SHT_75 with picbasic pro

**iw2fvo** · - 27th November 2010, 16:40

Good day to all of you !

I am going to interface the temperature _ humidity sensor type SHT_75 fron Sensirion to the Pic micro.
I will like to know if there is a pic basic pro code to read this sensor and the routines to compute:
> temperature in C° having + and - signs
> temperature compensated RH ( + lin too !) in %
> dew point in C° having + and - signs
Thanks in advance for any assistance.
Best regards,

Ambrogio

**mackrackit** · - 28th November 2010, 00:09

Never used on, but a quick lppk at the data sheet makes me think I2C or SHIFTIN might work.
Here is a little about both for working with an EEPROM, maybe it will get you started?
http://www.picbasic.co.uk/forum/cont...-EEPROM-Part-1

**iw2fvo** · - 28th November 2010, 16:49

Thanks mackrackit:
I do not have any problem to talk with the SHT_75.
I am able to get signed temperature tto.
My problem is to compute RHtc with picbasic pro and to compute DEW POINT with its sign.
Thanks for help
Ambrogio
IW2FVO

Originally Posted by mackrackit

Never used on, but a quick lppk at the data sheet makes me think I2C or SHIFTIN might work.
Here is a little about both for working with an EEPROM, maybe it will get you started?
http://www.picbasic.co.uk/forum/cont...-EEPROM-Part-1

**iw2fvo** · - 28th November 2010, 14:36

Good day to all of you ,

I am going to interface the Sensirion SHT_75 Temperature_Humidity sensor to my pic micro.
I would like to know how to compute the RH termally compensate and how to compute the dew point. I rerad Srh @12 bit and St @14 bit.
I expect that the RHtc will range from 0 to 99% RH
The dew point temperature in C° will have the sign ( + or - ).

I thank you in advance for any assistance on the coding .

regards,
Ambrogio
IW2FVO

**ScaleRobotics** · - 28th November 2010, 14:57

A quick google search got me to here for some basic stamp code. It may help. You could also search sht75 on this site for some more hits.

http://www.emesystems.com/OL2sht1x.htm

Please don't double post. I moved your post to this thread, even though it is pretty much the same as the first post.

Code:

'{$STAMP BS2}
' SHT15.bs2 version C
' (c) 2002, 2004 Tracy Allen, http://www.emesystems.com
' access the Sensirion model sht11 or sht15
' humidity and temperature chip
' temperature to xx.x degrees Celsius, RH to xx.x %
' Repeatedly shows raw data and converted values on debug screen.
' version B, corrects glaring error in humidity computation
' also adjusts timeout to allow use with faster stamps too.
' version C updates the math for temperature compensation of %RH
' and enables display of negative temperatures
' hookup sht11 or sht15 as follows for this program
'
' STAMP                 SENSIRION
'               220
' p1 ----o-----/\/\---o--pin 3 sck, clock
'                     |
'        ;---/\/\-----' pull-down
'        | 4.7k
' Vss--o-o---------------pin 1 common
'      |
'     ===  0.1uf
'      |             
' Vdd--o-o---------------pin 4 +5 volts
'        |    4.7k
'        '---/\/\-----; pull-up
'                     |
'              220    |
' p0 ----------/\/\---o--pin 2 dta, data
'                       
'
' The following code does not implement the CRC checking
 
 
sck PIN 1
dta PIN 0 ' note, 5k-10k pull-up, also 330ohm between the dta on stamp to dta on sht
dtain var in0
 
shtTR CON 3 ' read temperature
shtRH CON 5 ' read humidity
shtSW CON 6 ' status register write
shtSR CON 7 ' status register read
shtS0 CON 30 ' restore status register defaults (be sure to delay 11 milliseconds)
 
cmd VAR Byte
result VAR Word ' raw result from sht, also used as counter
r0 VAR result.byte0
r1 VAR result.byte1
degC VAR Word ' degrees Celsius * 100
RH VAR Word ' %RH * 10
RHtc VAR Word ' for temperature compensation of RH
 
initialize:
 outs=0
 dirs=%1111111111111101
 GOSUB shtrst ' reset communication with sht
 
