linux-kernel - Re: Accelerometer, Gyros and ADC's etc within the kernel.

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Message-ID: <20080527155641.GB29868@linux.intel.com>
Date:	Tue, 27 May 2008 08:56:41 -0700
From:	mark gross <mgross@...ux.intel.com>
To:	David Brownell <david-b@...bell.net>
Cc:	spi-devel-general@...ts.sourceforge.net,
	Jonathan Cameron <Jonathan.Cameron@...il.com>,
	linux-kernel@...r.kernel.org,
	LM Sensors <lm-sensors@...sensors.org>
Subject: Re: Accelerometer, Gyros and ADC's etc within the kernel.

On Wed, May 21, 2008 at 05:53:38PM -0700, David Brownell wrote:
> On Tuesday 20 May 2008, mark gross wrote:
> > > ST Micro LIS3L02DQ 3D accelerometer.  ...
> > 
> > FWIW: I have this device talking to a PIC-18 and pushing the results
> > over the serial port.
> 
> Jonathan neglected to mention that he's already sent a driver
> for this to the SPI list.  ;)
> 
> Which is part of the reason for asking this question.  Right
> now that driver sits in drivers/spi/lis3l02dq.c but that is
> probably not its best long-term domicile.
> 
> 
> > WRT linux support, I can't think of a generalized way to create a driver
> > that would be able to be used with this device in Linux.  You need to
> > know witch IRQ line the DR line is connected too, and if you are
> > bit-banging the SPI data off the thing, then you need to know which
> > GPIO's of the host CPU you'll be using.  If you have SPI hardware then
> > you need to know where to pull the data from.  The problem doesn't seem to
> > generalize well.
> 
> I guess I don't follow.  The SPI framework handles all that
> stuff already, even if you're bitbanging.  Though to be
> sure, if you're bitbanging SPI you want your platform to
> be able to inline those GPIO calls so those inner loops
> only take a few instructions per bit ... also solved!  :)
>

I'll take a look sometime soon.
 
> 
> > Also, If you are playing with accelerometer data, you likely need some
> > real time support or at lest a reliable time stamping of the data to do
> > anything interesting.
> 
> True.  I imagine a few other such issues will appear once
> more folk than Jonathan are using these sorts of sensors
> on Linux.  I'd expect the sample stream to have internal
> timestamps for truly critical systems, unless variability
> in when the host makes timestamps is not really an issue.
>  

Its on my list of things to do..

> 
> > Another problem area is around SPI itself.  There are variations of
> > device implementations around chip select polarity, clock biasing
> > (rising,falling, or midpoint) sampling from one SPI part to the next.
> 
> Midpoint?  That's not one I've come across before.  All four
> standard SPI clock/sample/shift modes are already supported
> in the SPI framework though.  Ditto active-high chipselects
> (vs normal active-low) etc.

Yeah, its one of the sampling modes for the PIC18F4455 its one of the
master mode sampling options  (see page 194 of
http://ww1.microchip.com/downloads/en/DeviceDoc/39632D.pdf )

--mgross

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