linux-kernel - Re: [PATCH] Bitbanging i2c bus driver using the GPIO API

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Message-ID: <20070309200850.GA8444@flint.arm.linux.org.uk>
Date:	Fri, 9 Mar 2007 20:08:50 +0000
From:	Russell King <rmk+lkml@....linux.org.uk>
To:	David Brownell <david-b@...bell.net>
Cc:	Haavard Skinnemoen <hskinnemoen@...el.com>,
	Jean Delvare <khali@...ux-fr.org>,
	Bryan Wu <bryan.wu@...log.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Deepak Saxena <dsaxena@...xity.net>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] Bitbanging i2c bus driver using the GPIO API

On Fri, Mar 09, 2007 at 11:30:12AM -0800, David Brownell wrote:
> On Friday 09 March 2007 10:48 am, Haavard Skinnemoen wrote:
> > This is a very simple bitbanging i2c bus driver utilizing the new
> > arch-neutral GPIO API. Useful for chips that don't have a built-in
> > i2c controller, additional i2c busses, or testing purposes.
> 
> That's the right idea!  But remember that not all GPIOs support
> reading back the actual value on SCL (it's an OUT pin, so lacking
> multidrive capability the values "should" be what you wrote), so
> getscl() support should depend on a flag in platform data.  In
> the same vein, if SCL is an output-only pin, you won't be able
> to change its direction ... but then, I'm not sure why you were
> changing its direction in setscl() rather than just its value.

That's a more correct I2C implementation.  If you read the specs, the
SDA and SCL signals are supposed to be driven by open-collector or
open-drain drivers, such that devices only pull the bus low.  Pull-up
resistors pull the signals high when undriven.

This avoids the possibility of damage caused when one device drives
a signal low and another device tries to drive it high.

Therefore, the correct I2C GPIO implementation is one where you drive
both SDA and SCL low by using a combination of the data direction
register and the output level register, but avoid driving the output
high.

-- 
Russell King
 Linux kernel    2.6 ARM Linux   - http://www.arm.linux.org.uk/
 maintainer of:
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