DO
 getTemperature:
 cmd=shtTR ' temperature command to sht
 GOSUB shtget16
 degC=result+5/10-400 ' from 100ths to 10ths of a degree with rounding
 DEBUG tab,REP "-"\degC.bit15,DEC ABS degC/10,".",DEC1 ABS degC
 getHumidity:
 cmd=shtRH ' humidity command to sht
 GOSUB shtget16
 RH=(26542-(54722**result+result))**result-40
 ' temperature compensation follows:
 RHtc=655+(result*5)+(result**15917) ' intermediate factor
 RHtc=(RHtc**(degC+2480))-(RHtc**2730)+RH ' compensated value
 DEBUG tab, DEC result,tab,"%RH=",DEC RH/10,".",DEC1 RH
 DEBUG tab,"%RHtc=",DEC RHtc/10,".",DEC1 RHtc,cr
 PAUSE 1000
LOOP
 
' initializes communication with sht 
shtRst:
 SHIFTOUT dta,sck,lsbfirst,[$ffff\16]
RETURN
 
' get 16 bits of data, enter with command in "cmd"
shtget16:
 gosub shtcmd ' send the command "cmd"
 gosub shtwait ' wait for command to finish
 shiftin dta,sck,msbpre,[r1] ' msbyte
 low dta ' acknowledge
 pulsout sck,10
 input dta
 shiftin dta,sck,msbpre,[r0] ' lsbyte
 input dta ' terminate communication
 pulsout sck,10
return
 
' send start sequence and command
shtcmd:
shtStart: ' send the start sequence
 ' dta: ~~~~~|_____|~~~~~~
 ' sck: ___|~~~|_|~~~~|____
 ' while dta is low, clock goes low and then high
 input dta ' pullup high
 high sck
 low dta
 low sck
 high sck
 input dta
 low sck
shtcmd1: ' send the command
 shiftout dta,sck,msbfirst,[cmd]
 input dta ' allow acknowledge
 pulsout sck,10
return
 
shtWait:
 ' wait for sht to pull data pin low
 ' or for time out
 result=4096
 DO
 result=result-1
 LOOP WHILE dta & result.bit11
 RETURN

</pre>

**iw2fvo** · - 28th November 2010, 16:51

Thanks very much for the code.
I am searching now for the dew point computation in picbasic pro !

Thanks a lot for the help

Ambrogio
IW2FVO

Originally Posted by scalerobotics

A quick google search got me to here for some basic stamp code. It may help. You could also search sht75 on this site for some more hits.

http://www.emesystems.com/OL2sht1x.htm

Please don't double post. I moved your post to this thread, even though it is pretty much the same as the first post.

Code:

'{$STAMP BS2}
' SHT15.bs2 version C
' (c) 2002, 2004 Tracy Allen, http://www.emesystems.com
' access the Sensirion model sht11 or sht15
' humidity and temperature chip
' temperature to xx.x degrees Celsius, RH to xx.x %
' Repeatedly shows raw data and converted values on debug screen.
' version B, corrects glaring error in humidity computation
' also adjusts timeout to allow use with faster stamps too.
' version C updates the math for temperature compensation of %RH
' and enables display of negative temperatures
' hookup sht11 or sht15 as follows for this program
'
' STAMP                 SENSIRION
'               220
' p1 ----o-----/\/\---o--pin 3 sck, clock
'                     |
'        ;---/\/\-----' pull-down
'        | 4.7k
' Vss--o-o---------------pin 1 common
'      |
'     ===  0.1uf
'      |             
' Vdd--o-o---------------pin 4 +5 volts
'        |    4.7k
'        '---/\/\-----; pull-up
'                     |
'              220    |
' p0 ----------/\/\---o--pin 2 dta, data
'                       
'
' The following code does not implement the CRC checking
 
 
sck PIN 1
dta PIN 0 ' note, 5k-10k pull-up, also 330ohm between the dta on stamp to dta on sht
dtain var in0
 
