How to set HPWM to generate 38khz signal? (16F870)

[mackrackit]

And this tells how to pass data.
https://web.archive.org/web/20120307...B_PIC_TEMP.htm

[CuriousOne]

Thanks Henrik. Is it possible to do with 4mhz oscillator? I'm testing on breadboard, and with 20mhz oscillator it behaves badly.

[HenrikOlsson]

Short answer is:
Yes.

Question is:
Have you tried yourself with 4MHz? What did you change and why? What did it do, why do you think it didn't work?

Long answer is:
First, PWM period:
* TMR2 "drives" the CCP module.
* TMR2 starts at 0 and counts upwards.
* When the content of TMR2 equals that of PR2, TMR2 starts over from 0 and on it goes. The time it takes for TMR2 to count from 0 to PR2 is the PWM period.

As before, if you want a PWM perdiod of 26.315us you need to set the PR2 register so that TMR2 resets and starts over after 26.315us. At 4MHz the fastest you increment TMR2 (on that devices) is 1MHz, ie it'll "tick" every 1us. So the closest you're going to get is a flat 26us (38.461Hz) and you do this by setting PR2 to 25, right? Because TMR2 will then Count from 0 to 25 (26us) before it starts over and does it again and again and again......

Second, the dutycycle:
This can be a little trickier to understand but basically the dutycycle value as a whole can be though of the value you put into PR2 times 4. That is, when PR2 is 25 as in this case the dutycycle value can go from 0 to 100 (for 0-100% dutycycle). In the previous example with 20MHz Clock and a PR2 value of 130 the dutycycle value, again for 0-100% would go from 0 to 520.

Now, the dutycycle value since it can theoretically be 10bits wide (0-1023) are spread across two registers, CCPR1L contains the 8 high bits and CCP1CON.5 and 4 (from memory) contains the two low bits,

If you want "roughly" 50% dutycycle with a PR2 value of 25 you simply write either 12 or 13 (half of 25) to CCPR1L and that's it. If you want better resolution I'm sure you can figure out what to do with the two least significant bits.


/Henrik.

[peterdeco1]

Hello. A while back I used this routine to generate 38KHZ with a 12F675 running at 4MHZ.

STARTBIT:
High GPIO.4 'GETTING 38KHZ WITH THIS SUBROUTINE
High GPIO.4
Low GPIO.4
Low GPIO.4

LET X = X + 001
IF X < 125 Then STARTBIT '125 = 3MS PULSE, 83 = 2MS pulse, 33 = 1MS pulse

Maybe you can do something with this.

[richard]

Knocked this up when the remote for my aircon died , never really finished it . got a clone remote on ebay for $10 (delivered)
why bother.

this can work for any osc >= 8 MHz

'****************************************************************
'*  Name    : ir38khz.BAS                                       *
'*  Author  : RICHARD                                           *
'*  Notice  :                                                   *
'*          :                                                   *
'*  Date    : 6/30/2016                                         *
'*  Version : 1.0                                               *
'*  Notes   : REPL BRA WITH GOTO  FOR NASTY OLD CHIPS           *
'*          : OSC MUST BE AT LEAST 8 MHZ                        *
'****************************************************************
 

 
 
 
DEFINE OSC 16       ; MIN=8     TESTED  OK 8,16,32
DEFINE IRPORT  LATC 
DEFINE IRPIN  4 
define IR_1   2000     ; 1 TIME IN uS   MAX 6500 ISH
define IR_0   900      ; 0 TIME IN uS
 
cycnt       var byte  bank0       ;set to numbr of 38k cycles req then call irout  RANGE 1-255
dly         var byte  bank0      ;system

goto overasm
 asm  
_irout1                 ;entry point FOR 1 pulse   
        movlw   (IR_1/26)
        movwf   _cycnt
        BRA     _irout    ;GOTO 
_irout0 movlw   (IR_0/26) ;entry point FOR 0 pulse   
        movwf   _cycnt       
        
_irout                 ;entry point for cycnt cycles
nxbt    CHK?RP IRPORT
        BSF IRPORT ,IRPIN
        CALL dly13
        CHK?RP IRPORT
        BCF  IRPORT ,IRPIN
        CALL dly13
        RST?RP
        DECFSZ _cycnt,F
        BRA nxbt             ;GOTO
        RETURN 
dly13   movlw   (OSC-(16/OSC))
        RST?RP 
        movwf   _dly
d13         
        DECFSZ _dly,F
        BRA d13           ;GOTO
        RETURN
 endasm        
   
overasm:         
  
 

   OSCCON=$78
   ANSELA=0
   ANSELC=0

   TRISA = %111110
   TRISC = %11000011         ' set PORTC I/O
 
 
  
main:
    cycnt = 100
    call irout
    pauseus 250
    call irout1
    pauseus 250
    call irout0
    pause 50
    
goto main

[CuriousOne]

Thanks, will have look.