Noob question with PWM

[kasamiko]

hi,

can somebody tells me how this guy got the value of

CCPR1L = 25 and
CCP1CON = 12

here's part of the code:

@ DEVICE HS_OSC,MCLR_OFF,LVP_OFF,WDT_OFF,PROTECT_OFF
DEFINE OSC 20 ' We're using a 20MHz crystal
CCPR1L = 25 ' 25=20% PWM Duty-Cycle
PR2 = 124 ' Set PWM for ~40KHz
T2CON = 4 ' Timer2 ON + 1:1 prescale
TRISB=%00000100 ' Set PortB TRIS register
PORTB=%00000011 ' LEDs off, pulsout pins set for logic 1 pulses

....

Begin: ' Eats up 1 line, but we need it
CCP1CON = 12 ' Turn hardware PWM On
LOW PortB.0 ' Turn on left IR LED
PAUSE 10 ' Wait 10mS to allow IR detector output to change
Hits[0]=IR_Output ' Read detector output into Bit0
Portb=%00000001 ' Turn off left IR LED, turn on right IR LED
PAUSE 10 ' Wait 10mS to allow IR detector output to change
Hits[1]=IR_Output ' Read detector output into Bit1
HIGH PortB.1 ' Turn off right IR LED
CCP1CON = 0 ' Turn hardware PWM Off

I tried using the online PWM calculator which can be found here:

PIC PWM Calculator & Code Generator

but got CCPR1L and CCP1CON values as follows:

CCPR1L = 0b00011000 or 24 decimal
CCP1CON = 0b00111100 or 60 decimal

which of them are correct?

TIA



`

---------- Post added at 11:02 ---------- Previous post was at 10:19 ----------

bit 7-6 Unimplemented: Read as '0'
bit 5-4 CCP1X:CCP1Y: PWM Least Significant bits
Capture Mode: Unused
Compare Mode: Unused
PWM Mode: These bits are the two LSbs of the PWM duty cycle. The eight MSbs are found in
CCPRxL.
bit 3-0 CCP1M3:CCP1M0: CCPx Mode Select bits
0000 = Capture/Compare/PWM off (resets CCP1 module)
0100 = Capture mode, every falling edge
0101 = Capture mode, every rising edge
0110 = Capture mode, every 4th rising edge
0111 = Capture mode, every 16th rising edge
1000 = Compare mode, set output on match (CCP1IF bit is set)
1001 = Compare mode, clear output on match (CCP1IF bit is set)
1010 = Compare mode, generate software interrupt on match (CCP1IF bit is set, CCP1 pin is
unaffected)
1011 = Compare mode, trigger special event (CCP1IF bit is set; CCP1 resets TMR1
11xx = PWM mode

11xx = PWM mode

Well I think this is constant for CCP1CON to be 12 for PWM mode?

 

[malik_zohaib]

first check datasheet for formula

given things are:
Fosc=20MHz F=40KHz, DC=20% TMR2 prescaler =1

PR2=[RWM period/4*Tosc]-1 =[25us/.02us]-1=124

NOW

CCPR1L:CCP1CON <4:5>= PWM DC /Tosc * TMR2 prescaler =.2*25us/.05us =100d =1100100

so CCP1CON<4:5>=0 and CCPR1L=11001 =25d

so it gives CCP1CON=0x0C

hope it will help u

 

[kasamiko]

I'll try your suggestion...