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loboartico
Joined: 10 Aug 2011
Posts: 3
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| ADC->PWM Frequency problems |
 Posted: Wed Aug 10, 2011 12:51 pm |
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Hello guys.
I am using a 16F877A microcontroller to do a motor speed control.
The speed can be set by a trimpot.
I used a program that I found here.
| Code: |
#include <16F877A.h>
#device adc=8
#fuses XT,NOWDT,NOPROTECT,BROWNOUT,PUT,NOLVP
#use delay(clock=4000000)
//================================
void main()
{
int8 value;
setup_ccp1(CCP_PWM);
// Set the PWM frequency for 244 Hz (with a 4 MHz crystal)
setup_timer_2(T2_DIV_BY_16, 255, 1);
setup_port_a(AN0);
setup_adc(ADC_CLOCK_DIV_8); // Divisor for 4 MHz crystal
set_adc_channel(0);
delay_us(20);
while(1)
{
value = read_adc();
set_pwm1_duty(value);
}
} |
But if the motor is running at medium/low speed, it makes a lot of audible noise.
I tried to make a higher frequency changing
| Code: | | setup_timer_2(T2_DIV_BY_16, 255, 1); |
but if i do this, the pwm stops to work correctly as before.
How can i have a frequency higher than 20khz?
Sorry for my english. |
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PCM programmer
Joined: 06 Sep 2003
Posts: 21708
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| Posted: Wed Aug 10, 2011 1:12 pm |
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That program runs the PWM at 244 Hz. If you change the prescaler
to T2_DIV_BY_1 (instead of divide by 16), and if you change the
crystal to 20 MHz, then you can get 19531 Hz. Is that good enough ?
This post has links to all PWM formulas:
http://www.ccsinfo.com/forum/viewtopic.php?t=45968&start=1
My guess is that you probably don't need 20 KHz to make the motor
noise go away. It will probably work OK with a much lower frequency.
You might have to experiment and find the frequency that gives
acceptably low noise and maximum torque. It might not be 20 KHz. |
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loboartico
Joined: 10 Aug 2011
Posts: 3
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| Posted: Wed Aug 10, 2011 2:10 pm |
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Thanks for your reply.
Actually my circuit is workin with a 20Mhz oscillator and i changed the setup of the timer to 15, so i could get a higher frequency.
But since i did this, the ADC saturates earlier than before.
Before i could set 0V -> 5V at the trimpot and the PWM changed proporcionaly.
But now, the PWM changes only with my trimpot as 0V->1V more or less, so the PWM gets 5V when my trimpot is only 1V.
What is wrong with it?
Thanks. |
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Ttelmah
Joined: 11 Mar 2010
Posts: 19466
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| Posted: Wed Aug 10, 2011 2:40 pm |
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Of course it would.
You must _not_ change the timer divider, only the _prescaler_.
The maximum value accepted by the PWM, is (timer divider *4)+1 if using a 'long' integer to control it, or just the timer divider value +1, if using an 8bit integer.
Change T2_DIV_BY_16, to DIV_BY_1, and with your clock frequency, the output will go up to 19520Hz, and the ADC range will still be correct.
Read the data sheet, or do an online search for the relationship between dividers, and available PWM resolution.
Best Wishes |
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loboartico
Joined: 10 Aug 2011
Posts: 3
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| Posted: Wed Aug 10, 2011 8:35 pm |
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Thank you so much for your help, PCM Programmer and Ttelmah.
I did what you said and my program is working perfectly now =)
I can't hear the noise anymore, and the PWM is working fine.
Thanks for all. |
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