smt160.c

Example code for 16F819(8xx), which measures temperature via SMT160-30, shows it on LCD and communicates with RS232 (sends values of temperature to terminal) (by Petr Mervart).


// Declaration file for temperatuere measuring via SMT 160/30 sensor
// Major rutine writed in ASM to minimalize measuring errors 
//
// This idea to measure temp. is a litlle bit different from clasic one:
// we don't measure a real DUTY CYCLE (DC) and time, when is log1,
// but we are reading sensor (and count if is in log1 -> stridaL) after the same 
// period and for the same time (aprox 108ms) then temperature is linear dependence
// on DC and result is in stridaH var.
// The temp. resolution is 0.5C (if stridaL=256 => stridaH=1 => 0.5C)
//
// This idea (and ASM rutine) is taken from Ondrej Vitous http://www.hw.cz/programovani/pic_temperature/vitous@hw.cz
// ( only  czech page)

#define PORT          PORTB        
#define PIN           2          // pin number where is sensor 
#define CIDLO           PORT,PIN
#define SMT160_INPUT   TRISB,PIN
#define MAX          80     
/* MAX [0-135] 
    Tmax = (MAX/2 + 60); ex. MAX=130  => Tmax=130/2 + 60=125 C
    Tmin = [(MAX+1)-136]/2  when MAX=130 then Tmin=-2,5C  
     => deltaT=127.5C, resolution 0.5C
       
    MAX         0      10      50      80     100    120     130  135
    Tmax [C]   60      65      85     100     110    120     125    127.5
    Tmin [C]  -67.5*  -62.5*  -42.5   -27.5   -17.5   -7.5    -2      0
       
       * - out of sensor range (-45C - +130C)
*/

char countL ;        //  counter lower byte 
char countH ;        //  counter higher byte 
char stridaL ;       //  log1 lower counter (if stridaL=256 => 0.5C)   
char stridaH ;        //  log1 higher counter 
char TEMP_J;              //  measured tempearture - tenths [C]
char TEMP_D;          //  measured tempearture - tens (hundreds) [C]
char minus ;            //  this is negative values flag, if is 1 we are under zero


void temperature()      // main rutine
{
  TEMP_D=0;
  TEMP_J=0;
asm
{
       MOVLW d'32'   ;  2 cycles
       MOVWF _countL
       MOVLW d'54'   ;    2
       MOVWF _countH

       CLRF _stridaL ;  1
       CLRF _stridaH ;  1

       BCF STATUS,Z  ;  1

repeatA     nop         ;  1  measuring loop (sampling time) - 13600x
repeatB

      BTFSC CIDLO       ;1/2
      INCF  _stridaL,1  ;1

      BTFSC STATUS,Z       ;1/2
      INCF  _stridaH,1    ;1

         DECFSZ _countL,1     ;3/2
      GOTO repeatA
      DECFSZ _countH,1     ;3/2
      GOTO repeatB

          ; 3x rotate meassured DC
       BCF STATUS,C
      RLF _stridaL,1
      RLF _stridaH,1
      RLF _stridaL,1
      RLF _stridaH,1
      RLF _stridaL,1
      RLF _stridaH,1
      ; RETURN
 }
     // and now convert temperature from DC to degrees (linear dependence)
     if (stridaH < MAX)     //60C - Tmax
    {
        minus=0;
         TEMP_D=stridaH>>1;
       TEMP_J=stridaH;
       TEMP_D=TEMP_D+60;
       if (TEMP_J % 2) TEMP_J=5;     // 0,5 C or 0.0C
       else TEMP_J=0;
    }
     else
     {
        if(stridaH>135)    //(Tmin - 60C)
       {
         minus=0;
           TEMP_D=(stridaH - 136);      // ex.TEMP_J=185-136 = 49C
         TEMP_J=TEMP_D;
         TEMP_D>>=1;                  // temp_d/2
         if (TEMP_J % 2) TEMP_J=5;    // 0,5 C
         else TEMP_J=0;
       }
        else              // (Tmin - 0C) ex. stridaH=130 => t=(136-130)/2=-3C
        {
           minus=1;
           TEMP_D=(136-stridaH);
         TEMP_J=TEMP_D;
         TEMP_D>>=1;
         if (TEMP_J % 2) TEMP_J=5;
         else TEMP_J=0;
      }
   }
}
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