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vsmguy
Joined: 13 Jan 2007
Posts: 91
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| Details on set_pwmX_duty values |
 Posted: Sat Feb 10, 2007 9:45 am |
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I am totally confused regarding the value I should put in set_pwmX_duty()
PCM Programmer had put forward that 50% duty cycle means 50% of 249 = 124.
But I got only 8% of the output value by setting the value to 124.
However, I get approx 50% duty cycle by keeping the value as 500, which is exactly the value suggested by PWMWizard.
My code is as follows :
| Code: | //CCS 3.249
#include <16F877a.h>
#fuses HS,NOWDT,NOPROTECT,NOLVP,PUT
//#fuses HS,NOWDT,PROTECT,NOLVP
#use delay(clock=20000000)
#define SET_LEFT_MOTOR_DC set_pwm1_duty
#define SET_RIGHT_MOTOR_DC set_pwm2_duty
/* LUT containing values computes as : (index * 249), where index denotes the duty cule percent. For example, the 0th value (0%) is 0, the 10th entry is 10 * 249 = 24, etc */
const unsigned int8 uiDutyCycleLUT[] = {0, 2, 4, 7, 9, 12, 14, 17, 19, 22, 24, 27, 29, 32, 34, 37, 39, 42, 44, 47, 49, 52, 54, 57, 59, 62, 64, 67, 69, 72, 74, 77, 79, 82, 84, 87, 89, 92, 94, 97, 99, 102, 104, 107, 109, 112, 114, 117, 119, 122, 124, 126, 129, 131, 134, 136, 139, 141, 144, 146, 149, 151, 154, 156, 159, 161, 164, 166, 169, 171, 174, 176, 179, 181, 184, 186, 189, 191, 194, 196, 199, 201, 204, 206, 209, 211, 214, 216, 219, 221, 224, 226, 229, 231, 234, 236, 239, 241, 244, 246, 249};
void main()
{
disable_interrupts(GLOBAL);
output_low(PIN_C1); // Set CCP2 output low
output_low(PIN_C2); // Set CCP1 output low
setup_ccp1(CCP_PWM); // Configure CCP1 as a PWM
setup_ccp2(CCP_PWM); // Configure CCP2 as a PWM
//setup_timer_2(T2_DIV_BY_4, 249, 1); // This gives 5 kHz
setup_timer_2(T2_DIV_BY_16, 249, 1); // This gives 1250 Hz
/* Start with normal state */
SET_LEFT_MOTOR_DC(uiDutyCycleLUT[50]);
SET_RIGHT_MOTOR_DC(uiDutyCycleLUT[50]);
}
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Could someone (maybe PCM Programmer himself, write a small detailed description of how to choose the value ? |
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PCM programmer
Joined: 06 Sep 2003
Posts: 21708
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| Posted: Sat Feb 10, 2007 11:28 am |
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You need to put a while(1); statement at the end of your program.
If you don't have it, the PIC will go to sleep and PWM will stop working.
If you need more help (from me), I can do it on tomorrow, on Sunday.
Then I'll have a scope available, etc. |
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vsmguy
Joined: 13 Jan 2007
Posts: 91
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| Posted: Sat Feb 10, 2007 11:55 am |
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Oh no... the code did not end there... i just copy pasted te relevant portion... you can very well put a while(1); at the end.
Yes... to play around with the timer values and the duty cycle values.
PWM is a section which I see is a PITA...
Most questions that surface are either PWM or LCD or ADC as far as I can see ...
And thanks PCM Programmer... you have been the most helpful. |
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vsmguy
Joined: 13 Jan 2007
Posts: 91
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| Posted: Sat Feb 10, 2007 11:59 am |
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@PCM
Also do comment on the snippet in general... for example... my idea of putting the LUT in ROM and so on..
I heard that you need to follow a certain init pattern to setup PWM ?
Do you know about it ?
But one thing is for sure... I want to put an end to all the PWM confusion I have... this duty cycle calculation is getting on my nerves...
- the formula that Microchip gives in their datasheet - does it yelid the value in time period or count/timer values ? |
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vsmguy
Joined: 13 Jan 2007
Posts: 91
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| Posted: Sun Feb 11, 2007 7:18 am |
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@PCM
I think that the idea of using the percentage value of the timer overflow value in setup_timer is valid ONLY when using a scalar value of 1.
For values of 4 and 16, the duty percent values need to be scaled accordingly...
Could you verify it on the scope... I may well work with you on it ...but I will have to work for it after office hours as I don't have a scope at home... |
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PCM programmer
Joined: 06 Sep 2003
Posts: 21708
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| Posted: Sun Feb 11, 2007 2:22 pm |
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I took the code that you posted, added a while(1); statement to the end,
and ran it on a PicDem2-Plus board, with a 16F877A and 20 MHz crystal.
I'm looking at pins C1 and C2 with my scope.
I see a 1.25 KHz signal on pins C1 and C2.
Both pins are at 50% duty cycle.
I have the scope set on 100 us/division. Each half cycle of the square
wave takes 4 major divisions, which is 400 us. Then, 2 x 400 us = 800 us,
which is 1.25 KHz.
I'm using PCM vs. 3.249 with MPLAB, and a Microchip ICD2 programmer. |
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vsmguy
Joined: 13 Jan 2007
Posts: 91
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| Posted: Sun Feb 11, 2007 8:13 pm |
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@PCM
Could you just run the duty cycle index from 0 to 100% (right now the index is set to 50%) and get back to me...
I always test my code before burning it and am sure I got the firmware right, but maybe I think the problem lies in the hardware I am using.. but still, it's reassuring to know my code is ok...
Do have a program ro run through the LUT from 0 to 100%
I have a favour to ask - could you PM me with your email address so I can send you in details the application I am applying the codee to ? It's basically a DC MCS with the LMD18201 and a 877A. |
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PCM programmer
Joined: 06 Sep 2003
Posts: 21708
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| Posted: Sun Feb 11, 2007 8:32 pm |
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I can do the test with incrementing the index on Monday. I don't want
to send my email address. I only want to help people through the forum.
--------
Edited to add:
On 2nd thought, I don't want to test running the LUT. My goal on this
forum is to help people get past a sticking point on their project, where
some piece of hardware doesn't work and the driver needs a few tweaks,
or perhaps a CCS function doesn't work properly and it needs some
work-around code. I don't want to be a co-partner on a project.
I don't want to be asked to "approve" someone's code. I don't want
to be asked to test someones code and tell them that it works. For those
reasons I'm going to decline your request. |
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vsmguy
Joined: 13 Jan 2007
Posts: 91
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| Posted: Mon Feb 12, 2007 1:02 am |
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| PCM programmer wrote: | My goal on this
forum is to help people get past a sticking point on their project, where
some piece of hardware doesn't work and the driver needs a few tweaks,
or perhaps a CCS function doesn't work properly and it needs some
work-around code. I don't want to be a co-partner on a project.
I don't want to be asked to "approve" someone's code. I don't want
to be asked to test someones code and tell them that it works. For those
reasons I'm going to decline your request. |
Very good points.
I will test my code and hardware in this week and get back to you. |
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vsmguy
Joined: 13 Jan 2007
Posts: 91
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| Posted: Fri Feb 16, 2007 9:46 am |
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@PCM
You were right at the start... from the VERY beginning....
To summarize what you said :
The duty cycle value is EXACTLY the percentage of DC you want of the timer count value.
Now I got to have the bit resolution worked out... do you have good docs on PIC PWM except the datasheet ?
If not... how should I start with the PWM datasheet ? |
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