• Doug Anderson's avatar
    mmc: core: Increase delay for voltage to stabilize from 3.3V to 1.8V · 7c5209c3
    Doug Anderson authored
    Since the regulator used for the SDMMC IO voltage is not expected to
    draw a lot of current, most systems will probably use an inexpensive
    LDO for it.  LDO regulators apparently have the feature that they
    don't actively drive the voltage down--they wait for other components
    in the system to drag the voltage down.  Thus they will transition
    faster under heavy loads and slower under light loads.
    During an SDMMC voltage change from 3.3V to 1.8V, we are almost
    certainly under a light load.  To be specific:
    * The regulator is hooked through pulls to CMD0-3 and DAT.  Probably
      the CMD pulls are something like 47K and the DAT is something like
    * The card is supposed to be driving DAT0-3 low during voltage change
      which will draw _some_ current, but not a lot.
    * The regulator is also provided to the SDMMC host controller, but the
      SDMMC host controller is in open drain mode during the voltage
      change and so shouldn't be drawing much current.
    In order to keep the SDMMC host working properly (or for noise
    reasons), there might also be a capacitor attached to the SDMMC IO
    regulator.  This also will have the effect of slowing down transitions
    of the regulator, especially under light loads.
    From experimental evidence, we've seen the voltage change fail if the
    card doesn't detect that the voltage fell to less than about 2.3V when
    we turn on the clock.  On one device (that admittedly had a 47K CMD
    pullup instead of a 10K CMD pullup) we saw that the voltage was just
    about 2.3V after 5ms and thus the voltage change would sometimes fail.
    Doubling the delay gave margin and made the voltage change work 100%
    of the time, despite the slightly weaker CMD pull.
    At the moment submitting this as an RFC patch since my problem _could_
    be fixed by increasing the pull strength (or using a smaller
    capacitor).  However being a little bit more lenient to strange
    hardware could also be a good thing.
    Signed-off-by: default avatarDoug Anderson <dianders@chromium.org>
    Acked-by: default avatarMark Brown <broonie@kernel.org>
    Signed-off-by: default avatarUlf Hansson <ulf.hansson@linaro.org>
core.c 66.9 KB