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PCM programmer
Joined: 06 Sep 2003
Posts: 21708
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 Posted: Fri Feb 06, 2004 6:13 pm |
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The following test program will give you a 50 KHz pwm frequency,
with 400 steps available. This gives you better than 8-bit resolution.
The key is to use a 16-bit variable as the parameter in the
set_pwm1_duty() function. Notice that I'm using a variable, below.
If you use a constant, be sure to declare it as a "long" by putting
a "L" on the end. Example:
set_pwm1_duty(200L);
When you specify it as a 16-bit value, this tells the compiler
to use 10-bit resolution mode when it compiles the code.
| Code: | #include <16F877.h>
#fuses HS,NOWDT,NOPROTECT,PUT,BROWNOUT,NOLVP
#use Delay(clock=20000000)
#use rs232(baud=9600, xmit=PIN_C6, rcv=PIN_C7, ERRORS)
main()
{
int16 pwm_duty;
output_low(PIN_C2); // Set CCP1 output low
setup_ccp1(CCP_PWM); // Configure CCP1 as a PWM
setup_timer_2(T2_DIV_BY_1, 99, 1);
pwm_duty = 200; // Set for 50% duty cycle
set_pwm1_duty(pwm_duty);
while(1);
} |
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Neutone
Joined: 08 Sep 2003
Posts: 839
Location: Houston
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| Posted: Fri Feb 06, 2004 9:19 pm |
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| PCM programmer wrote: | The following test program will give you a 50 KHz pwm frequency,
with 400 steps available. This gives you better than 8-bit resolution.
The key is to use a 16-bit variable as the parameter in the
set_pwm1_duty() function. Notice that I'm using a variable, below.
If you use a constant, be sure to declare it as a "long" by putting
a "L" on the end. Example:
set_pwm1_duty(200L);
When you specify it as a 16-bit value, this tells the compiler
to use 10-bit resolution mode when it compiles the code.
| Code: | #include <16F877.h>
#fuses HS,NOWDT,NOPROTECT,PUT,BROWNOUT,NOLVP
#use Delay(clock=20000000)
#use rs232(baud=9600, xmit=PIN_C6, rcv=PIN_C7, ERRORS)
main()
{
int16 pwm_duty;
output_low(PIN_C2); // Set CCP1 output low
setup_ccp1(CCP_PWM); // Configure CCP1 as a PWM
setup_timer_2(T2_DIV_BY_1, 99, 1);
pwm_duty = 200; // Set for 50% duty cycle
set_pwm1_duty(pwm_duty);
while(1);
} |
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Is that going to toggle the output on quarter cycles of the instruction clock? |
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PCM programmer
Joined: 06 Sep 2003
Posts: 21708
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| Posted: Fri Feb 06, 2004 9:56 pm |
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| Quote: | | Is that going to toggle the output on quarter cycles of the instruction clock? |
Yes. Microchip doesn't come right out and say it as plainly as they
should, but yes, it will.
In AN594, Using the CCP Module, on page 2, Microchip says:
"This gives a maximum accuracy of Tosc (50 ns when the device
is operated at 20 MHz)".
In the 16F877 data sheet they say that in 10-bit mode, the two
LSB's come from the pre-scaler for Timer2. That pre-scaler
is clocked at Fosc, so that means that 2-bit counter will count
at a 50 ns rate. They have this small note at the bottom of
the PWM block diagram:
"Note 1: The 8-bit timer is concatenated with 2-bit internal Q
clock, or 2 bits of the prescaler, to create 10-bit time-base". |
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davt
Joined: 07 Oct 2003
Posts: 66
Location: England
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| setting up for PWM |
| Posted: Sat Feb 07, 2004 1:48 pm |
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Many thanks for your help.
Have a good weekend!
Dave |
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