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AM4372: About Unused pin handling for MMC interface

user1201164
Expert 1770 points
Part Number: AM4372

HI,

We have received a question from our customer about the unused pins of DAT[4-7], pow, sdcd, and sdwp when using "SDIO 4-bit mode".
Referring to the AM437x Schematic Checklist (Rev. A), I tried to answer "When connecting a device (card or eMMC), include 10k pullups on RST#, CMD, and all DAT signals.
However, it seems to me that these pullups are for insertion and removal.
When using a module that is not plugged in, is it safe to open DAT[4-7] as well as pow, sdcd, and sdwp?
Or, when using MMC interface, is it necessary to pillup all DAT signals (and RST#, CMD) regardless of whether they are used or not, as described in the document?

Best Regards,

Kouji Nishigata

  • 0
    TI__Guru 62835 points

    The DAT[4-7], pow, sdcd, and sdwp signals are pin multiplexed with other signal functions. None of these signals are selected with the default pin multiplex configuration. Each pin which can be associated with one of these signals defaults to GPIO mode. So it is not clear why your customer is concerned with these signals.

    The AM437x device does not have a signal named RST#.

    Is your customer asking how to connect pins of the SDIO connector or attached SDIO device?  If so, they need to read the SDIO specification or SDIO device datasheet to determine what needs to be done with these signals.

    We do not make system-level design recommendations unless it affects AM437x. This should not be the case if they are not using these signals, as the pins associated with these signals would remain in GPIO mode.

    Regards,
    Paul

  • 0 in reply to peaves
    Expert 1770 points

    Hi, Paul

    Thanks for the answer.

    What the customer has asked is how to handle the pins that become unused in SDIO. I understood that these are GPIO at boot time, so there is no problem with open.
    However, I have found a different description in the MMC section of the AM437x schematic checklist (www.ti.com/.../sprace0a.pdf). That is, "When connecting a device (card or eMMC), include 10k pullups on RST#, CMD, and all DAT signals".
    Please tell me what this sentence means.Or what are the cases where pullups are needed?
    I thought this was for protection when plugging and unplugging, but I took it from your answer that this is not the case.
    Also, "The AM437x device does not have a signal named RST#." but this is the signal described in the earlier document. If the signal is not there, is it a mistake in the documentation?

    Best Regards,

    Kouji Nishigata

  • +1 in reply to user1201164
    TI__Guru 62835 points

    With respect to the comment "When connecting a device (card or eMMC), include 10k pullups on RST#, CMD, and all DAT signals", the reference to DAT signals is talking about actual signals connecting AM437x to the attached device. The comment only applies to the DAT[3:0] signals if they are the only data signals connecting AM437x to the attached device. Pull-ups are required to hold inputs in a valid logic state when they are not driven by an output buffer. This is necessary to prevent a mid-supply voltage from being applied to an input buffer if left to float as this can cause shoot-through current that flows from VDD through the input buffer to VSS. See https://www.ti.com/lit/pdf/scba004 for more information on this topic. Shoot-through current may damage the input buffer if the voltage applied to the input spends too much time at mid-supply.

    I suspect the reference to RST# is a warning that some attached devices may have a reset input that needs to be pulled high to prevent the signal from floating if sourced from a AM437x pin which is not driven until software initializes the pin to operate as an output.

    Regards,
    Paul

  • 0 in reply to peaves
    Expert 1770 points

    Hi, Paul
    Thanks for the answer.
    I was only thinking about the unused pins.
    Now I understand that the example in the schematic checklist is talking about connected pins.

    Best Regards,

    Kouji Nishigata

  • 0 in reply to user1201164
    TI__Guru 62835 points

    That portion of the schematic checklist you referenced in this thread is specifically addressing recommendations for MMC connectivity. However, you should also consider what needs to be done with unused pins/signals. 

    Many AM437x pins are associated with bi-directional IOs that have internal pulls turned on by default, so they can be left unconnected if not used. However, you should confirm all unconnected pins have an internal pull turned on by default to prevent the input associated with the IO from floating. If not, you will need to connect an external pull to hold the IO in a valid logic state. We also recommend external pulls on any unused/undriven signal connected to a pin that has an internal pull turned on. The internal pulls are weak and may not be able to hold a valid logic state if enough noise couples to unused/undriven signal. External pulls provides a lower impedance path to VDD or VSS which makes it harder for noise to induce a voltage change on the unused/undriven signal.

    This above recommendation is good design practice which should be applied to all devices in your system.

    Regards,
    Paul