Generating 38kHZ wave with a PIC13XX

[Krumlink]

I tried to create a 38kHZ square wave with a PIC18F1330, but it did not work. I was wondering if anybody had any code or etc on how to generate one. I will post my code later.

 

[raviram87]

i did generate a 38kHz square wave with my pic16f877a....

i used the hardware pwm feature to do this... the output i generated was a perfect 38kHz square wave with a 25% duty cycle....

however not sure has to whether your series of pic has this feature...

 

[Nigel Goodwin]

I tried to create a 38kHZ square wave with a PIC18F1330, but it did not work. I was wondering if anybody had any code or etc on how to generate one. I will post my code later.

Are you wanting to generate a continuous 38KHz carrier, or just 38KHz pulses for IR remote control?.

If it's continuous then use the PWM module, if it's for remote control it's usually easier and more accurate to use simple software loops.

 

[Krumlink]

Yes, I wanted to generate a continous square wave at 38kHZ for IR detection in a upcoming robot

I was wondering how I enable the PWM hardware feature in a 18F1320/18F1330. I also have a 18F4620 and a 18F2550 if that makes it easier.

 

[Nigel Goodwin]

Yes, I wanted to generate a continous square wave at 38kHZ for IR detection in a upcoming robot

Are you sure about that?, usually you send pulses, a continuous carrier reduces the sensitivity of the receiver - a LOT!.

You might try this very old webpage of mine?

**broken link removed**

 

[Krumlink]

What would be the frequency of the interrupts? I think it would be easier just to use the PWM software to easily generate the 38kHZ wave, then I could just upload the new software with an ICSP if I wanted to include interrupts

 

[Nigel Goodwin]

What would be the frequency of the interrupts? I think it would be easier just to use the PWM software to easily generate the 38kHZ wave, then I could just upload the new software with an ICSP if I wanted to include interrupts

Check the webpage I mentioned, it just uses an 8 pin PIC and makes it trivial to detect objects in front and to the left and right.

 

[blueroomelectronics]

I take it the PIC is the Junebugs 18F1320? You don't have to get 38KHz spot on, 36-40KHz will also work. The pin with PWM is RB3 (PORTB.3)
Assuming you're using the 8MHz internal osc then this is what you need.
from this site https://www.micro-examples.com/public/microex-navig/doc/097-pwm-calculator.html

/*
 * PWM registers configuration
 * Fosc = 8000000 Hz
 * Fpwm = 37735.85 Hz (Requested : 38000 Hz)
 * Duty Cycle = 50 %
 * Resolution is 7 bits
 * Prescaler is 1
 * Ensure that your PWM pin is configured as digital output
 * see more details on http://www.micro-examples.com/
 * this source code is provided 'as is',
 * use it at your own risks
 */
PR2 = 0b00110100 ;
T2CON = 0b00000100 ;
CCPR1L = 0b00011010 ;
CCP1CON = 0b00011100 ;

 

[Krumlink]

Its all in assembly! I cant read assembly! I just want to know a link our something that will show how to create a PWM 38kHZ wave with a 18F1320, 18F1330, a 18F4620 or a 18F2550?

EDIT: ok Bill that will work great, this was posted right after you so i never saw it

So this creates a 38kHZ wave? Thanks!

 

[blueroomelectronics]

Yep it's actually 37.7KHz buts that's close enough. Same code will work with any PIC with a PWM, just look for the CCP1 pin on the datasheet.

 

[August Treubig]

Have it in C at home

I have code using PWM to do it in C. But it is at home. Will post it later.

 

[Krumlink]

Here is the code i was talking about:

//This project is for producing a 38kHZ wave for a infrared detecting robot


#pragma config WDT = OFF
#pragma config OSC = INTIO2 
#pragma config PWRT = ON
#pragma config LVP = OFF

#include <p18f1330.h>

void main(void)
{
    
    // speed up the clock to 8MHz
    OSCCONbits.IRCF0=1; OSCCONbits.IRCF1=1;
    OSCCONbits.IRCF1=1;
    OSCCONbits.IRCF2=1;
    OSCCONbits.SCS0=0;
    OSCCONbits.SCS1=0;
    //18F1330 Speed clock up to 32MHz using PLL
    OSCTUNEbits.PLLEN=1;
    ADCON1 = 0; // make RA0 digital
    TRISA = 0xFE;
    while(1)
        {
        PORTA = 0xFF;
        delay_us(13);
        PORTA = 0x00;    
        delay_us(13);
    }    
}

It diddnt produce the 38kHZ wave, but I am thinking of just using bills submitted code with a 18F4620 for xtreme overkill

 

[3v0]

Step through the code with the debugger and MPLABSIM. You can look at the output on the logic analyizer. It will even give you the timing so you can figure out the frequency.

