PWD relationship of duty value

[Just a User]

I was under the impresion that the period and duty were directly related. For example if I set the following:
int16 period = 99; //
setup_power_pwm(PWM_FREE_RUN,1,0,period,0,1,0);

then to get 100% duty cycle I would run:
int16 duty_cycle2 = 100; //100%
set_power_pwm0_duty(duty_cycle2);

but that seems to not be the case. If fact with the code below, 4x the period seems to be the 100% duty cycle. Could someone explain the relationship between the two if there is one?


#include<18f4331.h>
#device adc=10
#fuses INTRC_IO, NOWDT, NOPROTECT, NOBROWNOUT, PUT, NOLVP
#use delay(clock=8000000) //8Mhz

int16 duty_cycle1, duty_cycle2, duty_cycle3, duty_cycle4;
int16 period = 99; //

void main()
{
setup_oscillator(OSC_8MHZ);
setup_power_pwm_pins(PWM_COMPLEMENTARY ,PWM_COMPLEMENTARY, PWM_OFF, PWM_OFF);
setup_power_pwm(PWM_FREE_RUN,1,0,period,0,1,0);

duty_cycle1 = 800; //???%
duty_cycle2 = 100; //100%
duty_cycle3 = 25; //25%

while (true) {
set_power_pwm0_duty(duty_cycle1);
delay_ms(500);

set_power_pwm0_duty(duty_cycle2);
delay_ms(500);

set_power_pwm0_duty(duty_cycle3);
delay_ms(500);

} //while

}//main()

[PCM programmer]

[Ttelmah]

They are directly related.
However the PWM, internally uses the master 'instruction' clock, for the period, and the original input clock for the 'width' (*4). This is why you can have period values up to 255 (256), but width values up to 1023. The actual PWM period control register, only accesses the 8 bit counter, that directly relates to the period value. There are though two 'extra' bits, that provide access to two bits corresponding to the higher speed counter.
CCS, 'changes mode', according to the value you use to control the PWM period. If you use an 8bit value (int8), then it only accesses the main PWM control register. If you use a 16bit value, then it automatically takes the top eight bits of this, and puts these into the main register, and the low two bits into the extra control.
Now, if you use a cast like this:

set_power_pwm0_duty((int8)duty_cycle1);

The PWM, will behave as you expect.
With your constants, the compiler automatically switches to using an int16 type, if the constant is over 255, forcing access to the extra control bits.

Best Wishes

[Just a User]

Thanks guys..