youngjkoh
Newbie level 3
Hi, i'm new to the programming and I was trying to set up the a/d converter. I found this pwm example code from ccs examples, and I was expecting pinC7 as an analog input and the picC6 as a modulated output. However, when I checked the output on the oscilloscope it looked like it is not dependent on what input I put in C7! It looked like it goes high whenever Vcc is high... I don't understand.. Can anyone help me explaining it? Also, I was wondering how the timer works, and why we need it. Why is period=127? Thank you!
Code:
#if defined(__PCM__)
#include <16F877A.h>
#fuses HS,NOWDT,NOPROTECT,NOLVP
#use delay(clock=20000000)
#use rs232(baud=9600, xmit=PIN_C6, rcv=PIN_C7, BRGH1OK) // Jumpers: 8 to 11, 7 to 12
void main() {
char selection;
byte value;
printf("\r\nFrequency:\r\n");
printf(" 1) 19.5 khz\r\n");
printf(" 2) 4.9 khz\r\n");
printf(" 3) 1.2 khz\r\n");
do {
selection=getc();
} while((selection<'1')||(selection>'3'));
setup_ccp1(CCP_PWM); // Configure CCP1 as a PWM
// The cycle time will be (1/clock)*4*t2div*(period+1)
// In this program clock=20000000 and period=127 (below)
// For the three possible selections the cycle time is:
// (1/20000000)*4*1*128 = 25.6 us or 39.062 khz
// (1/20000000)*4*4*128 = 102.4 us or 9.765 khz
// (1/20000000)*4*16*128= 409.6 us or 2.441 khz
switch(selection) {
case '1' : setup_timer_2(T2_DIV_BY_1, 127, 1);
break;
case '2' : setup_timer_2(T2_DIV_BY_4, 127, 1);
break;
case '3' : setup_timer_2(T2_DIV_BY_16, 127, 1);
break;
}
setup_port_a(ALL_ANALOG);
setup_adc(adc_clock_internal);
set_adc_channel( 0 );
printf("%c\r\n",selection);
while( TRUE ) {
value=read_adc();
printf("%2X\r",value);
set_pwm1_duty(value); // This sets the time the pulse is
// high each cycle. We use the A/D
// input to make a easy demo.
// the high time will be:
// if value is LONG INT:
// value*(1/clock)*t2div
// if value is INT:
// value*4*(1/clock)*t2div
// for example a value of 30 and t2div
// of 1 the high time is 12us
// WARNING: A value too high or low will
// prevent the output from
// changing.
}
}