Sensirion SHT11 with a PIC16F628-20

[NavMicroSystems]

attached is a working piece of code for the 16F628

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' Temperature, Relative Humidity and Dewpoint
' Processor : 16F628
' Sensor    : SHT1x / SHT7x
' Display   : 16x2

'** Pin-Assingments

dta     VAR PORTB.1     ' Sensor Data
clk     VAR PORTB.0     ' Sensor Clock

'** Constants

cmdtr     CON %00000011    ' Command "read temperature"
cmdhr     CON %00000101    ' Command "read humitity"

'** Variables

result     VAR WORD    ' RAW Data from Sensor
chksum     VAR BYTE     ' Checksum
cmd     VAR WORD    ' Sensor Command
RHlin     VAR WORD    ' Rel. Humidity (RH) Linear (% *10)
RHtc    VAR WORD    ' Rel. Humidity (RH) temp. compensated (% *10)
Temp     VAR WORD    ' Temperature (°C *100)
DP     VAR WORD    ' Dewpiont (°C *10)


'** Temp variables used in div. calculations

TempDP  VAR WORD
Rchk     VAR BYTE 
logEW     VAR WORD
sign     VAR BIT
wy        VAR WORD
wz        VAR WORD
wj        VAR WORD
wx      VAR WORD
i     VAR BYTE
ix     VAR BYTE
bt     VAR BIT


Pause 500
LCDOut $FE,1,"**   SHT 11   **"
PAUSE 1000

main:

GoSub init
cmd = cmdtr

GoSub readsensor
Temp=result-3995

GoSub init
cmd = cmdhr

GoSub readsensor
RHlin=(26542-(54722**result+result))**result-40  ' RH linear
RHtc=655+(result*5)+(result**15917)                 
RHtc=(RHtc**(Temp/10+2480))-(RHtc**2730)+RHlin   ' RH temp. compensated

GoSub dewpoint

LCDOut $FE,$80," ",$DF,"C","   ",$25,"RH","    DP "
LCDOut $FE,$C0,DEC2 (TEMP/100),".",DEC1 (temp/10),"  ",DEC2 RHTC/10,".",DEC1 RHTC,"  ",DEC2 ABS DP/10,".",DEC1 ABS dp
sleep 10
GoTo main

'** Init Sensor
init:                                             
High dta 
Low clk 
For i=1 to 10 
High clk
Pause 1 
Low clk
Pause 1 
Next i 
Call tstart 
Return 

'** start transfer
tstart:
High clk
Pause 1 
Low dta 
Pause 1 
Low clk
Pause 1 
High clk
Pause 1 
High dta 
Pause 1 
Low clk
Return 

'** get Data from Sensor
readsensor:
GoSub tstart
GoSub WaitSensor

ShiftOut dta,clk,1,[cmd\8]                  ' send command
Input dta                      ' wait acknowledge 
Low clk
While dta=1 
Wend
PulsOut clk,10                      ' send ack 
While dta=0 
Wend
While dta=1                      ' wait for conversion to complete
Wend 
Low clk
ShiftIn dta,clk,0,[result.highbyte\8]          ' get first byte
Low dta 
PulsOut clk,10                      ' send ack 
ShiftIn dta,clk,0,[result.lowbyte\8]             ' get second byte
Low dta 
PulsOut clk,10                      ' send ack 
ShiftIn dta,clk,0,[chksum\8]                     ' get third byte (checksum)
High dta 
PulsOut clk,10                      ' send ack 
Input dta                      ' End of Transmission
Input clk                          

Return

'** Dewpoint Calculation
    ' See: SENSIRION Application Note "Dewpoint Calculation"
    ' logEW = (0.66077+7.5*T/(237.3+T)+(log(RH)-2)
      ' DP = ((0.66077-logEW)*237.3)/(logEW-8.16077)

DewPoint:
  
  TempDP=Temp/10
  logEW=6608
  sign=TempDP.bit15
  wz=ABS TempDP
  wx=(7*wz)+(wz/2)                              
  wy=2373+wz
  wj=wx/wy
   For ix=15 to 0 step -1
    wx=(wx//wy)<<1
    wz.bit0(ix)=wx/wy
   Next
  wx=(-sign^((wj*10000)+(wz**10000)))+sign        
  logEW=wx+logEW
  wx=RHtc
  wj=(NCD wx) - 1
  wx=wx<<(15-wj)
  wz=0
  For ix=14 to 0 step -1
    wy=wx**wx
    wz.bit0(ix)=wy.bit15
    bt=~wy.bit15
    wx=(wy<<bt)+(bt && wx.bit15)
   Next
  
  wx=((wj*4000)**49321)+(wz**6021)
  logEW=wx+logEW-30000            
  sign=logEW.bit15
  logEW=(-sign^(((ABS logEW)+2)/4))+sign
  wx=1652-logEW
  sign=~wx.bit15
  wx=ABS wx
  wy=20402-logEW
  wj=wx/wy
  
   For ix=15 to 0 step -1
    wx=(wx//wy)<<1
    wz.bit0(ix)=wx/wy
   Next
  
  DP=((23730**wz)+5)/10
  DP=(-sign^DP)+sign

Return

WaitSensor:
  result=4096
Loop:
  result=result-1
   IF dta && result.bit11 Then Loop
  Return
End
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Regards

Ralph