To Thread or not to Thread....?

[Jack Keegan]

Hi there,

I'm currently writing an application to control and take measurements during
an experiments. This is to be done on an embedded computer running XPe so I
am happy to have python available, although I am pretty new to it.
The application basically runs as a state machine, which transitions through
it's states based on inputs read in from a set of general purpose
input/output (GPIO) lines. So when a certain line is pulled low/high, do
something and move to another state. All good so far and since I get through
main loop pretty quickly, I can just do a read of the GPIO lines on each
pass through the loop and respond accordingly.
However, in one of the states I have to start reading in, and storing frames
from a camera. In another, I have to start reading accelerometer data from
an I2C bus (which operates at 400kHz). I haven't implemented either yet but
I would imagine that, in the case of the camera data, reading a frame would
take a large amount of time as compared to other operations. Therefore, if I
just tried to read one (or one set of) frames on each pass through the loop
then I would hold up the rest of the application. Conversely, as the I2C bus
will need to be read at such a high rate, I may not be able to get the
required data rate I need even without the camera data. This naturally leads
me to think I need to use threads.
As I am no expert in either I2C, cameras, python or threading I thought I
would chance asking for some advice on the subject. Do you think I need
threads here or would I be better off using some other method. I was
previously toying with the idea of using generators to create weightless
threads (as detailed in
http://www.ibm.com/developerworks/library/l-pythrd.html) for reading the
GPIOs. Do you think this would work in this situation?
Another option would be to write separately programs, perhaps even in C, and
spawn these in the background when needed. I'm a little torn as to which way
to go. If it makes a difference and more in case you are wondering, I will
be interfacing to the GPIOs, cameras and I2C bus through a set of C DLLs
using Ctypes.

Any help or suggestions will be greatly appreciated,

Thanks very much,

Jack
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