Author Topic: SHT31 breakout with Davis ISS  (Read 7995 times)

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Offline belfryboy

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SHT31 breakout with Davis ISS
« on: May 24, 2016, 07:22:04 AM »
Having followed the Davis T/H sensor thread with interest for the last few weeks I decided to have a go at a homebrew sensor replacement, with the PCB following the SHT31 design guide with a heat island and manufacturers filter cap to improve response time. I have however run into a curious problem;

Having sourced a 7246.070 from Ryan at Scaled instruments I connected it up to the test rig that I use for testing the SHT15 homebrew sensors that I produce, It worked great, happy days! I then connected the Davis sensor to an Arduino microprocessor to read the I2C bus address, and that is where the problems started.

I cannot "see" the sensor on the I2C bus. At all.

I ordered a breakout board for the SHT31 from Closed Cube, and whilst being able to read data from the breakout using the I2C bus scanner, I couldn't read anything from it using my T/H sensor tester, using the firmware that reads the SHT15 (but does read the 7346.070).

Has anyone else tried a non Davis breakout board with the ISS?

Offline jerryg

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Re: SHT31 breakout with Davis ISS
« Reply #1 on: May 24, 2016, 10:37:36 AM »
Before the Davis sensor was up for sale i bought a 31 breakout board and tried it with the Davis iss and could not make it work. It will not read the b/o board at all. I wonder if there is some kind of jumpering needed to get it to be read by serial Davis iss. I also wondered if the 31 Davis is using is factory modified to work with the iss. The reason i tried it is i don't think there will be a 35 sensor from Davis anytime soon, if at all, and i would like to compare the 31 to the 35 without having to do it with some i2c circuitry stuff.  I was hoping that some of the guru's that are into the kit stuff will find a work around but not holding my breath lol.

Offline belfryboy

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Re: SHT31 breakout with Davis ISS
« Reply #2 on: May 24, 2016, 11:59:34 AM »
I have examined the Davis PCB and it appears to be wired identically to a breakout board set at address 0x44.

Offline jerryg

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Re: SHT31 breakout with Davis ISS
« Reply #3 on: May 24, 2016, 12:12:10 PM »
I tried changing the address like it said and still no joy. I am not good enough with the programming part of this stuff  but it makes me wonder if the chips are modified for Davis sensors to be retro capable. The sht1x can be read with i2c with program modification so why doesn't it work, beats me.

Offline belfryboy

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Re: SHT31 breakout with Davis ISS
« Reply #4 on: May 24, 2016, 12:23:36 PM »
I contacted Sensirion about compatibility and got this reply;

Quote
Thank you for your inquiry.

Unfortunately existing code can’t be reused for the SHT3x generation. SHT1x uses a different protocol called Sbus, also mentioned in the link that you’ve provided. SHT3x is based on I2C, has a different address and different commands.


Offline jerryg

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Re: SHT31 breakout with Davis ISS
« Reply #5 on: May 24, 2016, 12:34:18 PM »
Ok so how is Davis doing it?  All my stations or 2010 or earlier without any updates. So makes me wonder if they were capable using either protocol from the git go.  #-o
« Last Edit: May 24, 2016, 12:41:03 PM by jerryg »

Offline johnd

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Re: SHT31 breakout with Davis ISS
« Reply #6 on: May 24, 2016, 12:45:15 PM »
Ok so how is Davis doing it?

Speculating wildly and very possibly wrongly, I suppose one possibility is that recent VP2 SIM boards have been able to autosense I2C vs Sbus all along. (Perhaps the I2C sensor option has been mooted for quite a while, either from Sensirion or maybe from another sensor maker, and Davis built in the I2C option as a fallback.) Remember that the post-2006 SIM boards have always been able to autosense analogue T/H sensors and work with them IIRC (back in the day when such sensors were readily available still). But if this is the case then it's a shame that they couldn't also eliminate the need for the 0.5C offset.
Prodata Weather Systems
Prodata's FAQ/support site for Davis stations
Includes many details on 6313 Weatherlink console.
UK Davis Premier Dealer - All Davis stations, accessories and spares
Cambridge UK

Sorry, but I don't usually have time to help with individual issues by email unless you are a Prodata customer. Please post your issue in the relevant forum section here & I will comment there if I have anything useful to add.

