How to setup 38kHz PWM with PIC18F1330

[flotulopex]

Hi there,

I'm trying to setup a 38kHz PWM with a PIC18F1330.

My circuit is as simple as possible for tests: a blinking "I'm alive" LED and a 8MHz crystal.

Don't get it...

' Fuses PIC18F1330 for MPASM
@ __CONFIG _CONFIG1H, _OSC_HS_1H & _FCMEN_OFF_1H & _IESO_OFF_1H
@ __CONFIG _CONFIG2L, _PWRT_OFF_2L & _BOR_OFF_2L & _BORV_0_2L
@ __CONFIG _CONFIG2H, _WDT_OFF_2H & _WDTPS_512_2H
@ __CONFIG _CONFIG3L, _HPOL_HIGH_3L & _LPOL_HIGH_3L & _PWMPIN_OFF_3L
@ __CONFIG _CONFIG3H, _FLTAMX_RA7_3H & _T1OSCMX_LOW_3H & _MCLRE_OFF_3H
@ __CONFIG _CONFIG4L, _STVREN_ON_4L & _BBSIZ_BB256_4L & _XINST_OFF_4L & _DEBUG_OFF_4L
@ __CONFIG _CONFIG5L, _CP0_OFF_5L & _CP1_OFF_5L
@ __CONFIG _CONFIG5H, _CPB_OFF_5H & _CPD_OFF_5H
@ __CONFIG _CONFIG6L, _WRT0_OFF_6L & _WRT1_OFF_6L
@ __CONFIG _CONFIG6H, _WRTC_OFF_6H & _WRTB_OFF_6H & _WRTD_OFF_6H
@ __CONFIG _CONFIG7L, _EBTR0_OFF_7L & _EBTR1_OFF_7L
@ __CONFIG _CONFIG7H, _EBTRB_OFF_7H    

'==========================================================================
' Main Registers
'          76543210
OSCCON  = %00000000 'OSCILLATOR CONTROL REGISTER
OSCTUNE = %00000000 'OSCILLATOR TUNING REGISTER
ADCON0  = %00000000 'A/D CONTROL REGISTER 0
ADCON1  = %00000000 'A/D CONTROL REGISTER 1
ADCON2  = %00000000 'A/D CONTROL REGISTER 2
INTCON  = %00000000 'INTERRUPT CONTROL REGISTER
INTCON2 = %00000000 'INTERRUPT CONTROL REGISTER 2
INTCON3 = %00000000 'INTERRUPT CONTROL REGISTER 3
TRISA   = %00000000 'Data Direction Control Register (Input/Output)
PORTA   = %00000000 'State High (1) or Low (0)
TRISB   = %00000000 'Data Direction Control Register (Input/Output)
PORTB   = %00000000 'State High (1) or Low (0) 

' Set up PWM1 (PORTB.1)
PTPERL  = %11111111
PTPERH  = %11111111
T0CON   = %00000000
T1CON   = %01001010
PTCON0  = %00000000 'Postscale 1:1, Fosc/4 Prescale 1:1, Free Running Mode
PTCON1  = %10000000 'Time base in ON, Time base counts up
PWMCON0 = %00010000 'PWM1 enabled
PDC0H   = 30        'Duty Cycle control for PWM0/1


DEFINE OSC 8

LED var PORTA.2
'PORTB.1 is PWM1

MAIN:
    TOGGLE LED
    PAUSE 500
    GOTO MAIN

END

What is actually the highest PWM frequency I can get with the 8MHz crystal?

The DS is confusing me a lot :-(

[mark_s]

What is actually the highest PWM frequency I can get with the 8MHz crystal?

The DS is confusing me a lot :-(

Hi

If you look on page 129 of the data sheet. There is a chart of Fosc vs. PWM frequency.
They do not list 8mhz, but the highest 4mhz frequency is 3.9khz. So I guess you can expect
it to be double at 8mhz or 7.8Khz

If you configure the oscillator for 4x HSPLL you could get closer.

8mhz crystal 4x HSPLL - PWM freq = 31.2khz max

Did the PWM output anything, you didn't say?

[HenrikOlsson]

Hi,
If you set PTPER to 51 with the prescaler set to 1:1 and 8MHz clock you'll get a PWM period of 26us which 38461Hz, set it to 52 and you'll get 37736Hz.

Just remember that the resolution won't be 14 or even 10 bits at that PWM frequency and oscillator frequency. Forumla says you'll get 7.7bits which would be a dutycycle value of 0 to 246 or there abouts.

/Henrik.

EDIT: Oh, and the highest frequency possible would be when you set PTPER to 1 which would give you a PWM frequency of 1MHz. The resolution will be 2 bits.

[mark_s]

Henrik,

Thanks for correcting my wrong answer.

[flotulopex]

Thank you.

I'll try this out when I'm back home.

Did the PWM output anything, you didn't say?

Yes, it does work. With the code in my first post, I get a PWM signal around 430Hz (not sure about that frequency right now).

[HenrikOlsson]

Hi,

The PWM Period register has 12 bits of resolution and the PWM period is defined as (PTPER+1) * Prescaler / (Fosc/4)

In your original code you have the prescaler set to 1:1 and you're setting PTPER to 65535 but again, only the low 12 bits are used which equals 4095. Plug that into the formula and you'll get 4096/2000000=0.002048. That's a PWM period of 2.048ms or a frequency of 488.28Hz.

/Henrik.