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Simple question on how to calculate duty for pwm to motors

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PCM programmer

Joined: 06 Sep 2003
Posts: 21708

Posted: Sun Jul 25, 2010 4:31 pm

Here's an example program. When I turn the trimpot from 0 to +5v,
the PWM duty cycle goes from 0 to 100%.

This line calculates the duty cycle value for the set_pwm1_duty() function:

Quote:

duty = (adc_result / 255.0) * (PR2_VALUE +1);

The expression shown in bold changes from 0 to 1.0, when I turn the
knob on the trimpot. In terms of a percentage, that's from 0 to 100%.

If you want to use a percentage number (1 to 100), you will need
to divide it by 100 (to get 0 to 1.0) so it would be the same range as
the expression shown in bold above.

Code:

#include <16F877.H>
#device adc=8
#fuses XT, NOWDT, NOPROTECT, BROWNOUT, PUT, NOLVP
#use delay(clock=4000000)
#use rs232(baud=9600, xmit=PIN_C6, rcv=PIN_C7, ERRORS)

#define PR2_VALUE 254

//==========================================
void main()
{
int8 adc_result;
int8 duty;

setup_adc_ports(AN0);
setup_adc(ADC_CLOCK_DIV_8);
set_adc_channel(0);
delay_us(20);

setup_ccp1(CCP_PWM);
setup_timer_2(T2_DIV_BY_1, PR2_VALUE, 1);
set_pwm1_duty(0);

while(1)
{
adc_result = read_adc();
duty = (adc_result / 255.0) * (PR2_VALUE +1);
set_pwm1_duty(duty);
}

}

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