Offline jerryg

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Re: SHT31 breakout with Davis ISS
« Reply #7 on: May 24, 2016, 12:52:47 PM »
The thing is you can use an old sensor with a new iss by adding the offset back in. It sure seems like the software is dual purpose to me. I know i read on a forum that uses the kit gear that the processors were i2c and they used the sht series sensor for hum/temp by modifying the program. I wish i could find that again so someone with more smarts on this kind of stuff could read it and see what they were doing so they could use the sensor.

Offline belfryboy

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Re: SHT31 breakout with Davis ISS
« Reply #8 on: May 24, 2016, 12:59:16 PM »
And it wouldn't explain why an ISS can recognise a Davis sensor, but not a breakout board with the same wiring.

Offline jerryg

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Re: SHT31 breakout with Davis ISS
« Reply #9 on: May 24, 2016, 01:15:39 PM »
Here is a link that explains what was done to make the sht11 work on i2c.https://www.ccsinfo.com/forum/viewtopic.php?p=33089

Offline SystemsGuy

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Re: SHT31 breakout with Davis ISS
« Reply #10 on: May 24, 2016, 01:34:28 PM »
Ok so how is Davis doing it?  All my stations or 2010 or earlier without any updates. So makes me wonder if they were capable using either protocol from the git go.  #-o

Just so I understand - the "test rig" you are using is an ISS, right?

Offline belfryboy

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Re: SHT31 breakout with Davis ISS
« Reply #11 on: May 24, 2016, 01:41:06 PM »
The test rig I use is an Arduino running code that uses code that can read the SHT15 sensor. It can read both old and new Davis sensors, but not a breakout board wired the same as a Davis sensor.

I'm guessing jerryg is using an ISS.

Offline jerryg

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Re: SHT31 breakout with Davis ISS
« Reply #12 on: May 24, 2016, 01:46:50 PM »
Yes i am using iss and i also have a hand held meter that used the sht11 got sensor and i modified it to allow me to plug sensors into it and it will not read it either. The station is from 2008 without any updates and works fine with the 31 so it has to be, at least to me, that the iss reads both. That is if the 31 is off the shelf without special stuff being done to it.  ](*,)

Offline SystemsGuy

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Re: SHT31 breakout with Davis ISS
« Reply #13 on: May 24, 2016, 01:52:42 PM »
The test rig I use is an Arduino running code that uses code that can read the SHT15 sensor. It can read both old and new Davis sensors, but not a breakout board wired the same as a Davis sensor.

I'm guessing jerryg is using an ISS.

Sounds like time for the bus pirate.  My ISS board - 2012 vintage - is working fine with the 31 from davis as well, but I got lazy and didn't try it with my i2c rig.

Offline jerryg

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Re: SHT31 breakout with Davis ISS
« Reply #14 on: May 24, 2016, 02:04:30 PM »
I hope someone will come up with an answer how the old stations can read both protocols with just an offset being applied. When you think about it the new 2016 stations with the new sensor can also be fit with the old sensor, so that means they are still using the same setup but just making a change so the offset doesn't need to be entered unless going back to an old sensor.

Offline Vanadium

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Re: SHT31 breakout with Davis ISS
« Reply #15 on: May 25, 2016, 10:45:42 PM »
Hi Guys,

I too have ordered the SHT31 from scaled and have been unsuccessful in getting it to work with my custom hardware, even with it running the SHT15 code. My next step was to do as you have done and get the breakout board. It's almost as though they're using a custom sensor as I was seeing some really weird stuff using the logic analyzer.

Like everyone in the other thread was saying, you should be able to replace the SHT15 with an SHT31, apply a small offset and it’d still work. But according to the datasheet for the SHT31 there’s just no way that should work.

I plugged the new SHT31 into my hardware and ran it with the SHT15 code, with the logic analyzer on the SCL/SDA lines to see what was going on. It’s actually really strange, it’s almost like the SHT31 responds to ‘any’ command sent like you would the old protocol to start a temperature/humidity conversion, and that’s indicated in the way that it holds down the SDA (serial data) line for ~11.4ms (The datasheet says typical time for high repeat-ability measurement is 12.5ms). It then releases the SDA line, just as the SHT15 does when the measurement has completed, BUT it then fails to send any data when clocking the CLK line out. According to the SHT31 datasheet though, it shouldn’t do that at all. It uses a proper I2C implementation. The ‘start’ sequence is different, the addressing is different, the commands are different – everything is different, so how do people supposedly just replace the SHT15 with the 31 and it works? Unless they put in some hidden backward compatibility protocol or have a custom chip?

