max31855: add sensor and documentation on creation of a UPM sensor

Signed-off-by: Brendan Le Foll <brendan.le.foll@intel.com>

docs/max31855.md

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+Making a UPM module for MAX31855                         {#max31855}
+================================
+
+The Maxim Integrated MAX31855 is a thermocouple amplifier allowing you to read
+from a K type themocouple. My board comes from the Pmod kit form Maxim
+(MAX31855PMB1) but you can get this from many different sources. The adafruit
+people made arduino code already so we'll use that as a
+[reference](https://github.com/adafruit/Adafruit-MAX31855-library/blob/master/Adafruit_MAX31855.cpp).
+
+### Basics
+
+This is a spi module so we will use the maa spi functions to build our module.
+First thing to do is to create a tree structure like this in upm/src/max31855:
+
+* max31855.cxx
+* max31855.h
+* jsupm_max31855.i
+* pyupm_max31855.i
+* CMakeLists.txt
+
+And then an example file to use & test our lib with in upm/examples/max31855.cxx.
+
+### Swig
+
+The .i files are used by swig, there is one for each python & javascript. They
+contain essentially the same thing and are very simple. The only thing to
+change between the javascript & node.js one is the argument to %module.
+
+@snippet jsupm_max31855.i Interesting
+
+The %include parameter defines which functions will be available to the
+node/python module created, Whilst the headers inside %{} will be explicitly
+required during compilation. Typically only the top level header is required in
+either of those args.
+
+### API
+
+Then we create the header (max31855.h) , a very simple header in our case we
+will have only a very basic api. We provide a getTemp() function which will
+return the same type as in the arduino library, a double.
+
+@snippet max31855.h Interesting
+
+Note that the header contains both the io that we will use, the gpio is in this
+case used as the chip select pin.
+
+### Implementing our API
+
+In the adafruit library the read function (our chip is a 3pin SPI so only read
+is possible), the spiread32() does all the work. It starts by setting up the io
+so we will do the same in our constructor.
+
+Note unlike on Arduino, we'll just set a 2Mhz clock and let the chip do the
+work.
+
+@snippet src/max31855/max31855.cxx Constructor
+
+Then we also need to implement a nice cleanup in our destructor.
+
+@snippet src/max31855/max31855.cxx Destructor
+
+Then to read data, we will use spi_write_buf which will allow us to write a
+whole uint32_t in order to get one back, which is what the arduino code does in
+spiread32. Obviously we set our chip select to low first. Here is the start of
+the implementation of MAX31855::getTemp()
+
+@snippet src/max31855/max31855.cxx spi
+
+Then using the arduino code as reference we simply reconstruct form the 4
+uint8_t values a 32bit int value and select only the valuable parts of
+information from that. The MAX31855 datahseet explains exactly which bits are
+useful, we will just do the same as the adafruit code, first checking the error
+bit and then scrapping everything but the 14bit of thermocouple data that are
+useful to us and converting it to a double.
+
+@snippet src/max31855/max31855.cxx conversion
+
+### Finalizing
+
+Our final example, very easy to use api!
+
+@snippet examples/max31855.cxx Interesting
+
+### Building
+
+The we need to add it to the examples/CMakeLists.txt. Only three lines are required
+
+~~~~~~~~~~~
+add_executable (max31855-example max31855.cxx)
+include_directories (${PROJECT_SOURCE_DIR}/src/max31855)
+target_link_libraries (max31855-example max31855 ${CMAKE_THREAD_LIBS_INIT})
+~~~~~~~~~~~
+
+Note you dont have to rebuild everything, cmake keeps target lists so if you
+named your example target <modulename>-example you can simply do make
+max31855-example and both the library & example will build.