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

Eric Bénard eric at eukrea.com
Wed Sep 5 08:22:44 EDT 2012

Le Wed, 5 Sep 2012 13:56:46 +0200,
Thomas Petazzoni <thomas.petazzoni at free-electrons.com> a écrit :

> 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?
in your case maybe but that really depends on how the chip is wired to
the CPU so I'm not sure that can be a generic choice.

> > > 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.
that's a reason why the chip select can be a simple gpio (even if
it's wired on a SPI chip select capable pin)


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