38Khz IR carrier - best way to turn it on and off

[mark_s]

If you look in the manual under HPWM. You can see that pbp HPWM is only 8bits.
In your code you have the duty set at 254, which is nearly 100%. I think you would want to set
the duty to 50% or 127.

untested

TRISC.2 = 0
MAIN:
	HPWM 1,127, 38000 ' duty cycle 50% 
              pauseus 1000
              HPWM 1, 0, 38000 ' duty cycle 0
              pauseus 1000

	GOTO MAIN

[andywpg]

If you look in the manual under HPWM. You can see that pbp HPWM is only 8bits.
In your code you have the duty set at 254, which is nearly 100%. I think you would want to set
the duty to 50% or 127.

Wow! You are SO right - looks MUCH better now. Thanks!



Still has that slope on it though. I might try DT's Instant Interrupts and use a pin on and off to get it. Here's what the code is now:

HPWM 1, 127, 38000
MAIN:
        pauseus 1000
        trisc.2 = 1
        pauseus 1000
        trisc.2 = 0
	GOTO MAIN

[Art]

Have you tried turning it off and on again?

MAIN:
	HPWM 1, 127, 38000
        pauseus 1000
	HPWM 1, 0, 38000
        pauseus 1000
	GOTO MAIN

Strictly speaking of course, the HPWM takes some instructions so the loop takes longer than 2ms to execute,
but you could make your own time wasting routine for accurate delay, so that shouldn’t be a problem.
I mentioned that since I noticed you had more accurate timing before using HPWM only once.
But you have a scope, you don’t even have to do the math to calculate a time waste routine.

[richard]

Frequency
is the desired frequency of the PWM signal. On devices with 2
channels, the
Frequency must be the same on both channels. Not all frequencies
are available at all oscillator settings. For the non-long versions of PBP (PBP and
PBPW), the highest frequency at any oscillator speed is 32767Hz. The lowest
usable HPWM
Frequency at each oscillator setting is shown in the following table.

quote from the pbp3 manual

did you actually check the resulting freq, it can be done manually of course

would a pull down resistor (say 1k) make the trace look better

[ardhuru]

Hi,

Here's what had worked for me. Richard is right, you cant use HPWM for frequencies above 32.768khz.

Set the registers as follows.

TRISC.2 = 0 ' CCP1 (PortC.2 = Output)
PR2 = 25 ' Set PWM Period for approximately 38KHz
CCPR1L = 13 ' Set PWM Duty-Cycle to 50%
CCP1CON = %00001100 ' Mode select = PWM
T2CON = %00000100 ' Timer2 ON + 1:1 prescale

This would give you a continuous 38khz on the pwm pin, at 50% duty cycle.

Now, toggle TRISC.2 between an input and an output at 1ms.

Hope it works! If it does, thanks go to Bruce Reynolds. If doesnt, brickbats to me!

Regards.

[richard]

that would be for fosc=4
for my money I'd say :
CCPR1L = 52 ' Set PWM Duty-Cycle to 50%
{mister_e pic multi-calc}

[mark_s]

As Ardhuru said, credit to Bruce Reynolds for publishing some great projects.

Maybe you can gets some ideas here.
http://wayback.archive.org/web/20120...com/remote.htm

[andywpg]

Hi,

Here's what had worked for me. Richard is right, you cant use HPWM for frequencies above 32.768khz.

From what I understand of what was quoted, 32768Hz was maximum WITHOUT using longs. I'm using an 18F2550, and when I compile, I get the following message:

'[MESSAGE]pic18f2550.pbpinc(285): LONG Variables enabled (PBPL used)'

It seems like it used longs because of the frequency I specified, and it seemed happy. Anyway, the problem is solved, see my last post.

Thanks for the help!

[andywpg]

Thanks Everyone!

It worked!

The answer was a pull down resistor. It got rid of that slope. I tried a few, and worked out that 4.7K was the highest value I could use. That gave me a final voltage about 4.2V which should be fine.

Then I had to play with the frequency I specified to get exactly 38 pulses in 1mS. Even though the scope's measurement function still reports it as 38.46KHz - I told you it was a cheap scope!

Here are two images of what I have now.

Thanks again!

Andy

Attachment 8114

[andywpg]

did you actually check the resulting freq, it can be done manually of course

Well, the scope has measurement functions. Again, this is NOT an expensive scope ($75), but it was reporting 38.46KHz and 26uS period......both right in the ballpark of what I was looking for.

would a pull down resistor (say 1k) make the trace look better

And that was the answer! See my last post for images.

Thank you!!

[andywpg]

Have you tried turning it off and on again?

MAIN:
	HPWM 1, 127, 38000
        pauseus 1000
	HPWM 1, 0, 38000
        pauseus 1000
	GOTO MAIN

Strictly speaking of course, the HPWM takes some instructions so the loop takes longer than 2ms to execute,
but you could make your own time wasting routine for accurate delay, so that shouldn’t be a problem.
I mentioned that since I noticed you had more accurate timing before using HPWM only once.
But you have a scope, you don’t even have to do the math to calculate a time waste routine.

Hmm, tried it, but I couldn't get the frequency up high enough due to the 'setup time' of HPWM. See below though.....got it looking great now.

Thanks Art,

Andy