33Khz square wave generation with PIC16F676

[girichavana]

Hello everybody,

I am new to this forum. Presently I am using CCS PCH for programming my PIC16F676. Mine is the purely time based project, is this compiler is suitable ? If so how can I program my timers to get 33 KHz frequency wave ? Here I want to use the internal 4Mhz oscillator.

thanks and regards

[RLScott]

Set up Timer 2 so that it is clocked by the Fosc/4 (1 MHz), and set its period to 30. Set the duty cycle (CCPR1L) to 15. Then you will get a 50% duty cycle PWM signal at 33.33 kHz. But I think you will need to use the PCM compiler for the 16F series. The PCH compiler is for the 18F series.
_________________
Robert Scott
Real-Time Specialties
Embedded Systems Consulting

[girichavana]

Thanks for ur replay,

Sorry for my mistake, actually I am using PCM compiler. You suggested that to use the Timer 2, but its not there in PIC16f676 . Can i use the Timer1?

[Ttelmah]

Not really.
Unfortunately, it'll be more complex on this chip. Timer2, has the nice ability for you to program the count where it resets. This allows you to generate quite accurate timings for jobs like this. On the 676, you are going to have to do the job 'in software'. A lot depends on what else you want to do with the chip?. If you only want a 33KHz waveform, with nothing else going on, then the 'simplest' solution, is just:

[girichavana]

Thanks,

Your code is working properly, but my application includes some
other tasks. So here I want to use the Timer0 interrupt for this.

I want to calculate the Timer0 loadable value to get a certain frequency.

Regards

[Ttelmah]

You are not going to have any time.
The pulse width needed for each half cycle, is only _15_ instructions. You can simply run timer0, directly off the master clock (DIV_1), and add 241 to it, when it's interrupt flag is set. By the time you have done this, cleared the flag, and start waiting for it to set again, you are going to be running out of instruction.
You can save work, if you can accept a pulse, rather than a square wave. You then add 226 to the timer, and just pulse the output high, then one cycle latter, low. You then have 28 machine instructions to do things, till the next pulse is needed. Still not enough for anything much. Also, use the 'fast_io' mode, since this will save a couple of instructions on each change to the output.
To do other things, you really need to be clocking a lot faster. Clock the chip at 24MHz, and you have 182 instructions between leading edges (if you can accept a pulse output), for a 32967Hz output. Much better.
Alternatively, use a chip that does have a PWM output. The waveform, can then be generated by the hardware, leaving you time to do your other jobs.

Best Wishes

[girichavana]

The following is the code that i have written. I have simulated this by using the PROTEUS. But it is giving 6kHz. I tested with oscilloscope also. The result is same.

#include <16F676.h>
#fuses NOWDT,INTRC,NOPROTECT,BROWNOUT,NOMCLR,NOCPD,PUT

#use delay(clock=4000000,type=internal)

#int_timer0
void timer0interrupt()
{
output_toggle(PIN_C4);
set_timer0(0xF1);
}

//=========main======

void main()
{
enable_interrupts(GLOBAL);
enable_interrupts(INT_TIMER0);
setup_timer_0(RTCC_INTERNAL);
set_timer0(0xF1);
while(1)
{

}

}

[RLScott]

[Ttelmah]

Exactly.
You will get closer, if instead of physically interrupting, you _test_ the interrupt bit, and then add the offset, rather than just loading it, but I doubt if even like this, you will have more than one or two instructions 'in hand'. Certainly no time to actually do anything else.
I have posted about using polling of the interrupt bit inside the main code, as an alternative to an actual interrupt on several occassions. A search should find some stuff about this.

Best Wishes

[girichavana]

Thanks,

I got the 33khz with the ASM. How can I include the ASM routine in the CCS C ?

Regards

[RLScott]