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benoitstjean
Joined: 30 Oct 2007
Posts: 566
Location: Ottawa, Ontario, Canada
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| PWM PIC24 Calculations - help needed |
 Posted: Fri Sep 21, 2012 8:16 am |
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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:
| Code: |
PWM = Xtal frequency
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(mode) x (period + 1) x 4
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Mode can be only 1/4/16. Period is a value between 0-255, 4 is a fixed number required by the equation. Using a 24.576MHz crystal, I can get 128kHz flat using MODE=1, PERIOD = 47.
At the moment, my board with a PIC24HJ, it only has a 20Mhz crystal and on my oscilloscope I see 128.2kHz. I got to this PWM value by trial-and-error because I just can't wrap my head around the docs. I'll change the crystal to a 24.576MHz when I have a chance but I'll work and calculate for now using what I have...
So, given the hardware configuration, this is my code to get to 128.2kHz:
| Code: |
setup_compare( 1, COMPARE_PWM | COMPARE_TIMER2 );
setup_timer2( TMR_INTERNAL | TMR_DIV_BY_1, 77 );
set_pwm_duty( 1, 39 );
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This is where I'm confused with the PIC24 PWM calculations. I don't get all the parts of the calculations (as much as _I am reading_ the datasheets, I just can't wrap my head around it). The Microchip datasheet says the following:
PWM Period = [(PRy) + 1] * Tcy * (TMRy Prescale Value)
PWM Frequency = 1/PWM Period
For the frequency, that's fine: 1/0.0000078 = 128205 Hz
Where I'm having the problem is how to _get to_ 0.0000078 given that I am using a 20Mhz crystal.
PRy+1 is any value between 0 and 65535, in my case, 77 SO:
128205 = [(77)+1] * Tcy * (TMRy Prescale Value)
In the PIC18's calculations, there's the x4 "fixed" part which I don;t see for the PIC24 calculations. However, if I was to use the PIC18's calculations, I simply need to change the fixed part '4' to '2' and that works:
| Code: |
PWM = Xtal frequency
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(mode) x (period + 1) x 2 (for PIC18, it's 4)
128205 = 20,000,000
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1 x (77 + 1 ) x 2
128205 = 20,000,000 / 156
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But in the datasheets, I don't see anything pertaining to that "fixed" part and I'd like it if someone could perhaps shed some light.
For the PIC18, I made myself an Excel spreadsheet with two calculations:
1) I enter the crystal frequency, the mode and period and it gives me the PWM output frequency;
2) I enter the desired PWM frequency, mode and period and it gives me the required crystal;
I'd like to be able to do the same thing with the PIC24 but I'm confused.
Thank you.
Benoit |
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ck
Joined: 02 May 2012
Posts: 18
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| Posted: Fri Sep 21, 2012 9:37 am |
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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
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setup_timer2(TMR_INTERNAL|TMR_DIV_BY_1,40000);
setup_compare(1,COMPARE_PWM|COMPARE_TIMER2|COMPARE_CONTINUE_IDLE);
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Timer 2 is the Base frequency an 40000 is counter. so i have 25nsec*40000=1ms -> so f_PWM=1KHz
Duty cycle is easy. For example, Duty=25%
You have to 25*40000/100=10000
| Code: |
set_pwm_duty (1,10000);
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benoitstjean
Joined: 30 Oct 2007
Posts: 566
Location: Ottawa, Ontario, Canada
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| Posted: Fri Sep 21, 2012 10:03 am |
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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)... |
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benoitstjean
Joined: 30 Oct 2007
Posts: 566
Location: Ottawa, Ontario, Canada
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| Posted: Fri Sep 21, 2012 10:26 am |
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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. |
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Ttelmah
Joined: 11 Mar 2010
Posts: 19538
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| Posted: Sun Oct 14, 2012 9:06 am |
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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 |
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benoitstjean
Joined: 30 Oct 2007
Posts: 566
Location: Ottawa, Ontario, Canada
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| Posted: Mon Oct 15, 2012 5:38 am |
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| Good! I wasn't too sure if it was the /2 factor. So that all makes sense now. Thank you! |
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