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Original question:

TCAN4550: Why internal pull-up on SCK (SPI mode 0)

TCAN4550: TCAN SPI Clock bug

Ryan Stephen
Prodigy 30 points
Part Number: TCAN4550


Tool/software:

Very strange behaviour we are seeing in that the SPI SCK pin is being pulled high from its weak internal pull up giving an SPI bus that is normally high. With this the chip does not respond. If we force a pull down on the MCU driving the SPI bus the TCAN4550 responds. Albeit with a 400mV DC bias as the two pull resistors battle each other. Any fix for this?

  • 0
    TI__Mastermind 34170 points

    Hi Ryan,

    The internal pull-up resistance is greater than 50kohm and should require no more than 100uA from the MCU pin to overcome in the worst case and this should not be difficult to accomplish.

    What is the drive strength of the pin driving SCLK?  

    Do you have any other components on the SPI lines (resistors, caps, etc.)?

    Is there a schematic that can be shared for review?

    Regards,

    Jonathan

  • 0 in reply to Jonathan Nerger
    Prodigy 30 points

    Hi Jonathan

    The traces for SPI are rather long so we have 22ohm series resistors at the driving MCU pin. The SPI peripheral is configured as in the TCAN SPI datasheet as Mode 0. The drive strength is actually quite low so i can reconfigure that and try again. It is confusing because i would have thought Mode 0 would see a normally low clock but it isnt appearing that way. Other than this change i am a little lost.

    Thanks for your reply.

  • 0 in reply to Ryan Stephen
    TI__Mastermind 34170 points

    Hi Ryan,

    I'm not sure what to say because I can't recall any similar issue with a MCU not being able to drive correct voltage levels on our SPI interface.  I would also think that the the MCU would hold the SCLK line low by default for Mode 0 because it needs to be low when the nCS pin transitions low at the start of a SPI transaction.  The weak internal pullup ensures the MCU is fully controlling the SPI and SCLK signals and needs to have enough drive strength to overcome this >50k weak pull-up resistance. 

    Regards,

    Jonathan