Frequency counter

[Mike, K8LH]

Hi Nick,

Using the PIC hardware instead of a "divide by 1000" external prescaler would allow for more frequent LCD updates. Would that be an advantage for your application?

Here's one way I thought about implementing a frequency counter in a HLL (BoostC in this case). I might use periodic 1-msec timer 2 interrupts to check for TMR0 register overflow and gate the counter on or off every 200 msecs by toggling the T0CKI data direction pin. Timer 0 would alternate between counting for 200-msecs or being idle for 200-msecs.

Extracting the 8-bit timer 0 prescaler value and adding it to the 24 bit result is tricky. After T0CKI has been set to an output (gate 'off') we pulse the timer 0 edge select bit (in option_reg) which bumps the prescaler and we decrement the CountL byte until the prescaler overflows into the TMR0 register.

Maybe something like this;

;
;  unsigned char msctr = 200;   //
;  unsigned char CountL = 0;    //
;  unsigned char CountH = 0;    //
;  unsigned char CountU = 0;    //
;  unsigned long result = 0;    //
;
;  void interrupt()             // 1-msec TMR2 interrupts
;  { pir1.TMR2IF = 0;           // clear TMR2 interrupt flag
;    msctr--;                   // decrement 200-msec timer
;    if(msctr == 0)             // if end of 200-msec interval
;    { trisio.0 ^= 1;           // toggle GP2/T0CKI direction
;      msctr = 200;             // reset 200-msec timer
;    }
;    if(intcon.T0IF)            // if TMR0 overflow
;    { CountU++;                // bump b23..b16 byte
;      intcon.T0IF = 0;         // clear the overflow flag
;    }                          //
;  }
;
;  void main()                  //
;  { init();                    // initialize
;    while(1)                   // main.loop
;    { while(trisio.2 == 1);    // while 200-msec gate "on"
;      CountH = tmr0;           // result b15..b8 bits
;      do                       // extract the prescaler bits
;      { option_reg.T0SE = 1;   // pulse TMR0 edge select bit
;        option_reg.T0SE = 0;   // to increment the prescaler
;        CountL--;              // adjust b7..b0 result bits
;      } while(tmr0 == CountH); // until overflow into TMR0
;      result = CountU * 65536; // build 24-bit result
;      result += CountH * 256;  //
;      result += CountL;        //
;      PutResult();             // put result on LCD
;      CountU = 0;              // prep for next count cycle
;      CountH = 0;              //
;      CountL = 0;              //
;      while(trisio.2 == 0);    // while 200-msec gate "off"
;    }                          //
;  }
;