francis]

Is there a faster way to do a comparison than this?
The IF bit I mean.

My 2010 brain has not worked very well after New Years Eve.

Code: Select all

' X = random byte value between 10 and 255

For I=1 to 255
   If X<>0 Then
      Dec(X)
      OutPin = 1
   Else
      OutPin = 0
   EndIf
Next

It simply runs in a fixed loop. The value of X is random between 10 and 255 and whilst X>0 then the outpin is high. As soon as it hits zero the Outpin is switched off.
'Outpin' is declared elsewhere, its simply any old usable I/O pin as an ouput (e.g. portA.1)
If poss. I'd like to speed up the IF section. All variables are bytes.
Suggestions very welcome. Thanks.

[rmteo]

Try this?

Code: Select all

' X = random byte value between 10 and 255 

OutPin = 1
while X > 0
   dec(X)
wend
OutPin = 0

[Francis]

Thanks. I'll give that a go on the 'scope.

Actually, sorry, I didn't explain myself very well at all.
I was trying to produce a rough single duty cycle.

The For-Next loop is approx the total cycle period and the value X is essentially the duty-On bit. Super accuracy not needed, but it 'scopes well.
I was really after a faster comparison/decrement part to reduce the total period. (If you see what I mean). I wanted to avoid timer interrupts as they are going to be used elsewhere.

I can dig out Coreldraw if I've confused the issue

Oh, darn my typing; I didn't mean X= range of 10 to 255, I meant 0 (zero) to 255. Oops. i.e. the 'duty' can be anywhere between 0 and 255 / 0-100%.

[rmteo]

How about using one of these functions:

Code: Select all

delayms(X)
' or
delayus(X)

DelayMS

sub delayms(expression as word)
The delayms subroutine suspends program execution for up to 65535 milliseconds (ms).


DelayUS

sub delayus(expression as word)
The delayus subroutine suspends program execution for up to 65535 microseconds (μs).

[Francis]

Thanks for replying.

This is what I'm trying to achieve.
The For-Next Value defines the total period.
The random value X defines the Duty-On time.



I realise I can reduce the For-Next Value but that will also reduce my resolution. e.g. For-Next=20 only gives me 20 X steps obv.
If I can do the IF bit quicker then that is what I'm after.

Super-dooper timing accuracy isn't required but a fast-as-possible loop speed would be ideal. So no delays wanted. And remember, in some cases (i.e. X=Zero) the output will be off for the whole period.
It does it fairly quickly as it is but if there is a simple way to improve it then that would be really good.

[rmteo]

I don't understand. Executing a for-next loop is no different from a delay. Both delay functions allow up to 16-bit values - way more than the 8-bit that you need.

In any event, the best way to do this would be to use the COMPARE module. If you need a continuous train of pulses, then use the PWM module.

[Francis]

True-ish.
The For-Next loop gives an overall period.
The value X specifies the Duty-ON time within that period.

I'm using the PWM module for something else.
I hadn't considered the Compare module.
It certainly looks like I can use Compare and PWM independently.
Good idea. Thanks.

[rmteo]

Which PIC18 are you using? A lot of them have 2 PWM modules.

[Francis]

18FK20 . I've nearly got it sussed now.