FOR...NEXT strange counting

[flotulopex]

I'm using this routine to switch ON the backlight of my LCD display, gradually but quite quickly (around 1 second). If I "PAUSE 500", it will takes ages to have the backlight fully on.

As stated in the MicroCode help file, the max "duty" value can be 255. If I want to have the backlight fully ON, I should have 255.

If the condition is "FOR Duty = 0 to 255", Duty will turn back to value 0 (zero) after the "NEXT" since it has been declared as a BYTE variable.

Maybe, the best thing to do is to declare Duty as WORD sized...

Nevertheless, the strange thing is that the Duty variable gets incremented by 1 when it exists the FOR...NEXT cycle.

[Darrel Taylor]

Hi flotul,

It won't matter that the Duty variable ends up at 0 when it wraps around. The HPWM will stay at the last DutyCycle it was set to "Inside" the FOR/Next loop. So you can go the full 0-255.

It appears that you are using the "if duty = 0 then" statement to make the backlight only ramp-up once, when power is applied. Which also means that you have this section of code within the Main Loop of the program.

If you take it out of the main loop and put it up at the top of the program, that section of code will only run once.
<br>

[flotulopex]

I have my LCD-backlight controlled with two buttons; this is why it is in the main loop.

The HPWM command accepts the duty value from 0 to 255.

Actually, when the FOR...NEXT cycle is completed with a count up to 255, the value "wraps" from 255 to 0.

But I track this value to prevent a second press on the same button (it's not nice to see the backlight switched OFF instantaneously and dimm up again when it was already ON - looks like a bug to the user).

To get the HPWM at full range, I would count from 0 to 254 in the FOR...NEXT cycle and, when completed, add one more command to set the duty value to 255. This is not very "clean".

Actually I found another way around. I use REPEAT...UNTIL. Now it works.

I was just suprised to see how the FOR...NEXT cycle affects the count.

[Ioannis]

Since the for-next loop counting from 0 to 255 and the HPWM is inside the loop, where is the problem? At the end HPWM will be at 100% (255), the for-next will wrap to 0 since you have byte variable ant the loop will exit.

Thats how a for-next works in the first place. Always exits when variable will pass the upper limit.

Did I miss anything?

Ioannis

[flotulopex]

I need to get a "HPWM 1, 255, 1000" final command with a Duty value of 255.

...
Duty VAR BYTE
Duty = 0

FOR Duty = 0 TO 255
   HPWM 1, Duty, 1000
   LCDOUT $FE, 2, DEC Duty   'First read of Duty
NEXT
LCDOUT $FE, $C0, DEC Duty    'Second read of Duty
...

When this FOR...NEXT loop is completed, the HPWM command will be correct ("HPWM 1, 255, 1000" - see first read of Duty) BUT Duty's value will be 0 (see second read of Duty).

Even if the count is completed (Duty has reached 255), passing the last NEXT command will increment Duty by 1 and make it a 0.

This is what happens with my 16F88 programmed with PBP. I never expected such a result.

In my program, I use Duty as a reference to check if my LCD-backlight is fully OFF (Duty = 0) or if it is fully ON (Duty = 255).

If Duty is 0, I'm allowed to press only button 'A'; if Duty's value is 255, I'm allowed to press only button 'B'.

In other words, if Duty is 0, I don't want to allow a second press on button 'A'; I will only accept button 'B' and vice-versa if Duty is 255.

The solution I found now is this one:

...
SUBROUTINE:
    REPEAT
        Duty = Duty + 1
        HPWM 1, Duty, 1000
    UNTIL Duty = 255
    RETURN
...

[keithdoxey]

Alternatively you could add the following line

FOR Duty = 0 TO 255
HPWM 1, Duty, 1000
LCDOUT $FE, 2, DEC Duty 'First read of Duty
NEXT
Duty = Duty-1
LCDOUT $FE, $C0, DEC Duty 'Second read of Duty

Which would put it back the the final value inside the For/Next loop

If you are also dimming down then do the opposite

FOR Duty = 255 TO 0 STEP -1
HPWM 1, Duty, 1000
LCDOUT $FE, 2, DEC Duty 'First read of Duty
NEXT
Duty = Duty+1
LCDOUT $FE, $C0, DEC Duty 'Second read of Duty

[flotulopex]

Thank you keithdoxey,

It is not about to find a solution to make it work; it is about how the FOR...NEXT loop counts.

There are lots of differents solutions to solve my problem and I wonder how many people noticed this stange loop counting behaviour.