16F88 PWM

boris · Joined: 29 Nov 2007 Posts: 15 Location: Hungary

Hi!

I've encountered a problem with PIC16F88, and the PWM module.

I use compiler v4.068

The comiled program works fine in Proteus, but when I flash it on the real chip, it won't create a PWM.

I've set the PIN_B3 output in the code wizard.

The sample program I'm using is:

Matro · Guest

Your timer2 is configures as follow:

setup_timer_2(T2_DIV_BY_1,99,1);

So the "set_pwm1_duty" should be between 0 (= 0%) and 100 (= 100%).

All values greater than 100 (as the 200 you choose) will give a duty cycle of 100% i.e. a static high output.

Try a "set_pwm1_duty(50)" and it should generate a 50% PWM signal.

Regards,
Matro

boris · Joined: 29 Nov 2007 Posts: 15 Location: Hungary

HI Matro!

You ROCK!

I've would never think of it!!!
The joke is that the 200 was given by the code wizard, so I thought it was correct....

If I write 50 the PWM works like a charm!

Thanks again!

Charlie U · Posted: Fri Mar 14, 2008 11:33 am

The actual value used for the period is 4x the value in the setup function. If you use an int16 for the duty cycle then 0 to 400 would be correct. To use a constant you would need to add an "L" as a suffix to the constant: set_pwm1_duty(200L);.

This gives you the full resolution of 400.

Try this and let us know if it works for you.

boris · Joined: 29 Nov 2007 Posts: 15 Location: Hungary

Hi Charlie U!

Thank You! The 200L works fine, and if I use int16 for the PWM value, it accepts values between 0 and 400. But where does the 400 come from? It's in the thatasheet, or in the CCS examples, or can be calculated?

Ttelmah · Guest

Basically, the timer runs off the processor clock/4, but the actual PWM generator, runs directly off the processor clock. The PWM generator, takes an extra two bits, to access this faster part of its counter. If you look at the data sheets, you will find that there is a main register (holding an 8bit value), and these extra two bits in another register. Now, the compiler, when the pwm_duty command is accessed with a normal 8bit integer, takes the 'quick' route, and writes the 8bit value directly to the 8bit PWM register, allowing counts up to the value selected for the timer (hence 0 to 100 in the example given). However if it is given a 'long' value, it realises you want to access the extra two bits, and extracts these, writing them to the extra bits in the control register, so you can then use 0 to timer*4 as a count value.

Best Wishes

Gerhard · Joined: 30 Aug 2007 Posts: 144 Location: South Africa

HI.
I am strugling to understand what part of the PWM is 4 times faster than in the setup.
If you generate a PWM signal of say 5KHZ at 50% duty cycle in the setup function of the timer, does the actual generated PWM signal equal 5KHZ or 20 KHZ. I what scenario is the actual PWM freq and duty cycle different than what is setup in the timer setup?