Infrared HPWM setup

[astanapane]

Dear all,

just a quick question. I didnt want to open a new thread but most of the Infrared threads are closed.

I would like to set up the PIC18F26k22 in order to use HPWM for the 38khz IR.

First of all i would like to ask if i can use 64Mhz as the main PIC frequency.

i tried to use mister e's pic multi-calc and set 64Mhz. The following are the results i got.



but from the calculations i made:

(64Mhz / (4*TMR2 prescale value * 38Khz))-1 = PR2
(64Mhz / (4*1*38Khz)) - 1 = PR2
(64Mhz / (152000)) - 1 = PR2 <=>

            64,000,000
<=> PR2 =  ------------ -1 <=>
             152,000

<=> PR2 = 420

Also calculating the bits:

              log(FOSC / FPWM)
 Resolution = ------------------- bits
                  Log(2)

log(64,000,000 / 38,000) = 3.226

log(2) = 0.301

so we have:
3.226
------- = 10.17 ~ 10 bits
0.301

and calculating the CCPRL1:

(PR2 + 1) * TMR2 prescale * 50% Duty Cycle = value for CCPRL1, or
(420 + 1) * 1 * 0.50 = 421 * 0.50 = 210

So i set in the code the following:

TRISC.2 = 0 ' CCP1 (PortC.2 = Output)
PR2 = 420 ' Set PWM Period for approximately 38KHz
CCPR1L = 210 ' Set PWM Duty-Cycle to 50%
T2CON = %00000100 ' Timer2 ON + 1:1 prescale --> i know this might not be right as at 64Mhz there is no 1:1 scale based on mister's e multi-calc.
CCP1CON = %00001100 ' Mode select = PWM

could you please help me to clear things up?

I dont mind using 16Mhz as a base frequency, but still the calculations i get are not match with mister's e multi-calc.

Thanks for your help in advance.

[richard]

prescaler is 4 , 16 is also a possibility
(64Mhz / (4*TMR2 prescale value * 38Khz))-1 = PR2
(64Mhz / (4*4*38Khz)) - 1 = PR2
(64Mhz / (152000*4)) - 1 = PR2 <=>


Code:
64,000,000
<=> PR2 = ------------ -1 <=>
152,000*4
<=> PR2 = 104

(PR2 + 1) * TMR2 prescale * 50% Duty Cycle = value for CCPRL1, or
(104+ 1) * 4 * 0.50 = 421 * 0.50 = 210

T2CON = %00000101

PR2 = 104 ' Set PWM Period for approximately 38KHz
CCPR1L = 210>>2 ' Set PWM Duty-Cycle to 50%

note
PR2 is a 8bit reg it cannot ever be 420 or anything larger than 255
CCPR1L are the upper 8 bits of the duty cycle word value
CCP1CON bit 4 and 5 are the lower two bits

[astanapane]

Hi Richard,

thanks for the clarification and answer.

I found in the manual what you said regarding the Resolution bits.

[richard]

the duty cycle is a 10 bit value at best for that chip, not all frequencies can have the full 10bit range available
it totally depends on choices made with regard to prescaler and period.
no matter what bit resolution is possible the CPRxL value is the upper 8 bits of the duty cycle word valueCCP1CON bit 4 and 5 are the lower two bits

[astanapane]

i've also noticed you've shifted 2 bits on the right the CCPR1L

CCPR1L = 210>>2 ' Set PWM Duty-Cycle to 50%

So the CCPR1L value will be 52.

(11010010) 210>>2 = 00110100 = 52

why have we shifted?

[richard]

why have we shifted?
CPRxL value is the upper 8 bits of the duty cycle word valueCCP1CON bit 4 and 5 are the lower two bits



(011010010) 210>>2 = 00110100 = 52 ;high 8bits
(011010010) 210 & 3 = 00000010 = 2 ;low 2 bits

00000010<<4 = 00100000
CCP1CON = 00100000 |12 to set low bits in pwmmode

C version from microchip

void PWM1_LoadDutyValue(uint16_t dutyValue)
{
   // Writing to 8 MSBs of pwm duty cycle in CCPRL register
    CCPR1L = ((dutyValue & 0x03FC)>>2);
    
   // Writing to 2 LSBs of pwm duty cycle in CCPCON register
    CCP1CON = ((uint8_t)(CCP1CON & 0xCF) | ((dutyValue & 0x0003)<<4));
}

[astanapane]

thanks Richard. now is clear.

i will get back once i have a working code and establish an IR communication.