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ritchie
Joined: 13 Sep 2003
Posts: 87
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| Need a suggestion on PWM application |
 Posted: Sat Jul 10, 2004 8:30 am |
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Hi,
I need a suggestions and/or comments on my PWM application.
I have setup a PWM mode on my PIC18F452 and the code is listed below:
| Code: |
#include <18F452.H>
#device *=16
#fuses HS,NOPROTECT,NOWDT,NOLVP,PUT
#use delay(clock=20000000)
void init_mcu()
{
delay_ms(3000);
set_tris_c(0b11111011); // set PIN_C2 as output
setup_adc_ports(NO_ANALOGS); // set PortA to digital I/O
setup_ccp1(CCP_PWM); // set CCP1 to PWM mode
setup_timer_2(T2_DIV_BY_1,39,1); // (1/20000000)*4*1*40
// t2=8uSec. or f2=125KHz
set_pwm1_duty(20); // 50% duty cycle at 125KHz
}
void main()
{
init_mcu(); // initialize MCU
while (TRUE);
}
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the code above will provide a 125KHz square wave at 50% duty cycle on PIN_C2.
The catch here I want to turn-on my PWM for 100ms then turn-off for 100ms... this cycle will repaet periodically...
what would be a good approach in doing this? Is it good practice to use setup_ccp1(CCP_OFF) such that my PWM is turn-off for 100ms then turn-on again.
Do I need to use two(2) timer interrupt to handle this turn-on or turn-off thing. I prefer to use interrupt on this....
I need your comments and suggestions... a simple code snippet will provide me help.
thank u. |
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Ttelmah
Guest
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| Re: Need a suggestion on PWM application |
| Posted: Sat Jul 10, 2004 11:47 am |
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| ritchie wrote: | Hi,
I need a suggestions and/or comments on my PWM application.
I have setup a PWM mode on my PIC18F452 and the code is listed below:
| Code: |
#include <18F452.H>
#device *=16
#fuses HS,NOPROTECT,NOWDT,NOLVP,PUT
#use delay(clock=20000000)
void init_mcu()
{
delay_ms(3000);
set_tris_c(0b11111011); // set PIN_C2 as output
setup_adc_ports(NO_ANALOGS); // set PortA to digital I/O
setup_ccp1(CCP_PWM); // set CCP1 to PWM mode
setup_timer_2(T2_DIV_BY_1,39,1); // (1/20000000)*4*1*40
// t2=8uSec. or f2=125KHz
set_pwm1_duty(20); // 50% duty cycle at 125KHz
}
void main()
{
init_mcu(); // initialize MCU
while (TRUE);
}
|
the code above will provide a 125KHz square wave at 50% duty cycle on PIN_C2.
The catch here I want to turn-on my PWM for 100ms then turn-off for 100ms... this cycle will repaet periodically...
what would be a good approach in doing this? Is it good practice to use setup_ccp1(CCP_OFF) such that my PWM is turn-off for 100ms then turn-on again.
Do I need to use two(2) timer interrupt to handle this turn-on or turn-off thing. I prefer to use interrupt on this....
I need your comments and suggestions... a simple code snippet will provide me help.
thank u. |
The last number in the Timer2 setup, is the number of counts between it's interrupt. If (for instance), you set this to '10', then you will get an interrupt every 1/12,500th second. In this, implement a counter, and when it gets to zero, set the duty cycle to '0'. You can turn the PWM off, but assuming that for your 'off', just wants a logic '0', setting the duty cycle does a similar thing, with less operations involved.
The cycle will update on the next PWM cycle.
The same will be true if you set the pwm cycle back to 20, after the same time interval.
So you can use the same timer for both jobs, with just an extra counter, and a 'flag', to say which way to change the counter.
Best Wishes |
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Neutone
Joined: 08 Sep 2003
Posts: 839
Location: Houston
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| Posted: Sat Jul 10, 2004 7:44 pm |
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I don't think you can service an interupt every 40 instruction cycles. I would sugest using timer4 to time the on and off intervals.
| Code: | #int_TIMER4
void TIMER4_isr()
{ Miliseconds++;
if(Miliseconds>99)
{ Miliseconds=0;
if(On_Cycle)
{ set_pwm1_duty(20);
On_Cycle=0;
}
else
{ set_pwm1_duty(0);
On_Cycle=1;
}
}
}
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| Code: | setup_timer_4(T4_DIV_BY_4,249,5); // Set 1mS period on a 20Mhz xtl
enable_interrupts(INT_TIMER4);
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ritchie
Joined: 13 Sep 2003
Posts: 87
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| Posted: Sat Jul 10, 2004 8:14 pm |
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| Neutone wrote: | I don't think you can service an interupt every 40 instruction cycles. I would sugest using timer4 to time the on and off intervals.
| Code: | #int_TIMER4
void TIMER4_isr()
{ Miliseconds++;
if(Miliseconds>99)
{ Miliseconds=0;
if(On_Cycle)
{ set_pwm1_duty(20);
On_Cycle=0;
}
else
{ set_pwm1_duty(0);
On_Cycle=1;
}
}
}
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| Code: | setup_timer_4(T4_DIV_BY_4,249,5); // Set 1mS period on a 20Mhz xtl
enable_interrupts(INT_TIMER4);
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Thank you Ttelmah and Neutone for such a great idea.... btw, I don't think a PIC18F452 has a timer4? Anyway, I will use timer3 for this ON-OFF thing..
once again... tengkyu |
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Ttelmah
Guest
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| Posted: Mon Jul 12, 2004 3:24 am |
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| Neutone wrote: | I don't think you can service an interupt every 40 instruction cycles. I would sugest using timer4 to time the on and off intervals.
| Code: | #int_TIMER4
void TIMER4_isr()
{ Miliseconds++;
if(Miliseconds>99)
{ Miliseconds=0;
if(On_Cycle)
{ set_pwm1_duty(20);
On_Cycle=0;
}
else
{ set_pwm1_duty(0);
On_Cycle=1;
}
}
}
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| Code: | setup_timer_4(T4_DIV_BY_4,249,5); // Set 1mS period on a 20Mhz xtl
enable_interrupts(INT_TIMER4);
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If you use the 'counter' for the interrupts on Timer2, then set to 10, the interrupt will be every 400 instructions, which is achievable. The maximum allowed is 16, which gives one interrupt every 640 instructions, which is fine. It does save a timer, if this is important.
Best Wishes |
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Neutone
Joined: 08 Sep 2003
Posts: 839
Location: Houston
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| Posted: Mon Jul 12, 2004 7:47 am |
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| ritchie posted a PWM period of 40 instruction cycles. It's my understanding that the interupt period of the timer and the PWM period are the same. Does the overflow counter not figure into the PWM period? |
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Ttelmah
Guest
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| Posted: Mon Jul 12, 2004 2:19 pm |
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| Neutone wrote: | | ritchie posted a PWM period of 40 instruction cycles. It's my understanding that the interupt period of the timer and the PWM period are the same. Does the overflow counter not figure into the PWM period? |
Timer2, has a post divider, between the timer, and the interrupt. This is the last number in the timer definition, and can be 1 to 16. Hence with a 40 instruction pwm, you can have interrupts at 40 instructions (with it set to 1), 80 instructions (set to 2), up to 640 instructions (with it at 16). It is a very useful feature, when you want to do something 'synchronous' to a fast PWM waveform, but need a longer interval.
I use this on a PWM system at 80KHz (with a 40MHz processor clock), to synchronise the servo update loop driving the PWM, to 8 cycles of the PWM.
Using this, avoids the need for another timer, and would work at a 'reasonable' frequency.
Best Wishes |
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