If I try to use it just like the datasheet says, using the I2C procotol shown, I can’t even get it to respond at all. It won’t even pull down the first acknowledge bit after sending the start sequence and sending the address.

If, as you say, it works with the SHT31 on the breakout board but not on the Davis SHT31, then there has to be a difference in the sensor, surely..

I'd be happy if I could just get it working using the SHT15 protocol. The arduino code you use, is it in the public domain? Are you just bit-banging the clock and data lines?

And it wouldn't explain why an ISS can recognise a Davis sensor, but not a breakout board with the same wiring.

Exactly.. there has to be more to it

Offline ValentineWeather

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Re: SHT31 breakout with Davis ISS
« Reply #16 on: May 26, 2016, 04:08:21 AM »
Sounds like they used something different for the Davis sensor making it compatible with the small offset. You aren't the first that questioned how it even works per data sheet.
Has anyone asked Sensirion whats different? They may or may not be able to answer.
Randy

Offline rdsman

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Re: SHT31 breakout with Davis ISS
« Reply #17 on: May 28, 2016, 11:36:26 AM »
Quote
I hope someone will come up with an answer how the old stations can read both protocols with just an offset being applied.

The product code on the Davis SHT31 begins with LS (maybe supports Sensibus??)  The product code on the SHT31 breakouts that I have seen begin with DI (Digital Interface).

Just my 2 cents worth........

 
Ray

Offline mcrossley

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Re: SHT31 breakout with Davis ISS
« Reply #18 on: May 28, 2016, 05:10:00 PM »
I can't find any reference for an 'LS' code with the SHT3x, as you say DI = digital, AR = analogue. LS could well be an SBus version which would explain a lot.

Edit, found a reference to an SHT30 legacy SBus model... http://uk.futureelectronics.com/en/Technologies/Product.aspx?ProductID=SHT30LSSPSENSIRIONAG4071228
« Last Edit: May 28, 2016, 05:14:18 PM by mcrossley »
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Offline ValentineWeather

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Re: SHT31 breakout with Davis ISS
« Reply #19 on: May 28, 2016, 05:45:58 PM »
I think what he's saying on the Davis chip itself it starts with a LS, you may be able to see in this image.
Randy

Offline belfryboy

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Re: SHT31 breakout with Davis ISS
« Reply #20 on: May 28, 2016, 06:13:08 PM »
That's interesting, both my sht31 breakout boards have DI prefix, the Davis chips have the LS prefix. We may be on to something.

Offline mcrossley

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Re: SHT31 breakout with Davis ISS
« Reply #21 on: May 29, 2016, 10:01:18 AM »
I think what he's saying on the Davis chip itself it starts with a LS, you may be able to see in this image.
yes, I should have said I could not find any reference to that chip on the Sensirion web site. It would appear it is an S Bus version Davis are using. That SHT30 SBus version uses the LS prefix.
Mark

Offline kobuki

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Re: SHT31 breakout with Davis ISS
« Reply #22 on: May 29, 2016, 10:50:20 AM »
Interesting. IIRC the SHT11/15 also uses the proprietary Sensirion interface. That would explain how the old SIM boards can communicate with the new sensor. However if that were the case, it'd complicate things a bit. An implementation of the S-Bus would be required in an auxiliary chip to interface the SHT31 to the SIM (old or new). An Atmel ATTiny or a very cheap PIC, maybe... Finally, some "real" hacking again ;)

Offline jerryg

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Re: SHT31 breakout with Davis ISS
« Reply #23 on: June 13, 2016, 08:45:53 AM »
I emailed the manufacturer of the sensor and as we suspected the 31lss chip is made to replace the sht11 and sht75 series sensor. The reason we could not find any info on it is because it is not publicly available yet. He sent me a copy of the data sheet for it. So it looks like sometime in the future it will be on the market and i am sure someone will come out with the sht35 on a breakout board that we can play around with. So there it is, coming soon lol.  =D>

Offline kobuki

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Re: SHT31 breakout with Davis ISS
« Reply #24 on: June 13, 2016, 09:03:58 AM »
Jerry, are you allowed to share the datasheet? I'd like to see it as well but I think it would be useful for all of those hoping for a Belfryboy solution :)