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

Ole Kristian Western
Prodigy 85 points
Part Number: TCAN4550

In the datasheet I read that the SCK terminal has an internal pull-up. But the TCAN4550 operates in SPI mode 0, which has a low idle clock. Wouldn't it make more sense to have an internal pull-down?

  • +1
    TI__Guru 53515 points

    Hi Ole,

    This is a good question, and I'm afraid I don't have a definitive answer for you. The SCLK is an input for TCAN4550 that may be shared with other SPI peripheral devices. Because the signal is controlled solely by the SPI controller, the internal bias of this line from the TCAN4550 is only impactful when the controller is not yet active. For TCAN4550, this is still not terribly important as the SCLK signal is ignored until the nCS line has been pulled low by an active SPI controller. 

    My assumption would be that the pull-up was chosen to be consistent with other devices that may share the SCLK signal line as SPI peripherals. This way there would be no contention in weak biases that result in an intermediate voltage when the controller is high-z. However, I do see how this is not ideal for the operating case when the controller is active and holding the signal low in idle. It seems that the extra µA of leakage in this idle case was deemed less impactful than the potential problems with contentious biases from various peripherals. 

    I hope this helps expand a bit on the decision that was made regarding your questions. Let me know if there's anything else you are curious about. 

    Regards,
    Eric Schott