PWM PIC24 Calculations - help needed

[benoitstjean]

Compiler: 4.132
PIC: 24HJ128GP306

Please, I *know* that there's documentation to read, but I have the docs opened in front of me but I'm a bit lost and need help, which is what this board is for so please, bear with me:

So on a previous circuit with a PIC18, the calculations for the PWM frequency was the following:

[ck]

Hi,
I have pic24hj and i have setup my PWM in this way:
I have 10MHz External Cristal with PLL enabled so fcycle=40MHz
so Tcycle=25nsec

[benoitstjean]

Hello, thanks for your info but it's not quite what I'm asking or perhaps, it's not very clear.

So, your "frequency" is 40MHz (25ns). If I follow your example and make the same calculation using my 20MHz crystal, this is what I get:

0.00000005 (50ns) * 78 = 0.0000039 (3.9us)

1/3.9us = 256410 <--- PROBLEM, it's twice what I'm currently seeing on my oscilloscope.

So again, using MY code in MY example, how did I get to 128205Hz? Your calculations don't jive with my calculations although you seem to be getting the PWM value you're looking for and I'M getting what I'm looking for, but I don't understand the calculation:

setup_compare( 1, COMPARE_PWM | COMPARE_TIMER2 );
setup_timer2( TMR_INTERNAL | TMR_DIV_BY_1, 77 );
set_pwm_duty( 1, 39 );

With the above three lines AND the information I provided, what makes it that I get to 128.205KHz? Again, I'm using a 20MHz crystal and the period value is 77 (the PWM duty is 50% therefore the value 39)...

[benoitstjean]

UPDATE:

I changed my crystal from a 20MHz to a 38.4MHz and now, the value for setup_timer2( TMR_INTERNAL | TMR_DIV_BY_1, ulPWMValue ); is 149. This gives me a perfect, on the spot, 128kHz PWM @ 50% duty (set_pwm_duty value = 75).

So now, how is the calculation done (referring to my PIC18 calculation I posted above)? If I use CK's calculation, it doesn't make any sense:

1/38400000 = 26.041667ns * 149 = 3.8802083ms -> 257.718kHz, which doesn't make any sense at all (when I change CK's values for mine) because my oscilloscope shows me a perfect 128kHz.

Thanks.

[Ttelmah]

Your original calculation is exactly right.

So:

38,400,000/((149+1)*2)

= 38400000/(300 = 128000

The key point about the factor of two, is the peripheral clock. In the PIC18, this is Fosc/4. On the PIC24, it is Fosc/2.

It is shown in the oscillator diagram for most of the PIC24's, with 'Fp', coming from Fosc/2.

On the PIC18, it is slightly less obvious, being instead shown on the timer block diagrams.

Best Wishes

[benoitstjean]

Good! I wasn't too sure if it was the /2 factor. So that all makes sense now. Thank you!