 

[August Treubig]

That isnt PWM

To do true PWM, you let the hardware do it.

The delay functions are not that accurate. I got it going with software with nops, but what I have is all hardware. It is for a 16F628A at 4 mhz. But I know it is right. I have viewed it on my USB scope and it turns on and off my TV.

This is written in Boost C.

#include <system.h>

//Target PIC16F628A configuration word

#pragma DATA _CONFIG, _CP_OFF & _LVP_OFF & _BODEN_OFF & _MCLRE_ON & _PWRTE_ON & _WDT_OFF & _INTOSC_OSC_NOCLKOUT

//Set clock frequency
#pragma CLOCK_FREQ    4000000


// Period = CCP1Period * PreScaler * clock
//  3.2 ms = 200 * 16 *1

// Duty Cycle
//  Period * Duty% / 4 * Osc
//   3.2 * .5 = 1.6  
//     1600 / 4 = 400

// 3.2 ms
/*
#define CCP1Period    199
#define    CCP1DutyCy    399
*/
// 1.6 ms
/*
#define CCP1Period    99
#define    CCP1DutyCy    199
*/
// 1.25 ms  = 800 hz
/*
#define CCP1Period    77
#define    CCP1DutyCy    155
*/
// Set Prescaler to 1:1
// 32 khz
/*
#define CCP1Period    30
#define    CCP1DutyCy    63
*/
// 38 khz

#define CCP1Period    25
#define    CCP1DutyCy    51

/*
// 40 khz
#define    CCP1Period    24
#define    CCP1DutyCy    49
*/



bit        running = 0;

unsigned char    turnon[6] = { 0x40, 4, 1, 0, 0xbc, 0xbd};

void interrupt( void )
{
    // Handle timer1 interrupt
    // Used to time the length of pulses
    if( pir1.TMR1IF )
    {
        running = 0;
        t1con.TMR1ON = 0;    // Turn TIMER1 off
        pir1.TMR1IF = 0; //clear timer 1 interrupt bit
    }
}

void xmit_pulse(short micro_pulse)
{
    static short    clock_count;
    clock_count = 65536 - micro_pulse;
    LOBYTE( tmr1l, clock_count);
    HIBYTE( tmr1h, clock_count);
    pir1.TMR1IF = 0;        // Reset overflow bit
    t1con.TMR1ON = 1;    // Turn TIMER1 on
    running = 1;

    trisb.3 = 0;
    t2con.TMR2ON = 1;
    while (running) ;        // Wait for pulse time to end    
    t2con.TMR2ON = 0;
    tmr2 = 0;
    portb.3 = 0;
    trisb.3 = 1;
}

void send_on(void) {
    static char    i;
    static char    j;
    static char    t;
    static unsigned char    *ovalue;
    static unsigned char    k;

//    Send it 3 times
    for (k = 0; k < 3; k++) {
        ovalue = &turnon;
        xmit_pulse(3550);    // 4 ms header
        delay_100us(10);    // 1 ms
        delay_10us(6);        // .6 ms
        for (i = 0; i < 6; i++) {
            for (j = 8; j > 0; j--) {
                t = j - 1;
                if (test_bit(*ovalue, t)) {
//                    A one
                    xmit_pulse(512);        // 400 micro s 
                    delay_100us(10);        // 1 ms
                    delay_10us(2);            // .2 ms
                }
                else {
//                    A zero
                    xmit_pulse(512);        // 400 micro s
                    delay_100us(3);            // 400 micro s
                }
            }
            ovalue++;
        }
//        End bit
        xmit_pulse(400);
        delay_ms(70);
    }
}
void main( void )
{
    static unsigned char    Low;
    static unsigned char    High;
    static unsigned int        Duty = CCP1DutyCy;
    ccp1con = 0;        // CCP Module off
    tmr2 = 0;            // Clear Timer 2
    //Configure port B
    trisb = 0x00;
    trisb.3 = 1;
    option_reg.NOT_RBPU = 0;
    //Initialize port B
    portb = 0;
    cmcon = 7; //disable comparators

