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From: Jerome Brunet <jbrunet@baylibre.com>
To: Ulf Hansson <ulf.hansson@linaro.org>
Cc: Kevin Hilman <khilman@baylibre.com>,
	"linux-mmc@vger.kernel.org" <linux-mmc@vger.kernel.org>,
	"open list:ARM/Amlogic Meson..."
	<linux-amlogic@lists.infradead.org>
Subject: mmc: UHS Voltage switch and operating frequency question
Date: Mon, 31 Jul 2017 18:20:04 +0200	[thread overview]
Message-ID: <1501518004.2759.5.camel@baylibre.com> (raw)

Hi Ulf,

I am working on adding signal voltage switch callback to the meson mmc driver.
While testing, I noticed that a few cards fail to exit the busy state after the
voltage switch.

After tinkering with the driver a bit, I noticed that increasing the clock
frequency from 400kHz to 1MHz solve the problem. Strange, isn't it ?

I'm don't know MMC that much but is it possible that some card require a minimum
operating frequency to enter UHS mode ?

The simplified spec (Part 1 - Physical Layer) says that before CMD11, we should
have had CMD41 (Init Command) . After CMD41, the card should be operating at
Default Speed or SDR12. 

I believe that in both case, the frequency is "up to 25Mhz" ? This would be the
maximum, but is there a minimum ? 400kHz seems pretty low compared to 25Mhz ...

I hope you will able to shed light on this :)

Best Regards
jerome

             reply	other threads:[~2017-07-31 16:20 UTC|newest]

Thread overview: 6+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2017-07-31 16:20 Jerome Brunet [this message]
2017-08-01  0:38 ` mmc: UHS Voltage switch and operating frequency question Shawn Lin
2017-08-01 16:56   ` Jerome Brunet
2017-08-02  2:54     ` Shawn Lin
2017-08-02  9:20       ` Jerome Brunet
2017-08-02 10:07         ` Shawn Lin

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