[PATCH 5/8] gpio: 74x164: Add output pin support

Thomas Petazzoni thomas.petazzoni at free-electrons.com
Wed Sep 5 07:56:46 EDT 2012

Le Wed, 5 Sep 2012 12:26:52 +0200,
Eric Bénard <eric at eukrea.com> a écrit :

> Le Wed, 5 Sep 2012 12:09:06 +0200,
> Thomas Petazzoni <thomas.petazzoni at free-electrons.com> a écrit :
> > I don't think the /OE pin can be used as a chip select, because when it
> > is high, the state of the output pins is not maintained to their
> > previous state: the pins are turned into the high impedance state. If
> > used as a chip select, it would mean that the value of the output pins
> > transition from their old state to high impedance at the beginning of
> > the transfer, and then from the high impedance to their new state at
> > the end of the transfer.
> OK that's exactly what I was thinking to ;-)

Good. So, do you think it's reasonable to use the STCP as a chip-select
for this device?

> > And of course, this signal is inverted compared to a normal chip select.
> polarity of the chip select can vary (mc13xxx-spi has an active high CS
> for example).

Sure, but I guess it depends on capabilities of the SPI controller in
your SoC. I haven't seen a way of configuring the polarity of the chip
selects in the i.MX28 SPI controller, but maybe I've missed it.
Anyway, /OE isn't usable as a chip-select, so we don't care.

Best regards,

Thomas Petazzoni, Free Electrons
Kernel, drivers, real-time and embedded Linux
development, consulting, training and support.

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