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DRA750: MMC1 timings in Default Speed Mode

Manfred Becker
Expert 4570 points

Part Number: DRA750

A customer identified a timing violation in his system using MMC1 in Default Speed mode. Based on our DM, the required hold time in this mode is 20.46ns. Measured hold time is only in the range of 15ns. So far connected SD cards are working without any issues even with this hold time violation. Anyhow, this problem needs to be fixed for an automotive system in production.

Can anybody explain why DRA75x needs a hold time of more than 20ns only in DS mode (see DM extract below)?

Isn't that violating the SD Specification Part 1 (Physical Layer Specification) which defines an input set-up and hold time for CMD and DAT of always 5ns?

Best regards, Manfred

  • 0
    TI__Genius 10180 points

    Manfred,

    The spec is for the device and the hold time mentioned there is not applicable to the host.  The host hold time is an equation of multiple timing, including pulse width, jitters, device's output delay timing, and lane skew matching.

    Thanks & Regards,

    Shiou Mei

  • 0 in reply to Shiou Mei Huang
    TI__Expert 4570 points

    Shiou Mei,

    would it be possible to use the MMC1 - SD Card High Speed Mode instead where we define a hold time of just 2.6ns but still configure a mmc1_clk frequency of 25MHz? HS mode is defined in the DM, but when looking at bit HSPE of register MMCHS_HCTL in TRM, there's a note saying: "Do not set this bit to 0x1 as timings cannot be met at high speeds. It is recommended to not modify it and keep its default value of 0x0."

    Regards,

    Manfred

  • 0 in reply to Manfred Becker
    TI__Genius 10180 points

    Manfred,

    A device's setup/hold time are typically not frequency dependent.  As a result, if the device can work with a smaller hold time at a higher frequency, it likely will function with the same hold time at a slower frequency.  This could explain why the customer's device has not seen failures so far.

    As for HSPE, the mode is not supported and the TRM has the correct statement.

    Have a great day!

    Best Regards,

    Shiou Mei