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TMP107: Strange problem with TMP107 dependent on number of sensors

Dominic Stratten
Prodigy 35 points
Part Number: TMP107
Other Parts Discussed in Thread: , , SN74LVC1G125

Hi All

I came across a weird one last week. I'm using the EVM software with a homebuilt interface (PL2303 to TTL and LV125 buffer) which work perfectly for up to 20 temperature sensors but not 21 or 22 ......

We'd instrumented a large module with plenty of cables and connectors so assumed issues with the wiring. If we took off the 21st sensor in the chain, it would all work, add the 21st  and / or 22nd sensor and nothing at all. We also moved these last two sensors elsewhere in the chain and the same thing. With sensor 21 and or 22 in the chain it would not initialise. We had another chain of 26 sensors on the module which read fine and one of 17 which also read fine.

I naturally assumed there was some kind of current limitation or similar with my el cheapo PL2303 5V out but scoping the leads and signals shows everything as fine. I then made up another 21 boards / sensors at the weekend and hooked them up on the bench - same result. 

Maybe an issue with the connectors / FPC I'm using ? Tried it with some old boards I'd made up with five sensors to a board. 5 sensors = ok, 10 sensors = ok, 15 = ok, 20 = ok, 21 = no go, 22 = no go, 23 = ok ! 

Hang on, so <=20 works fine >=23 works fine. I tested all the way up to 32 sensors without an issue.

I then used my own hardware / software to try it on a string of 21 sensors (CAN BUS -> PIC controller -> galvanic isolator -> buffer -> TMP107 string) and it worked with no issues with any number of sensors.

This might still be an issue with using the PL2303 TTL interface instead of maybe a FTDI but though I'd mention it in case there was a software issue (Dec 32 -> Hex 20 - just a thought ... ) or if it is just this particular setup that has caused an issue. I ran out of project time to debug any further.

  • 0
    TI__Mastermind 35895 points

    Hi Dominic,

    Are you using the LabVIEW GUI that accompanies TMP107EVM? Described here: https://www.ti.com/lit/sbou158

    When developing your own software, I recommend the library included with BOOSTXL-TMP107 software examples. www.ti.com/.../BOOSTXL-TMP107

    thanks,

    ren

  • 0 in reply to Ren Schackmann
    Prodigy 35 points

    Hi - yes, This issue occurs when using the LabVIEW GUI.

    My own software works without an issue with 20/21/22 sensors but it is pretty integrated into a module (using a PIC18F2685 + CAN)  that we don't have easy access to so I thought I'd use the LabVIEW GUI to check the sensors were all working when I noticed this issue.

    My module / software doesn't see this issue. 

  • 0 in reply to Dominic Stratten
    TI__Mastermind 35895 points

    Understood. Thanks for your feedback. We'll need some time to assess the potential issue with the LabVIEW GUI.

    thanks,

    ren

  • 0 in reply to Ren Schackmann
    Prodigy 35 points

    Thanks for the prompt replies Ren. This could just be an anomality with the PL2303 / LabVIEW GUI / sn74lvc1g125 / Direction of the wind etc. but if there is a software glitch with the GUI then this post could help others with the same issue I had on Friday. 

    Exact test procedure was to add TMP107's in chunks of 5, restart the software, initialise the COM port and see how many were enumerated. Up to the 20 mark worked fine but the moment you add an additional one (21) it would report zero devices detected. Same with 22nd but on adding the 23rd sensor (and onwards) it would work correctly. If I wasn't up against it time wise, I'd have grabbed some logic traces to see exactly what was going on but unfortunately couldn't manage this. I scoped the signal with a Picoscope and looked for noise etc on the power lines but these all seemed stable. I also added a 10uF tantalum on the power rails of the last device (which was moved around in the chain later on) and this had no effect. Total physical chain length on the 21 devices was only about 3 foot (four PCBs with 5 x TMP 107 on each, one PCB with a single TMP107 on for the 21st unit test). I also tried various baud rates without success although they all worked with the 20 or 23 modules.  Just a really strange issue that had us pulling our hair out for a bit.

  • 0 in reply to Dominic Stratten
    TI__Mastermind 35895 points

    I'm sorry. If it's any consolation, I do agree with your assessment that its a software issue. I just won't be able to confirm with hardware in the short term.

    thanks,

    ren

  • 0 in reply to Ren Schackmann
    Prodigy 35 points

    Cheers Ren, if I get a chance in the meantime to investigate more, I'll update this thread.