Code for SHT21

Hello all,

I have been using SHT11 and Tracy Allen's code for some time. I recently got a SHT21 and found the conversion does not work.

I found this on the Basic Stamp Forum:

Quote:

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Just to compare in red and blue, the formulae for the SHT2x:
°C = -46.85 + 175.72 * rawT / (2^resT) resT is 14 bits default
%RH = -6 + 125 * rawH / (2^resH) resH is 12 bits default
res = resolution, tradeoff with speed, set in the user register.

with formulae for SHT1x:
°C = -40.1 + 0.01 * rawT on 14 bit setting and 5V supply
%RH1 = -2.0468 + 0.0367 * rawH - 1.5955E-6 * rawH^2 on 12 bit setting
then temperature compensation
%RH = (°C - 25) * (.01 + 0.00008 * rawH) + RH1
The SHT1x too has both high and low resolution modes.

---

Does anyone else use a SHT21 and what does the code actually look like?

Thanks in advance

Mike2545

Not familiar with the SHT11, but at a glance I can say the PIC will not work with decimals or negatives. However. if you wanted to add -46.85 + 175.72 it could be structured

(17572 - 4685) / 100

Multiplication works similarly; Var * 1.75 becomes (Var * 175) / 100.

Something tells me I'm not even getting close to the jist of your question. If you can elaborate some, I'll see if I can help.

mpgmike,
Thanks for the response. I'll link the code for the sht11 on the parallax site and the data sheet for the sht21.
https://www.parallax.com/sites/defau...ample-Code.zip
http://www.mouser.com/ds/2/682/Sensi..._V4-469715.pdf

If you don't have the BS2 editor here is the code:

Code:

---

' ==============================================================================
'
'  File...... SHT11.BS2
'  Purpose... Interface to Sensirion SHT11 temperature/humidity sensor
'  E-mail.... [email protected]
'  Date...... 12/30/02
'
'  {$STAMP BS2}
'
' ==============================================================================


' ------------------------------------------------------------------------------
' Program Description
' ------------------------------------------------------------------------------
'
' This program demonstrates the interface and conversion of SHT11 data to
' usable program values.
'
' For detailed information on the use and application of the ** operator,
' see Tracy Allen's web page at this link:
'
' -- http://www.emesystems.com/BS2math1.htm
'
' For Tracy's SHT1x code [very advanced]:
'
' -- http://www.emesystems.com/OL2sht1x.htm
'
' For SHT11/15 documentation and app notes, visit:
'
' -- http://www.sensirion.com


' ------------------------------------------------------------------------------
' I/O Definitions
' ------------------------------------------------------------------------------

ShtData        CON    1                      ' bi-directional data
Clock          CON    0


' ------------------------------------------------------------------------------
' Constants
' ------------------------------------------------------------------------------

ShtTemp        CON    %00011                  ' read temperature
ShtHumi        CON    %00101                  ' read humidity
ShtStatW        CON    %00110                  ' status register write
ShtStatR        CON    %00111                  ' status register read
ShtReset        CON    %11110                  ' soft reset (wait 11 ms after)

Ack            CON    0
NoAck          CON    1

No              CON    0
Yes            CON    1

MoveTo          CON    2                      ' for DEBUG control
ClrRt          CON    11                      ' clear DEBUG line to right

DegSym          CON    186                    ' degrees symbol for DEBUG


' ------------------------------------------------------------------------------
' Variables
' ------------------------------------------------------------------------------

ioByte          VAR    Byte                    ' data from/to SHT1x
ackBit          VAR    Bit                    ' ack/nak from/to SHT1x
toDelay        VAR    Byte                    ' timeout delay timer
timeOut        VAR    Bit                    ' timeout status

soT            VAR    Word                    ' temp counts from SHT1x
tC              VAR    Word                    ' temp - celcius
tF              VAR    Word                    ' temp - fahrenheit

soRH            VAR    Word                    ' humidity counts from SHT1x
rhLin          VAR    Word                    ' humidity; linearized
rhTrue          VAR    Word                    ' humidity; temp compensated

status          VAR    Byte                    ' SHT1x status byte


' ------------------------------------------------------------------------------
' EEPROM Data
' ------------------------------------------------------------------------------


' ------------------------------------------------------------------------------
' Initialization
' ------------------------------------------------------------------------------

Initialize:
  GOSUB SHT_Connection_Reset                    ' reset device connection

  PAUSE 250                                    ' let DEBUG window open
  DEBUG CLS
  DEBUG "SHT1x Demo", CR
  DEBUG "----------", CR

  ' GOTO Main                                  ' skip heater demo

' ------------------------------------------------------------------------------
' Program Code
' ------------------------------------------------------------------------------

Sensor_Demo:
  GOSUB SHT_Measure_Temp
  DEBUG MoveTo, 0, 3
  DEBUG "tF...... "
  DEBUG DEC (tF / 10), ".", DEC1 tF, DegSym, ClrRt, CR

