How to change PWM resolution from 10bits to 8bits ?

[daviz]

Hi everyone,

I using PIC18f2331. I wish to increase the frequency of the PWM, therefore, i wish to lower the PWM resolution to 8bits.

Can someone guide me on how to change PWM resolution from 10bits to 8bits ?


Regards,
Daviz

[MikeValencia]

In general, the CCPR1L register contains the duty cycle value, which is 8 bits. These are the most significant 8 bits in a 10 bit duty cycle. The least significant two bits are in the control register (CCP1CON?). If you keep the least significant two bits as '00', then you have 8-bit resolution.

You raise your frequency by decreasing the value in PR2, because now your period will be smaller, and thus a higher frequency.

[PCM programmer]

This thread has code examples for PWM in 8 or 10 bit mode, and links to more examples:
http://www.ccsinfo.com/forum/viewtopic.php?t=24055

[Ttelmah]

Mike has described what is involved, but it may not be obvious from this what has to be done in CCS.
The Timer2 value, controls the total 'period' length. Then the PWM value, can be adjusted up to this value*4. The '*4', comes because the counter accesses an extra two bits for the comparison, which correspond to the prescaler feeding the counter.
There are then two ways of getting '8 bit resolution'. The first is simply not to access these extra bits. The 'set_pwmx_duty' commands do this automatically, if given an integer value, instead of a long. So, if you have:

setup_timer_2(T2_DIV_BY_16,249,10);

and then use:

set_pwm1_duty(125);

You will get 50:50 duty, because the compiler automatically puts the 'short' value it is given into the most significant 8 bits. However if you use:

set_pwm1_duty(125L);

You will now only get a 12.5% 'on' period, as the full 10 bits are accessed.

Note also, that because timer_2 is only counting to 249, the 50:50 point is at 125, not the 128 for the full count. This is the 'key' to using higher frequencies with lower resolution.

So if you now change the setup to:

setup_timer_2(T2_DIV_BY_16,63,10);

The frequency used will increase to 3.9* the previous rate (250/64), and a duty setting of:

set_pwm1_duty(128L);

will now give 50:50 duty cycle, corresponding to an 8 bit resolution.

The highest frequency PWM, with an 8bit duty cycle control, will be:

setup_timer_2(T2_DIV_BY_1,63,10);

Note that in all these, the last byte (10), does not affect the pwm width or the duty, but how many cycles occur between interrupts. If you are not using the timer2 interrupt, the value here does not matter.

Using a 20MHz clock for example, this will give a 78.125KHz output.

So your available resolution, is the value used as the second parameter in the setup_timer_2 function+1, *4.

Best Wishes

[sseidman]

Is there anything special that needs knowing with respect to setup_power_pwm on the 18fx331?

Scott