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Need help with Adafruit MAX31855 thermocouple breakout

  • I am trying to connect to Adafruit's MAX31855 SPI thermocouple breakout board. I can't boot the Omega2+ with the board connected. If I disconnect the CS1 connection, it boots up fine. Does this suggest the board doesn't float the SP1 connection? Has anybody here used the Adafruit board successfully?

    As it happens, I just received some cheaper generic MAX31855 breakouts from Amazon which do allow the Omega to boot. I'll pursue my project with this board for the time being and see how far I get -- this is my first attempt at SPI on the Omega.

    As a general question, how does one get the pin to float if the target isn't floating? Would I be better off to go to software SPI instead of using the hardware SPI pins (I'm just reading a few bytes)

  • @Ian-Smith The MAX31855 breakout board has/is "3.3 to 5v power supply and logic level compliant!" writes Adafruit.
    According to the schematic this is correct - there are a 5V-to-3.3V LDO (IC2) and two 5V-to-3.3V level shifters for the input pins of MA31855 (D1, R1 for SCK and D2, R2 for /CS).

    Omega2's boot problem is caused by the R1, R2 pull up resistors because the SPI SCK and CS1 pins must be floating at boot time.

    Your options:

    • connect this board to "normal" GPIOs and run some software SPI program
    • use any cheap boards which are don't have level shifters for the input pins (ie. not 5V tolerant board)
    • remove/desolder R1, R2 and use 0 Ohm or a piece of wire instead of D1, D2

    I'm sure the above suggestions are correct although I don't have such Adafruit board.

  • @György-Farkas
    Thanks for the advice--It all sounds totally on-target. I have acquired a couple of cheap breakouts and am pursuing that solution.

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