PIC16F628A 50MHz Frequency Meters Using Timer1 External Counting

CCS PICC have a ready to use example, demonstrating the technique of using timer 1 external pulses counting to make a 50 MHz frequency meter.

The ex_freq.c explained the the process of making frequency meter using timer 1 of PIC16F877 with a one second delay. The output data is presented in serial terminal.

We can use timer 1 of PIC16F628A to count external digital input pulses. With this advantage, we can make a simple frequency meters, measuring the number of pulses per second.

In this example, I switch the CPU to PIC16F628A with the same build-in timer 1. A 16x2 character LCD uses to present the frequency and period.

PIC16F628A supplied at +5V, clocks at 20 MHz.
Digital Clock pulse feds into pin RB6 Timer 1 Clock In.
SW1 hold the current frequency/period Value,
when shorted to ground

C Source code and simulation file could be download here:

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#include <16F628A.h>

#fuses HS,NOWDT

#use delay(clock=20000000)    

//one instruction=0.2us

#bit t1_overflow=0x0C.0

#define LCD_ENABLE_PIN  PIN_B2                                        

#define LCD_RS_PIN      PIN_B0                                        

#define LCD_RW_PIN      PIN_B1 

#define LCD_DATA4       PIN_B3                                       

#define LCD_DATA5       PIN_B4                                       

#define LCD_DATA6       PIN_B5                                        

#define LCD_DATA7       PIN_B7                                    

#include<LCD.C>

#define holdButton PIN_A0

void main() {

   int cycles8, cycles;

   long freq;

   long freqc_high;

   long freqc_low;

   set_tris_a(0x01);

   lcd_init();

   

   printf(LCD_PUTC,"Sampling!!!!!!");

   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

      lcd_gotoxy(1,1);

      printf(LCD_PUTC,"FREQ:");

      lcd_gotoxy(6,1);

      

      float period;

      if(freq==0){

         printf(LCD_PUTC,"Zero Signal");

         printf(LCD_PUTC,"\nT: -------------");

         

      }

      else if(freq<100000)  {

         printf(LCD_PUTC," %LU Hz      ",freq);

         period=1000.0/freq;

         printf(LCD_PUTC,"\nT: %.6f mS  ",period);

      }

      else if(freq<1000000){

         printf(LCD_PUTC," %3.3f kHz       ",(float)freq/1000.0);

         period=1000.0/freq;

         printf(LCD_PUTC,"\nT: %3.6f mS    ",period);

      }

      else{

         printf(LCD_PUTC," %3.3f MHz       ",(float)freq/1000000.0);

         period=1000000.0/freq;

         printf(LCD_PUTC,"\nT: %.6f uS   ",period);

      }

      

      while(input(holdButton)==0);

   }

}

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Simulation Result:

Simulation Result,
signal feed from a virtual signal generator.
The picture show a sampling signal of 8.299 kHz.

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