linux-kernel - RE: MMC: Card detection through pin CD/DAT3 issue

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Date:	Thu, 20 Mar 2008 08:27:13 +0100 (CET)
From:	Bodo Eggert <7eggert@....de>
To:	Wappler Marcel <Marcel.Wappler@...dgeco.net>
cc:	7eggert@....de, Pierre Ossman <drzeus-mmc@...eus.cx>,
	linux-kernel@...r.kernel.org
Subject: RE: MMC: Card detection through pin CD/DAT3 issue

On Wed, 19 Mar 2008, Wappler Marcel wrote:
> Bodo Eggert wrote:

> >> 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?
>  
> > If the VCC pin is connected to 0 V, the "pullup" will pull 
> > down to 0V. 
> > If the controller reverses it's logic while VCC=0, it might work.
> 
> I am concerned  that in this case there would be neither a pull 
> up nor a pull down at this signal. Due to floating gates that input
> could lead to millions of "card detection" events a second, if

As long as no card is inserted, the pin on the controller would be floating 
and generating the same amount of interrupts, unless there is a (weak)
internal pulldown. You can do the same for the reversed logic, using
an internal pullup and abusing the card's pullup as a pulldown.

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