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Subject: RE: MMC: Card detection through pin CD/DAT3 issue
Date: Tue, 18 Mar 2008 13:24:33 +0100
Message-ID: <83116F0A4FF67A4F97BA0B6E408C48E301DAB4DF@zuerich.BC-Int.NET>
In-Reply-To: <20080318122033.020b45b5@mjolnir.drzeus.cx>
Thread-Topic: MMC: Card detection through pin CD/DAT3 issue
Thread-Index: AciI6jPX7I/PkEJmSoy49MsreeApSgAB7Mvw
References: <83116F0A4FF67A4F97BA0B6E408C48E301DAB4AA@zuerich.BC-Int.NET> <20080318122033.020b45b5@mjolnir.drzeus.cx>
From: "Wappler Marcel" <Marcel.Wappler@bridgeco.net>
To: "Pierre Ossman" <drzeus-mmc@drzeus.cx>
Cc: <linux-kernel@vger.kernel.org>
Sender: linux-kernel-owner@vger.kernel.org
Content-Length: 1656
Lines: 39

Hi Pierre,

>> With this detection scheme I have some dificulties togehter
>> with the internal working of the current MMC stack:
>> The controller cannot detect a newly inserted card if the card power 
>> is switched of. This is due to the fact that the SD card applies the 
>> internal card detection pull up after it powered up. If there is no 
>> power -> no powering up -> no pull up. (besides the fact that a pull 
>> up needs power to for doing its work :-) )
>> 

> Then you have broken hardware. The specs clearly state that the 
> controller must be able to detect cards without powering up the
> cards. Also, the cards must implement the pull-up in a way that
> behaves like a resistor. So there should be no requirement for 
> full power there either.

I have found this requirement in the SD host controller spec:
"The card detect is active regardless of the SD Bus Power."

I wonder how the card should be able to pull up the CD/DAT3 Pad to VCC
if its only VCC pad is not connected to VCC due to the fact that the host
controller has switched off the cards power - any idea how this could be
managed?

There is a SD card spec section somewhere which statet that:
"If one of the supply pins (VDD or VSS) is not connected properly, then
the current is drawn through a data line to supply the card."

Maybe this is the key in understanding ho the pull up without VCC connected
works?

Thanks,
marcel
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