linux-kernel - Re: [patch 4/4] sdio: disable CD resistor

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Message-ID: <46B885CC.6030404@csr.com>
Date:	Tue, 07 Aug 2007 15:46:36 +0100
From:	David Vrabel <david.vrabel@....com>
To:	Pierre Ossman <drzeus-list@...eus.cx>
CC:	linux-kernel@...r.kernel.org
Subject: Re: [patch 4/4] sdio: disable CD resistor

Pierre Ossman wrote:
> On Tue, 07 Aug 2007 13:55:59 +0100
> David Vrabel <david.vrabel@....com> wrote:
> 
>> sdio: disable CD resistor
>>
>> Disable the card detect resistor to ensure all data lines are equally
>> loaded. Not doing this can have a negative impact on buses with
>> marginal signal quality.
>>
>> Signed-off-by: David Vrabel <david.vrabel@....com>
> 
> I'm not sure about this. I've seen several hosts which lack a
> mechanical detect switch.

If host is correctly designed it will have identical pull-ups on all the
DAT lines, and therefore this internal pull-up should be disconnected.

However, given that disabling the pull-up may cause 4 bit transfers to
fail on hardware that omits a host-side pull-up on DAT3 I agree that the
resistor should remain enabled.

Is there an example driver for a host that uses pin 1/DAT3 sensing for
card detection?  I'm curious about how removal detections work.

App Note on card detection goes into the use of this resistor in more
detail but I don't believe this document is publicly available :(.

David
-- 
David Vrabel, Software Engineer, Drivers group  Tel: +44 (0)1223 692562
CSR plc, Churchill House, Cambridge Business Park, Cowley Road, CB4 0WZ

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