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LMP90080 - Switching Scan Modes

Tracy Whittaker1
Prodigy 90 points
Other Parts Discussed in Thread: LMP90080, LMP90100

I am using the LMP90080 with 5 thermocouples connected. I all works well except now I need to test for open circuits. I can do this in the normal Mode 3 with Burnout enabled because a use one input as a thermocouple common. So any current injected into the previous TC would affect the one that is being converted. So I need to switch scan modes every so often to single channel scan with Burnout enable and check the flag.

My question is when is the minimum registers I would need to change in order to switch scan modes back and forth?


Thanks,

  • 0
    TI__Expert 3350 points
    I suggest that you operate the device in two states in mode 3; data acquisition and diagnostics states on periodic intervals.
    In the data acquisition state, the burnout injection feature should be disabled using BURNOUT_EN bit, see section 9.4.2.2.1 on page 30 of the LMP90080 datasheet. In this state accurate data is collected. In the diagnostic state, the burnout injection should be enabled, checking the diagnostics flags only and discarding data.
  • 0 in reply to Bahram Mirshab
    Prodigy 90 points

    Once the LMP90080 is acquiring data in mode 3, what steps need to be taken and what registers need to be changed to change it to the diagnostic state. When I try changing registers after acquiring data in mode 3, it does not behave correctly. I am assuming there is a sequence I need to follow, but there is nothing in the documentation that I can find to explain the sequence for changing states after the part is initialized and running.

  • 0 in reply to Tracy Whittaker1
    TI__Expert 3350 points

    Software configuration examples  are provided in application note  SNAA075A "AN-2181 Microcontroller Firmware Considerations for the National LMP90100 Sensor AFE" at the link below. For register informations, please refer to section 9.6 Register Maps of the LMP90080 on starting on page 43 of the datasheet.  Channel scan mode configurations are provided in Table 16 on page 47.

  • 0 in reply to Bahram Mirshab
    Prodigy 90 points
    While I already know how to program the LMP90080 in mode 3 and acquire data using the Data Only Mode, what I want to know is if I have to completely reset the chip and reprogram it in order to switch to mode 3 with BURNOUT_EN bit enabled.
  • 0 in reply to Tracy Whittaker1
    TI__Expert 3350 points
    You should be able to change the bit and restart the conversion without a complete reset command, will confirm this on the bench and let you know.
  • 0 in reply to Bahram Mirshab
    Prodigy 90 points

    Have you had a chance to verify this concept? I have had limited success with it, but it seems that there needs to be some delay or something when restarting the conversion. The first data reading after the conversion starts seems to be wrong. Also if the input is shorted to VA, it seems to lock up the chip.

  • 0 in reply to Tracy Whittaker1
    TI__Expert 3350 points
    I will try to create the condition in the lab, will provide you my feedback.
  • 0 in reply to Bahram Mirshab
    Prodigy 90 points

    I am having one other problem with the chip. The main power to our board is 12VDC. The chip has VA=5VDC, and VIO=3.3VDC. If a Thermocouple is accidentally shorted to 12VDC, the input that is common to all the thermocouples stop reading. I still get data from the other pins, but not the common pin. The is a 10k series resister to limit the current to the chip and any voltage on the pin is clamped to the 3.3VDC rail with a diode. Sometimes the pin will start reading again after the fault is removed and other times it will not and a power cycle must be performed. Any ideas on what might be causing this and how to prevent it from locking up?

    Thanks,

  • 0 in reply to Tracy Whittaker1
    TI__Expert 3350 points
    I cannot verify the condition using the EVM. Just to see if you can get proper operation, please try a minimum delay of two DRDYs (maximum three) before reading the results after restart. Regarding the latch up condition due to short to supply, I have replied to your posting, please check it out.
  • 0 in reply to Bahram Mirshab
    Prodigy 90 points
    Thank you for checking. I don't see any reply to my posting regarding the short to supply.
  • 0 in reply to Bahram Mirshab
    Prodigy 90 points

    What is the differences between the LMP90080Q and the LMP90080? I have boards with both parts and they seem to handle the thermocouple break detection differently with the same firmware.


    Thanks,

  • 0 in reply to Tracy Whittaker1
    TI__Expert 3350 points

    The LMP90080Q is the automotive grade version with higher (150 C Max) operating temperature rating. The LMP90080 is the industrial grade version with maximum operating temperature range of 125 C. I recommend to use the LMP90080 unless you are working on a high temperature automotive application.

  • 0 in reply to Bahram Mirshab
    Prodigy 90 points
    The problem I am having is that code to detect thermocouple breaks was working on the LMP90080Q. When they switched to the LMP90080, the detection is no longer working. It seems there are timing differences when making changes to register settings that are now not working.
  • 0 in reply to Tracy Whittaker1
    TI__Expert 3350 points

    We need to check the characterization data to determine if there is a variation in the parameters influencing the functionality of the diagnostic feature in the automotive grade version. That would take some time, thanks in advance for your patience