[docs] python-smbus and repeated starts

Zachary Ware zachary.ware+pydocs at gmail.com
Thu Jan 22 06:38:02 CET 2015


Hi Roger,

This is not the appropriate venue for usage questions such as this;
this list is meant for discussion of the Python documentation.  You'll
have better luck asking on the main Python mailing list,
python-list at python.org.

Regards,
--
Zach

On Tue, Jan 20, 2015 at 3:18 PM, Roger <roger at kistruck.co.uk> wrote:
> Greetings,
>
>         I am a moderately ignorant near-beginner with Python. I have a problem with trying to read a particular sensor chip attached to the i2c bus connections of a Raspberry Pi. The sensor is the MMA8451Q by Freescale Semiconductor; a well-established device with good performance and more features than I am likely to need in a long while.
>
> I have managed to read a different sensor (ADXL345) on the same i2c bus, using the standard tools available in Linux (I am using Raspbian on the Pi), so I have successfully used a handful of commands from the smbus repertory. I had to learn how to do it by copying from a previous piece of code written by someone else.
>
> Unfortunately for me, the MMA8451 _requires_ repeated start commands to read its data registers, and I cannot find any documentation that covers it. I have looked at the Python official documentation, and a search for 'smbus' or 'i2c' yields no search results at all. I have spent a good many hours thrashing through Google too, and also Github, and failed to find any useful leads. See :
>
> <http://wiki.erazor-zone.de/wiki:linux:python:smbus:doc>  and
> <http://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/plain/Documentation/i2c/smbus-protocol>  and
> <http://www.raspberrypi.org/forums/viewtopic.php?p=603520#603520>
>
> What seems to be missing is anything about the Python SMBus module to explain how it handles the crucial elements of the i2c protocol, and when it uses repeated-start signals in particular.
>
> It may be that I have not been very clever in finding what is there, but if so I would be glad to know.
>
> With thanks for any help you can give.
>
> Roger
> --
> Roger Kistruck
> 5 The Limes
> Long Melford
> Sudbury
> Suffolk
> CO10 9SX
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