Re: [PATCH] RFC: mmc: core: Increase delay for voltage to stabilize from 3.3V to 1.8V

From: Ulf Hansson
Date: Wed May 13 2015 - 09:06:54 EST

On 13 May 2015 at 13:09, Mark Brown <broonie@xxxxxxxxxx> wrote:
> On Tue, May 12, 2015 at 02:46:11PM -0700, Doug Anderson wrote:
>> 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.
> What a LDO is doing is basically just charging up a capacitor - the
> regulation consists of monitoring the voltage on the capacitor and
> opening a transistor to charge the capacitor when the voltage droops too
> much.
>> 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.
> Right, and this is probably going beyond the delays that the regulator
> API is handling since it's not something the regulator hardware is
> actively managing.

Thanks for elaborating from the regulator perspective. So may I apply
your ack for this one?

Kind regards
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