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    SubjectRe: [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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