  GOSUB SHT_Measure_Humidity
  DEBUG "rhLin... "
  DEBUG DEC (rhLin / 10), ".", DEC1 rhLin, "%", ClrRt, CR, CR

Heater_On:
  DEBUG "SHT1x heater on", CR
  status = %00000100                            ' heater bit = On
  GOSUB SHT_Write_Status
  DEBUG "Waiting 2 seconds", CR
  PAUSE 2000

Heater_Off:
  DEBUG "SHT1x heater off", CR, CR
  status = %00000000                            ' heater bit = Off
  GOSUB SHT_Write_Status

  GOSUB SHT_Measure_Temp
  DEBUG "tF...... "
  DEBUG DEC (tF / 10), ".", DEC1 tF, DegSym, ClrRt, CR

  GOSUB SHT_Measure_Humidity
  DEBUG "rhLin... "
  DEBUG DEC (rhLin / 10), ".", DEC1 rhLin, "%", ClrRt, CR, CR

  PAUSE 5000

Main:
  DEBUG CLS
  DEBUG "SHT1x Demo", CR
  DEBUG "----------", CR

Main2:
  GOSUB SHT_Measure_Temp
  DEBUG MoveTo, 0, 3
  DEBUG "soT...... "
  DEBUG DEC soT, ClrRt, CR
  DEBUG "tC....... "
  DEBUG DEC (tC / 10), ".", DEC1 tC, DegSym, ClrRt, CR
  DEBUG "tF....... "
  DEBUG DEC (tF / 10), ".", DEC1 tF, DegSym, ClrRt

  GOSUB SHT_Measure_Humidity
  DEBUG MoveTo, 0, 7
  DEBUG "soRH..... "
  DEBUG DEC soRH, ClrRt, CR
  DEBUG "rhLin.... "
  DEBUG DEC (rhLin / 10), ".", DEC1 rhLin, "%", ClrRt, CR
  DEBUG "rhTrue... "
  DEBUG DEC (rhTrue / 10), ".", DEC1 rhTrue, "%", ClrRt

  PAUSE 1000                                    ' minimum delay between readings
  GOTO Main2

  END


' ------------------------------------------------------------------------------
' Subroutines
' ------------------------------------------------------------------------------

' connection reset: 9 clock cyles with ShtData high, then start sequence
'
SHT_Connection_Reset:
  SHIFTOUT ShtData, Clock, LSBFIRST, [$FFF\9]

' generates SHT1x "start" sequence
'          _____        _____
' ShtData      |_______|
'              ___    ___
' Clock    ___|  |___|  |___
'
SHT_Start:
  INPUT ShtData                                ' let pull-up take line high
  LOW Clock
  HIGH Clock
  LOW ShtData
  LOW Clock
  HIGH Clock
  INPUT ShtData
  LOW Clock
  RETURN


' measure temperature
' -- celcius = soT * 0.01 - 40
' -- fahrenheit = soT * 0.018 - 40
'
SHT_Measure_Temp:
  GOSUB SHT_Start                              ' alert device
  ioByte = ShtTemp                              ' temperature command
  GOSUB SHT_Write_Byte                          ' send command
  GOSUB SHT_Wait                                ' wait until measurement done
  ackBit = Ack                                  ' another read follows
  GOSUB SHT_Read_Byte                          ' get MSB
  soT.HIGHBYTE = ioByte
  ackBit = NoAck                                ' last read
  GOSUB SHT_Read_Byte                          ' get LSB
  soT.LOWBYTE = ioByte

  ' Note: Conversion factors are multiplied by 10 to return the
  '      temperature values in tenths of degrees

  tC = soT / 10 - 400                          ' convert to tenths C
  tF = soT ** 11796 - 400                      ' convert to tenths F
  RETURN


' measure humidity
'
SHT_Measure_Humidity:
  GOSUB SHT_Start                              ' alert device
  ioByte = ShtHumi                              ' humidity command
  GOSUB SHT_Write_Byte                          ' send command
  GOSUB SHT_Wait                                ' wait until measurement done
  ackBit = Ack                                  ' another read follows
  GOSUB SHT_Read_Byte                          ' get MSB
  soRH.HIGHBYTE = ioByte
  ackBit = NoAck                                ' last read
  GOSUB SHT_Read_Byte                          ' get LSB
  soRH.LOWBYTE = ioByte

  ' linearize humidity
  '  rhLin = (soRH * 0.0405) - (soRH^2 * 0.0000028) - 4
  '
  ' for the BASIC Stamp:
  '  rhLin = (soRH * 0.0405) - (soRH * 0.004 * soRH * 0.0007) - 4
  '
  ' Conversion factors are multiplied by 10 and then rounded to
  ' return tenths
  '
  rhLin = (soRH ** 26542)
  rhLin = rhLin - ((soRH ** 3468) * (soRH ** 3468) + 50 / 100)
  rhLin = rhLin - 40