    //Set timer 1
    //prescaler rate 1:1
    //Internal clock (FOSC/4)
    t1con = 00001000b;

    // Configure Capture/Compare/PWM for PWM
    // at 38khz.  
    //Set timer 2 prescaler rate
    // T2CPS = 1:1
    t2con = 01001000b;
    //t2con = 0;
    pr2 = CCP1Period;
    Low = Duty & 3;
    High = Duty >> 2;
    ccpr1l = High;
    ccp1con = 00001100b;
    ccp1con.5 = Low.1;
    ccp1con.4 = Low.0;

    intcon.PEIE = 1;        // Enable peripheral interrupts
    intcon.GIE = 1;         // Enable global interrupt
    pie1.TMR1IE = 1;        // Enable Timer 1 interrupts
    
//    Main Loop
    send_on();
    while (1) {
    }
}

Remember that this is running at 4 mhz.

It takes one instruction to turn it on and one to turn it off.

  trisb.3 = 0;
    t2con.TMR2ON = 1;    // Turn 38khz on
    while (running) ;        // Wait for pulse time to end    
    t2con.TMR2ON = 0;   //  Turn 38khz off
    tmr2 = 0;

Use the hardware..

There are commented out definitions for 40 khz and other odds and ends.

 

[August Treubig]

Software at 4mhz

This is what it looks like with a nop-loop.

void xmit_pulse(short micro_pulse)
{
    static short    clock_count;
    clock_count = 65536 - micro_pulse;
    LOBYTE( tmr1l, clock_count);
    HIBYTE( tmr1h, clock_count);
    pir1.TMR1IF = 0;        // Reset overflow bit
    t1con.TMR1ON = 1;    // Turn TIMER1 on
    running = 1;
    while (running) {
         asm {
            bsf        _irout,1
            nop        // 1
            nop        // 2
            nop        // 3
            nop        // 4
            nop        // 5
            nop        // 6
            nop        // 7
            nop        // 8
            nop        // 9
            nop        // 10
            nop        // 11
            nop        // 12
            bcf        _irout,1
            nop        // 1
            nop        // 2
            nop        // 3
            nop        // 4
            nop        // 5
            nop        // 6
            nop        // 7
            nop        // 8
        }
    }
    irout = 0;
}

Again at 4mhz on a 628A. But looked at in the scope and Boost C.

 

[August Treubig]

Here are the numbers for 18F1320 at 8 mhz

These are the goodies to set up for 38khz

// 38 khz

#define CCP1Period    12
#define    CCP1DutyCy    25

In the main init routine

    // Configure Capture/Compare/PWM for PWM
    // at 38khz.  
    //Set timer 2 prescaler rate
    // T2CPS = 1:4
    t2con = 00000001b;
    pr2 = CCP1Period;
    Low = Duty & 3;
    High = Duty >> 2;
    ccpr1l = High;
    ccp1con = 00001100b;
    ccp1con.5 = Low.1;
    ccp1con.4 = Low.0;

And to turn it on permanent like for a test
// test    
    trisb.3 = 0;
    t2con.TMR2ON = 1;
    while (1) ;

Here is the wave form:

https://www.electro-tech-online.com/attachments/capture4-7-2008-8-22-38-pm-jpg.18506/

 

[3v0]

This is all nice but for now we will be useing software generated PWM. Walk first then run.

 

[3v0]

To do true PWM, you let the hardware do it.

Why are you saying that PWN generated with software delays is not true PWM. Sort of like saying that bit bang rs232 in not rs232.

 

[August Treubig]

Actually much harder

It is actually much harder to get the software loop de loop correct. I have done it both ways as you saw. I would be willing to bet that my count of nops would be different on a different C compiler.

Whereas the hardware PWM will be consistent. If you notice that the off section has fewer nops because of the time doing the while calculations.


For beginner teaching, (which I understand is what you are doing), the loops may be easier to explain, but very hard to keep straight.

You really cannot use delay_us() calls because of the overhead of the call and expect easy results.

Of course, I cheated. I stuck the Parallax scope on the thing and made it right. If you call that cheating. Nearly 40 years of doing this stuff tells me you use whatever weapons you have at your disposal.




Bit banging. Yea, that is sort of it.

 

[Krumlink]

We are using instruction cycles as a delay.