shtTR CON 3 ' read temperature
shtRH CON 5 ' read humidity
shtSW CON 6 ' status register write
shtSR CON 7 ' status register read
shtS0 CON 30 ' restore status register defaults (be sure to delay 11 milliseconds)
 
cmd VAR Byte
result VAR Word ' raw result from sht, also used as counter
r0 VAR result.byte0
r1 VAR result.byte1
degC VAR Word ' degrees Celsius * 100
RH VAR Word ' %RH * 10
RHtc VAR Word ' for temperature compensation of RH
 
initialize:
 outs=0
 dirs=%1111111111111101
 GOSUB shtrst ' reset communication with sht
 
DO
 getTemperature:
 cmd=shtTR ' temperature command to sht
 GOSUB shtget16
 degC=result+5/10-400 ' from 100ths to 10ths of a degree with rounding
 DEBUG tab,REP "-"\degC.bit15,DEC ABS degC/10,".",DEC1 ABS degC
 getHumidity:
 cmd=shtRH ' humidity command to sht
 GOSUB shtget16
 RH=(26542-(54722**result+result))**result-40
 ' temperature compensation follows:
 RHtc=655+(result*5)+(result**15917) ' intermediate factor
 RHtc=(RHtc**(degC+2480))-(RHtc**2730)+RH ' compensated value
 DEBUG tab, DEC result,tab,"%RH=",DEC RH/10,".",DEC1 RH
 DEBUG tab,"%RHtc=",DEC RHtc/10,".",DEC1 RHtc,cr
 PAUSE 1000
LOOP
 
' initializes communication with sht 
shtRst:
 SHIFTOUT dta,sck,lsbfirst,[$ffff\16]
RETURN
 
' get 16 bits of data, enter with command in "cmd"
shtget16:
 gosub shtcmd ' send the command "cmd"
 gosub shtwait ' wait for command to finish
 shiftin dta,sck,msbpre,[r1] ' msbyte
 low dta ' acknowledge
 pulsout sck,10
 input dta
 shiftin dta,sck,msbpre,[r0] ' lsbyte
 input dta ' terminate communication
 pulsout sck,10
return
 
' send start sequence and command
shtcmd:
shtStart: ' send the start sequence
 ' dta: ~~~~~|_____|~~~~~~
 ' sck: ___|~~~|_|~~~~|____
 ' while dta is low, clock goes low and then high
 input dta ' pullup high
 high sck
 low dta
 low sck
 high sck
 input dta
 low sck
shtcmd1: ' send the command
 shiftout dta,sck,msbfirst,[cmd]
 input dta ' allow acknowledge
 pulsout sck,10
return
 
shtWait:
 ' wait for sht to pull data pin low
 ' or for time out
 result=4096
 DO
 result=result-1
 LOOP WHILE dta & result.bit11
 RETURN

</pre>

**ScaleRobotics** · - 28th November 2010, 17:28

Here is some code for the SHT11 for calculating dewpoint. I have not compared datasheets to know how similar (or not) they are. But this may help:

http://www.picbasic.co.uk/forum/show...=3851#post3851

That is quite the equation! From NavMicroSystems post above:

Code:

'** 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<
   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

And attached is an application note for dew point calculations.

**iw2fvo** · - 29th November 2010, 07:10

Hi scalerobotics,

yes, the sht_11 has the same interface characteristics of the sht_75 so the code would be OK,

Thanks for helping me,

Regards,
Ambrogio
IW2FVO

Originally Posted by scalerobotics

Here is some code for the SHT11 for calculating dewpoint. I have not compared datasheets to know how similar (or not) they are. But this may help:

http://www.picbasic.co.uk/forum/show...=3851#post3851

**iw2fvo** · - 29th November 2010, 17:10

Good day to all in the forum.

I am using the sht15.bs2 program together with the sht_75 sensor.
I found that the temperature compensated relative humidity RHtc is not correct for temperature below 0 degC.
I think that there is a problem in the code related to the use of the degC variable;
This variable becomes negative if the temperature result is less than 4000 ( this is equivalent to 0degC ): in the RHtc computation this negative value could generate problem...

