EX_FREQC.C
---------------------------------------------------------------------------
/////////////////////////////////////////////////////////////////////////
//// EX_FREQC.C ////
//// Counts the 'zero' crossings of an input signal for 1 second, ////
//// therefore counting the frequency of the signal. The input ////
//// signal's frequency is then displayed over the serial port. ////
//// ////
//// The 1s delay is created by generating a perfect 5,000,000 ////
//// cycles (it takes 0.2us for one cycle at 20Mhz). The trick is ////
//// that conditional statements have different values of cycles ////
//// depending on a TRUE or FALSE jump. ////
//// ////
//// The input signal is to be supplied to PIN C0 ////
//// ////
//// This example will work with the PCB and PCM compilers. The ////
//// following conditional compilation lines are used to include a ////
//// valid device for each compiler. Change the device and clock ////
//// for your hardware if needed. ////
/////////////////////////////////////////////////////////////////////////
//// (C) Copyright 1996,2001 Custom Computer Services ////
//// This source code may only be used by licensed users of the CCS ////
//// C compiler. This source code may only be distributed to other ////
//// licensed users of the CCS C compiler. No other use, ////
//// reproduction or distribution is permitted without written ////
//// permission. Derivative programs created using this software ////
//// in object code form are not restricted in any way. ////
/////////////////////////////////////////////////////////////////////////
#include <16f877.h>
#fuses HS,NOWDT
#use delay(clock=20000000) //one instruction=0.2us
#use rs232(baud=9600, xmit=PIN_c6, rcv=PIN_c7)
#bit t1_overflow=0x0C.0
// #bit t1_overflow=0xF9E.0 (PIC18, Reminder)
void main() {
int cycles8, cycles;
int32 freq;
long freqc_high;
long freqc_low;
while (TRUE) {
cycles8=0;
cycles=0;
freqc_high=0;
t1_overflow=0;
set_timer1(0);
setup_timer_1(T1_EXTERNAL|T1_DIV_BY_1);
/* ___ wait one second ___ */
while (cycles!=0xFF) { //true=3, false=4
cycles8=0; //1 cycle
//start inner loop
while (cycles8!=0xFF) { //true=3, false=4
if (t1_overflow)//true=2,false=3 //----|
{t1_overflow=0;freqc_high++;}//6 cycles // |
else // |-- 8 cycles
{delay_cycles(5);} //----|
delay_cycles(62); //x
cycles8++; //1
///2 cycles to jump to top
//math: end inner loop
//math: total inner loop=((3+8+x+1+2)*255 + 4)*255
//math: if x=62.87781 then inner loops takes 5mil instructions
//math: if x=62 then inner loop takes 4942920, have to fill 57080 cycles
}
delay_cycles(216); //y
cycles++; ///1 cycle
///2 cylces to jump to top
//math: outer=(3+1+y+1+2)*255+4=57080
//math: y=(57080-4)/255)-(3+1+0+0+1+2)
//math: if y=216.827450980392156862745098039216 then outer loop cylces is 57080
//math: if y=216 then outer loop cycles is off by 211 cycles. z=211
}
delay_cycles(211); //z
/* ___ end waiting 1 second ___ */
setup_timer_1(T1_DISABLED); //turn of counter to prevent corruption while grabbing value
if (t1_overflow) //check one last time for overflow
freqc_high++;
freqc_low=get_timer1(); //get timer1 value as the least sign. 16bits of freq counter
freq=make32(freqc_high,freqc_low); //use new make32 function to join lsb and msb
printf("%LU Hz\r\n",freq); //and print frequency
}
}
---------------------------------------------------------------------------