Issue with PWM DAC using built-in PWM Module of PIC16F690

yasir9909 · Joined: 04 Jan 2010 Posts: 9

I have searched out this help forum extensively, many of my queries regarding PWM & built-in PWM module of PIC Micro-controller have been addressed through this search, but here is an issue for which I am unable to find solution through search of this help forum.

I am trying build a simple DAC with PIC16F690 using its built-in PWM module. The PWM module is working at my desired frequency but the issue I am facing is that when I try to generate Sawtooth waveform, the PWM output stays high at 100% duty cycle for a long span of time and then starts again from zero & proceeds to upward count.

Here is my code:

PCM programmer · Joined: 06 Sep 2003 Posts: 21708

yasir9909 · Joined: 04 Jan 2010 Posts: 9

PCM programmer thank you for your reply. I have been able to deduce following the formulas from the post whose link you have shared :

1. PWM Period = 1/(Clock Frequency)*4*t2div*(Period+1)

or

PWM Frequency = (Clock Frequency)/( 4*t2div*(Period+1) )

where in function setup_timer_2(T2_DIV_BY_1,79,1);

t2div = 1,Period = 79

or

Ttelmah · Joined: 11 Mar 2010 Posts: 19539

Value = (%age of Duty Cycle/100) *(Period+1) *4 -1

You are missing one critical thing.
You are setting the Period value (PR2), to 79.

Try the formula again, with this, and understand what PCM_Programmer is telling you.

yasir9909 · Joined: 04 Jan 2010 Posts: 9

I think,I have mentioned in summary of my findings regarding PWM and Timer2 settings that

PR2=Period=Middle Value = 79

Ok let me revisit the links provided by PCM Prgrammer.

Actually,I need to read analog signal through input pin of built-in analog-to-digital converter and then after mapping it to some other values saved in lookup table,I shall output it as analogue output through PWM Module and low pass filter.

Thanks for your help

regards
m.yasir

Ttelmah · Joined: 11 Mar 2010 Posts: 19539

I'm trying to make a point about the way you are thinking.

Key to understand is that you are dealing with _embedded_ languages. You have the 'core' C functions, and then dozens of other parts that deal directly with hardware.
As such, it is the _hardware_ data that you need to look at.

Now the formula from me, that you found, is copied directly from the chip data sheet. If you look at the 16F690 data sheet, go to section 11, and look at Equation 11-2, and 11-3, you get the formulae for the pulse width, and the duty cycle. Then even a nice little table giving example periods, and how the resolution decreases as the PR2 value drops (table 11-2).

Then step back and realise that the CCS functions only set the hardware. Nothing else. If you look again at the data sheet, you see that the time for the PWM, is set by timer2. Look at timer2, and you see it has a 'prescale' setting, a period setting, and a postscale setting. Then look at the setup_timer_2 function, and the three values this takes, set the prescale, period, and postscale - the same!....

The problem is that you are working with hardware, and as such it is the hardware data you need to start with.

I posted some time ago, in another thread, the sources of data that you needed to look at when working with CCS, and a PIC. The 'C programming language' (K&R), was there, but the chip data sheet came _above_ it in the list.

Though these languages have as their core, 'C', it is an understanding of what the hardware abilities are, that has to 'lead' your use of the functions, and where you look for answers.