From mboxrd@z Thu Jan 1 00:00:00 1970 From: eric@eukrea.com (Eric =?ISO-8859-1?B?QuluYXJk?=) Date: Wed, 5 Sep 2012 14:22:44 +0200 Subject: [PATCH 5/8] gpio: 74x164: Add output pin support In-Reply-To: <20120905135646.3bbf98fa@skate> References: <1346834457-6257-1-git-send-email-maxime.ripard@free-electrons.com> <1346834457-6257-5-git-send-email-maxime.ripard@free-electrons.com> <20120905112012.13ce2c86@skate> <20120905114645.57bfe591@eb-e6520> <20120905120906.12d08bf5@skate> <20120905122652.7c752e96@eb-e6520> <20120905135646.3bbf98fa@skate> Message-ID: <20120905142244.003e5e4b@eb-e6520> To: linux-arm-kernel@lists.infradead.org List-Id: linux-arm-kernel.lists.infradead.org Le Wed, 5 Sep 2012 13:56:46 +0200, Thomas Petazzoni a ?crit : > Le Wed, 5 Sep 2012 12:26:52 +0200, > Eric B?nard a ?crit : > > > Le Wed, 5 Sep 2012 12:09:06 +0200, > > Thomas Petazzoni a ?crit : > > > 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) Eric