PIC18F26K20 - 40 KHz PWM with defined # of periods

[chrhartz]

Hi,

I need to pulse a red laser diode with a fixed number of 40 KHz,
50 % duty pulses. Clock is a 16 MHz Crystal. The modulation input
of the laser driver is connected to the CCP1 pin. Also I can enable the
Laser Power input via RC3.

Timer2 Setup for 40 KHz and 50 % duty is perfect. I can simply switch
on and off the pwm to generate the desired signals.

My problem is that I am not synchronised with the pwm timers if
I simply switch the pwm on and off. The PWM periods sometimes
are not complete.

The idea was to generate an interrupt at begin or end of the pwm
period but I didn´t find a good solution. It would be perfect to trigger
an additional timer with each period so I can watch this and stop the pwm
after a specified number of cycles.

40 KHz is a lot if the interrupt is called permanently. This could be a
no go for the main program. But if the main program stops during laser
transmission, this does not care.

Does anyone have a good idea how to solve this?

Best wishes,
Chris

[Mike Walne]

Change the duty period to either zero of greater than PWM period.
(Choice depends on the output state you want when stopped.)
Current cycle will run to completion then pulses will stop.
It's all in the data sheet.
Should give you what I understand you've asked for.


Mike

[asmboy]

1- connect the PWM output to the input of Timer0 T0CKI input pin.
setup with prescalar as bypassed and counter in 8 bit mode

2 - before you enable the 40khz PWM pulse generator :
a- preload timer0 with a count of 256-(number_of pwm pulses) you want.
ENABLE the timer0 T0IF interrupt handler.
b- inside the TOIF handler -disable the pwm generator
3- start the pwm generator

this will only make ONE INT at pulse count completion, and cancel the generator until you trigger it again-

i can even see how to use another timer( of which there are plenty on this PIC) to decide how often this complete routine wants to execute- if auto repeat of the same pulse count is needed,

making external connections to other functions of your PIC is a powerful way to get the job done.

rinse, repeat as needed .....

[PCM programmer]

[Ttelmah]

Lots of bits already given.

The simplest I think is to use CCP2.

As Mike says, don't start/stop the PWM. Just change it's duty from 0% to the 50% to start sending, and back to 0% to stop (or use 100% if you want the signal to stay high). When you change the PWM duty cycle, this change occurs on the _next_ PWM cycle.

Now your PWM output uses Timer2.
Setup your PWM 'permanently running', but with the duty cycle set to 0% (or 100%).

CCP's in 'capture' or 'compare' mode, use Timer1 or (on your chip), optionally Timer3.

So feed the CCP1 output, back in to the T13CKI pin.

Program Timer1 for 'T1_EXTERNAL'.

Now Timer1, will count the PWM pulses.

Setup CCP2, with 'CCP_COMPARE_INT'.

Now just before you enable the PWM output, load CCP_2, with the number of pulses you require, and set_timer1(0).

Enable the CCP2 interrupt, and start the PWM output (set duty to 50%).
When the CCP2 interrupt occurs, set the duty cycle back to the 'off' state (0% or 100%). On the _next_ cycle this change will occur (so at the end of the current cycle).

Now the only extra complexity, is if you need more than 65535 pulses?. If so, then you have to let the timer wrap a number of times _before_ setting the CCP to interrupt. Have a look at how I generate a servo pulse in this thread:

<http://www.ccsinfo.com/forum/viewtopic.php?t=54739>

Where I count to 100000 on a CCP.

[chrhartz]

Many thanks for your answers!

I didn´t know that I can set the duty to 0% or 50% to change
the output behaviour of the pwm.

Feeding back the output to a timer input is a nice idea. This gives
more flexibility without side-effects.

Best wishes,
Chris