PWM 12f683

[Milhaus]

Hi all,
i need to generate ~22KHz PWM with 50% duty cycle. I have connected LED on the CCP1 pin to the +5V. But the LED does shining only when i have
on the line below SETUP_TIMER_2(T2_DIV_BY_1,180,1); When i use SETUP_TIMER_2(T2_DIV_BY_1,91,1) the LED is turned off.

Please can you check my code:

void main()
{
setup_oscillator(OSC_8MHZ);
setup_adc_ports(NO_ANALOGS|VSS_VDD);
setup_adc(ADC_OFF);
setup_comparator(NC_NC);
set_tris_a(0b11111111);
setup_ccp1(CCP_PWM);
set_pwm1_duty(128); //50% duty cycle
SETUP_TIMER_2(T2_DIV_BY_1,91,1); //91=21 978Hz ,180=11 049Hz
set_tris_a(0b11111011);

while(1)
{
}

[bungee-]

[Ttelmah]

Try one critical change:

Either:

[Guest]

Thanks guys,

[Ttelmah]

The 45, _will_ give 50% duty cycle (close to). Try them as I posted them.

What 'L' means, will be explained in a general 'C' book, rather than by CCS.

The key is how numbers are handled.

The PWM, has two separate registers used to control the reset point. The first is an 8bit register, while the other is two extra 'bits' stored in the CCPCON register. If you don't put anything into these extra two bits, then the value put into the top 8bit register is used for the count, and is effectively multiplied by 4, for the comparisons. So in your case, the PWM, would count up to 91, if you only access the top register, and 45 or 46 would give 50%.

In the help file:
"Writes the 10-bit value to the PWM to set the duty. An 8-bit value may be used if the least significant bits are not required. If value is an 8 bit item, it is shifted up with two zero bits in the lsb positions to get 10 bits.".
So if the value you are sending is only an 8bit value, it is multiplied by 4 (shifted left 2 places), and used. 45%4 = 180. To write the 10bit value, you have to actually call the function with a value having more than 8bits.

So the question then becomes, 'How do I get the compiler to treat a number with less than 8 significant bits (a value < 255), as if it had more bits'?. This is what 'L' does. It tells the compiler to treat the value _as if it is a 'Long'_, no matter what size it actually is.

So you either want to pass 1/4 the value you want, without the 'L', or add the 'L' to the value.

Best Wishes

[Milhaus]

Hi Ttelmah,
Thank you very much for the explanation. Now I understand it !!!