I will appreciate very much any help or comment .
Thanks
Ambrogio
Iw2fvo

**Dave** · - 29th November 2010, 18:12

iw2fvo , I believe that would be 0% RH. (freezing) Check out this site for more info...
http://www.techtoys.com.hk/8051/AT89...SHTxx_demo.pdf

Dave Purola,
N8NTA

**iw2fvo** · - 29th November 2010, 21:26

Hi Dave,

Thanks a lot for your reply on the matter.
I will try to better clarify the problem that I have using the SHT15.bs2 ver.C program from Tracy Allen ( emesystems).

Point _1.
The program reads the raw temperature and store it on the " result " word variable.
The temperature is obtained using the following : degC=result+5/10-400.
This means that if the result is 5000 we have +10°C, if the result is 4000 temp is 0°C
and if 3000 the temp is -10°C. I modified the program in order to "display" the minus sign
when result is less then 4000. Result is the 14 bit raw temperature delivered by the sensor.
Up to here everythings is correct and my system shows the correct temperature. I did some comparison with the airport control tower close to my home : it is consistent.

Point _2
the program then reads the raw RH (12 bit ) and stores that value on the "result" word variable.
RH is then obtained as follows: RH=(26542-(54772**RESULT+RESULT))**RESULT-40.
At this point everything is ok to me : my PIC micro gives me the correct RH value.

Point_3
The program computes now the temperature compensated RH in two steps:
First step > RHtc=655+(RESULT*5)+(RESULT**15917)
Second Step > RHtc=(RHtc**(degC+2480))-(RHtc**2730)+RH
>>> At this point the RHtc is correct only if the temperature is above 0°C or, if the raw temperature data from the sensor is more than 4000 !.
If the temperature is negative e.g. -1.7 °C the RHtc is not correct. It is about 172%.
The control tower gives me T=-1.7°C , rh=85% and Dew point of -3.9°C.
>>> This is my problem.

I will appreciate very much if someone can give me an help to solve the troble.
May be that some other programmer ( I am not a programmer ! ) have already solved it !
Thanks in advance

Regards,
73!
Ambrogio
IW2FVO

P.s.: English is not my first language and I am not sure to have express my problem correctly: I am Italian !

Originally Posted by Dave

iw2fvo , I believe that would be 0% RH. (freezing) Check out this site for more info...
http://www.techtoys.com.hk/8051/AT89...SHTxx_demo.pdf

Dave Purola,
N8NTA

**Dave** · - 30th November 2010, 11:46

iw2fvo, It is not possible to have this sensor or that equation to give you acurate results below freezing. Please read this artical: http://www.rhsystems.net/papers/RH_WMO.pdf. I have used a chilled mirror in the past for this type of measurement sampling air from a test chanber at - 20 F. However the type of sensor you are using is limited as far as humidity measurements and that is why the formula is not acurate below 0C.

Dave Purola,
N8NTA

**iw2fvo** · - 30th November 2010, 14:57

Dave,
thanks a lot for the document you have indicated to me.
I will read it carefully.
I noted that all the weather station here on the web and the control towers broadcast RH even if the temperature is below 0°C. All of them diclare a consistent RH. At this point I do not understand well. I must study a little bit more.
At the same time I am notice that my Oregon weather station gives a correct RH for negative temperature too.
The SHT15.BS2 program has a bug somewhere : I would like to know if someone solved it !
Please let me know... just in case you know a fix.
Thanks again Dave for the help.
73 !
Ambrogio
IW2FVO
North Italy

Originally Posted by Dave

iw2fvo, It is not possible to have this sensor or that equation to give you acurate results below freezing. Please read this artical: http://www.rhsystems.net/papers/RH_WMO.pdf. I have used a chilled mirror in the past for this type of measurement sampling air from a test chanber at - 20 F. However the type of sensor you are using is limited as far as humidity measurements and that is why the formula is not acurate below 0C.

Dave Purola,
N8NTA

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