  ' temperature compensated humidity
  '  rhTrue = (tC - 25) * (soRH * 0.00008 + 0.01) + rhLin
  '
  ' Conversion factors are multiplied by 100 to improve accuracy and then
  ' rounded off.
  '
  rhTrue = ((tC / 10 - 25) * (soRH ** 524 + 1) + (rhLin * 10)) + 5 / 10
  RETURN


' sends "status"
'
SHT_Write_Status:
  GOSUB SHT_Start                              ' alert device
  ioByte = ShtStatW                            ' write to status reg command
  GOSUB SHT_Write_Byte                          ' send command
  ioByte = status
  GOSUB SHT_Write_Byte
  RETURN


' returns "status"
'
SHT_Read_Status:
  GOSUB SHT_Start                              ' alert device
  ioByte = ShtStatW                            ' write to status reg command
  GOSUB SHT_Read_Byte                          ' send command
  ackBit = NoAck                                ' only one byte to read
  GOSUB SHT_Read_Byte
  RETURN


' sends "ioByte"
' returns "ackBit"
'
SHT_Write_Byte:
  SHIFTOUT ShtData, Clock, MSBFIRST, [ioByte]  ' send byte
  SHIFTIN  ShtData, Clock, LSBPRE, [ackBit\1]  ' get ack bit
  RETURN


' returns "ioByte"
' sends "ackBit"
'
SHT_Read_Byte:
  SHIFTIN  ShtData, Clock, MSBPRE, [ioByte]    ' get byte
  SHIFTOUT ShtData, Clock, LSBFIRST, [ackBit\1] ' send ack bit
  INPUT ShtData                                ' release data line
  RETURN


' wait for device to finish measurement (pulls data line low)
' -- timeout after ~1/4 second
'
SHT_Wait:
  INPUT ShtData                                ' data line is input
  FOR toDelay = 1 TO 250                        ' give ~1/4 second to finish
    timeOut = INS.LOWBIT(ShtData)              ' scan data line
    IF (timeOut = No) THEN SHT_Wait_Done        ' if low, we're done
    PAUSE 1
  NEXT

SHT_Wait_Done:
  RETURN


' reset SHT1x with soft reset
'
SHT_Soft_Reset:
  GOSUB SHT_Connection_Reset                    ' reset the connection
  ioByte = ShtReset                            ' reset command
  ackBit = NoAck                                ' only one byte to send
  GOSUB SHT_Write_Byte                          ' send it
  PAUSE 11                                      ' wait at least 11 ms
  RETURN

---

What I am trying to do is, write code to read the SHT21 temperature and humidity.

mpgmike, I got the sensor to respond to my finger temp difference. Here is the code so far:

Yes, yes, I am prototyping on a Stamp :/

Code:

---

' {$STAMP BS2p}
' {$PBASIC 2.0}

  tf    VAR Byte
  tc    VAR Byte
  rawT  VAR Word
  rawRH VAR Byte

  PAUSE 250                                    ' let DEBUG window open
  DEBUG CLS
  DEBUG "SHT21 Demo", CR
  DEBUG "----------", CR

Main:

I2COUT 0 ,%10000000, [%11100011]
  PAUSE 50
I2CIN  1 , %10000001, [rawT]
  DEBUG 2, 0, 3
  DEBUG "rawT...... "
  DEBUG  DEC rawT, 11, CR
  rawT= rawT <<2
  tC = (rawT & $fffc) ** 17572 - 4685
  DEBUG "Deg C......", DEC tC
  PAUSE 500


  GOTO main

---

Getting elevated readings on the SHT21
The SHT21 temp says my room is 42°C (107.6°F) Actual temperature is ~ 23.8°C (75°F)
rawT @ 42°C = 111

don't know much about stamp code but here goes

from the sensiron web example

Quote:

---

//================================================== ============================
float SHT2x_CalcTemperatureC(u16t u16sT)
//================================================== ============================
{
ft temperatureC; // variable for result
u16sT &= ~0x0003; // clear bits [1..0] (status bits)
//-- calculate temperature [°C] --
temperatureC= -46.85 + 175.72/65536 *(ft)u16sT; //T= -46.85 + 175.72 * ST/2^16
return temperatureC;
}

---

the reading is not shifted <<2 places the two lsb are just masked off

so
rawT= rawT <<2
becomes
rawT= rawT & ~3 ;ie %1111111111111100

Quote:

---

I2COUT 0 ,%10000000, [%11100011]

---

is hold master mode is that what you need ?

try
I2COUT 0 ,%10000000, [%11110011]

might be better

Quote:

---

I2COUT 0 ,%10000000, [%11100011]
PAUSE 50 is this long enough at max resoloution ?
I2CIN 1 , %10000001, [rawT]

---

Quote:

---

rawT @ 42°C = 111

---

by my calcs rawT should be 33132 @ 42°C

what rawT do you actually get ?