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AN #116 - Reading the SHT11 Humidity sensor Print
Reading the SHT11 Humidity sensor

This application note was submitted by Stuart Leslie. I am very pleased with it since I tried to get the SHT11 working too but failed. After a few days I gave up on it and was ready to test it with a 8051 chip. The SHT11 does not follow the I2C specifications. Good for us that Stuart was so clever to use the SHIFT routines.

The SHT11 is a very good dual sensor, it is a temperature and humidity sensor. It is cheap when you consider the performance and there are free samples available from Sensirion.


 

You can download the sample BASCOM-AVR file.
It is also shown below.


'Routine to read the SHT11 Humidity sensor chip
'By Stuart Leslie
'Contact stu@4sightinc.com with any questions
'Uses BascomAVR
'a .01 uf capacitor across VCC and Ground on the SHT11 really cleans up the data
'a pullup is required on "data" pin as shown in the data sheet

Dim Ctr As Byte
Dim Dataword As Word
Dim Command As Byte
Dim Dis As String * 20

Dim Calc As Single
Dim Calc2 As Single
Dim Rhlinear As Single
Dim Rhlintemp As Single
Dim Tempc As Single
Dim Tempf As Single

Const C1 = -4
Const C2 = 0.0405
Const C3 = -0.0000028
Const T1c = .01
Const T2 = .00008
Const T1f = .018

Sck
Alias Portb.0
Dataout
Alias Portb.1
Datain
Alias Pinb.1
Redled
Alias Portb.2

Declare Sub Getit()

Ddrb = &B11111111  'all port b are output
Config Pinb.0 = Output 'sck
Config Pinb.1 = Output 'datain

'reset the serial communications first, it is easily confused!
Set Dataout
For Ctr = 1 To 12
 
Set Sck
 
Waitus 2
 
Reset Sck
 
Waitus 2
Next Ctr


Do 'continually read the tempfature and humidity

 Command
= &B00000011
 
Call Getit 'Get the temperature, puts result in "dataword" for us
 
'
 Tempf
= T1f * Dataword
 Tempf
= Tempf - 40

 Tempc
= T1c * Dataword 'get celcius for later calculations and for "the rest of the world"
 Tempc
= Tempc - 40

 Dis
= Fusing(tempf , "###.##")
 
Print "Temperature = " ; Dis ; " (F)"

 Command
= &B00000101
 
Call Getit 'get the humidity
 Calc
= C2 * Dataword
 Calc2
= Dataword * Dataword 'that "2" in the datasheet sure looked like a footnote for a couple days, nope it means "squared"!
 Calc2
= C3 * Calc2
 Calc
= Calc + C1
 Rhlinear
= Calc + Calc2

 
'Dis = Fusing(rhlinear , "##.##")
 
'Print "Humidity adjusted for linear = " ; Dis


 Calc
= T2 * Dataword
 Calc
= Calc + T1c
 Calc2
= Tempc - 25
 Calc
= Calc2 * Calc
 Rhlintemp
= Calc + Rhlinear

 Dis
= Fusing(rhlintemp , "##.##")
 
Print "Humidity adjusted for temperature = " ; Dis
 
Print

 
Wait 1
Loop


Sub Getit()

 
Local Datavalue As Word
 
Local Databyte As Byte

 
'start with "transmission start"
 
Set Sck
 
Reset Dataout
 
Reset Sck
 
Set Sck
 
Set Dataout
 
Reset Sck


 
'now send the command
 
Shiftout Dataout , Sck , Command , 1

 
Ddrb = &B11111101 'datain is now input
 
Config Pinb.1 = Input 'datain
 
Set Sck 'click one more off
 
Reset Sck
 
Waitus 10 'no idea why, but it doesn't work without it!
 
Bitwait Pinb.1 , Reset 'wait for the chip to have data ready

 
Shiftin Datain , Sck , Databyte , 1 'get the MSB
 Datavalue
= Databyte

 
Ddrb = &B11111111
 
Config Pinb.1 = Output

 
Reset Dataout 'this is the tricky part- Lot's of hair pulling- have to tick the ack!
 
Set Sck
 
Reset Sck

 
Ddrb = &B11111101 'datain is now input
 
Config Pinb.1 = Input

 
Shiftin Datain , Sck , Databyte , 1 'get the LSB
 
Shift Datavalue , Left , 8
 Datavalue
= Datavalue Or Databyte
 
'don't tick the clock or ack since we don't need the CRC value, leave it hanging!
 Dataword
= Datavalue

 
Ddrb = &B11111111
 
Config Pinb.1 = Output

 
Reset Dataout
 
Set Sck
 
Reset Sck

 
Ddrb = &B11111101 'datain is now input
 
Config Pinb.1 = Input

 
Shiftin Datain , Sck , Databyte , 1 'not using the CRC value for now- can't figure it out! Anybody know how to impliment?
 
'Print "CRC value was - " ; Databyte

 
Ddrb = &B11111111
 
Config Pinb.1 = Output

 
Set Dataout
 
Set Sck
 
Reset Sck
End Sub

End