120khz square wave generation

[mischa]

Hey. Here is my code. Please let me know if this code is right. I have decided to use a 12Mhz crystal instead of the 7.805Mhz.

Just to recap:
I want to generate a 120Khz square wave that will last for 1ms (done by gating the RC2 pin) on RC2 when a switch connected to portD.5 is pressed.

Using the formulaes:
PWM period= [((PR2)+1)*4*Tosc*TMR2 prescalar value] and
PWM duty cycle = (CCPR1L:CCP1CON<5:4>)*Tosc*TMR2 prescalar value,

I decided to use the prescale value =1, this then gave me the following values:
PR2= 24 (0b00011000)
CCPR1L:CCP1CON<5:4> = 50 (0b0000110010)-{10 bit value}
Since CCP1CON<5:4> is the 2 LSb value = "10"--> CCP1CON="0b00101100"
therefore: CCPR1L=8 Msb="0b00001100"

CODE
------------------------------------------------------------------------------------

#include "16F877.h"
#include "DEFS_877.H"

#include "string.h"
#include <stdlib.h>
#include <stdio.h>

#use delay (clock=12000000)
#fuses HS,NOWDT,PROTECT,NOLVP,BROWNOUT,PUT
#use rs232(baud=56000,parity=N,xmit=PIN_C6,rcv=PIN_C7)

set_tris_d(0xff); //setting port d as input

void squarewave (void)
{

PR2=0b00011000; //set PWM period to 8.3333us
CCPR1L=0b00001100; //sets Duty cycle to 50%
CCP1CON=0b00101100; //Dc=50%
T2CON=0b00000100; //enables timer 2 and sets the prescalar value
set_tris_C(0x00); //output pwm to pin RC2
}


void main (void)
{
//just initailises and clears port c initially
set_tris_c(0xFF); //set port c as input
PortC=0; //clear portC

while(1)
{
if (!input(PIN_D5)) //if switch is closed/pressed
{
squarewave();
delay_ms(1); //wave on RC2 for a period of 1ms
set_tris_c(0xFF); //sets RC2 as input, so no more
//Pwm present on this pin
}
}
}
-----------------------------------------------------------------------------------

Please take a look at my code and let me know if its accurate and correct. Thanks a lot

 

[Pommie]

Re your PM. I tried the above code in BoostC and it appears to work as intended. There was some problems but I think that might be just compiler differences. However, the line,

set_tris_d(0xff); //setting port d as input

will never get executed.

Mike.

 

[mischa]

Why would that line not be executed?

 

[Pommie]

Because execution always starts at main. I'm surprised the complier doesn't error with that line.

Mike.