From mboxrd@z Thu Jan 1 00:00:00 1970 From: Russell King - ARM Linux Subject: Re: configuration for nc pins without pull Date: Sun, 22 Mar 2015 14:40:05 +0000 Message-ID: <20150322144004.GN8656@n2100.arm.linux.org.uk> References: <20150322100059.GH5664@pengutronix.de> Mime-Version: 1.0 Content-Type: text/plain; charset=iso-8859-1 Content-Transfer-Encoding: QUOTED-PRINTABLE Return-path: Received: from pandora.arm.linux.org.uk ([78.32.30.218]:34989 "EHLO pandora.arm.linux.org.uk" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751373AbbCVOkV (ORCPT ); Sun, 22 Mar 2015 10:40:21 -0400 Content-Disposition: inline In-Reply-To: <20150322100059.GH5664@pengutronix.de> Sender: linux-gpio-owner@vger.kernel.org List-Id: linux-gpio@vger.kernel.org To: Uwe =?iso-8859-1?Q?Kleine-K=F6nig?= Cc: Linus Walleij , linux-gpio@vger.kernel.org, linux-arm-kernel@lists.infradead.org On Sun, Mar 22, 2015 at 11:00:59AM +0100, Uwe Kleine-K=F6nig wrote: > Hello, >=20 > for a machine I want to configure a pin that is actually not connecte= d > to minimize floating. (I think this is sensible, isn't it?) Definitely. It's a design error to leave a pin floating, especially if they're MOS inputs (which most are). The reason is that many MOS inputs are a pair of MOS transistors - one from +V to their output (into the device) the other between their output and ground. If the input floats, then both transistors are partially turned on, which gives a wasteful flow of current between +V and ground - hence this increases the current draw and dissipation of the device - and is a total waste of energy. Generally, when designing MOS circuitry using standard gates, it is very much a design error to leave any unused gate inputs unconnected for this very reason - not only that, but the input is also sensitive to static electricity. A floating input has a very high input resistance, and a static discharge into it can destroy those input transistors I've mentioned above - turning *both* hard on, which gives you a dead short between +V and ground. That can result in the device effectively being fried, even if the fried input is not being used (because it drags the supply down.) =46loating inputs are *always* bad. (Just make sure that the designer hasn't already thought of this and tied the inputs high or low, possibly through a resistor. Even so, I'd say that it's good practice if the chip has biasing to bias them accordingly, so that should there be a bad joint on the board, it doesn't allow the pin to start floating. Bad joints happen, and sometimes show after a few months of otherwise good operation...) --=20 =46TTC broadband for 0.8mile line: currently at 10.5Mbps down 400kbps u= p according to speedtest.net. -- To unsubscribe from this list: send the line "unsubscribe linux-gpio" i= n the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html