1 Making a UPM module for MAX31855 {#max31855} 2 ================================ 3 4 The Maxim Integrated MAX31855 is a thermocouple amplifier allowing you to read 5 from a K type thermocouple. My board comes from the Pmod kit form Maxim 6 (MAX31855PMB1) but you can get this from many different sources. The adafruit 7 people made arduino code already so we'll use that as a 8 [reference](https://github.com/adafruit/Adafruit-MAX31855-library/blob/master/Adafruit_MAX31855.cpp). 9 10 ### Basics 11 12 This is a spi module so we will use the mraa spi functions to build our module. 13 First thing to do is to create a tree structure like this in upm/src/max31855: 14 15 * max31855.cxx 16 * max31855.h 17 * jsupm_max31855.i 18 * pyupm_max31855.i 19 * CMakeLists.txt 20 21 And then an example file to use & test our lib with in upm/examples/max31855.cxx. 22 23 ### Swig 24 25 The .i files are used by swig, there is one for each python & javascript. They 26 contain essentially the same thing and are very simple. The only thing to 27 change between the javascript & node.js one is the argument to %module. 28 29 @snippet jsupm_max31855.i Interesting 30 31 The %include parameter defines which functions will be available to the 32 node/python module created, Whilst the headers inside %{} will be explicitly 33 required during compilation. Typically only the top level header is required in 34 either of those args. The upm.i is just a shortcut to include some commonly 35 used swig wrappers for UPM sensors, it's not obligatory but recommended. 36 37 ### API 38 39 Then we create the header (max31855.h) , a very simple header in our case we 40 will have only a very basic api. We provide a getTemp() function which will 41 return the same type as in the arduino library, a double. 42 43 @snippet max31855.h Interesting 44 45 Note that the header contains both the io that we will use, the gpio is in this 46 case used as the chip select pin. 47 48 ### Implementing our API 49 50 In the adafruit library the read function (our chip is a 3pin SPI so only read 51 is possible), the spiread32() does all the work. It starts by setting up the io 52 so we will do the same in our constructor. 53 54 Note unlike on Arduino, we'll just set a 2Mhz clock and let the chip do the 55 work. 56 57 @snippet src/max31855/max31855.cxx Constructor 58 59 Then we also need to implement a nice cleanup in our destructor. 60 61 @snippet src/max31855/max31855.cxx Destructor 62 63 Then to read data, we will use spi_write_buf which will allow us to write a 64 whole uint32_t in order to get one back, which is what the arduino code does in 65 spiread32. Obviously we set our chip select to low first. Here is the start of 66 the implementation of MAX31855::getTemp() 67 68 @snippet src/max31855/max31855.cxx spi 69 70 Then using the arduino code as reference we simply reconstruct form the 4 71 uint8_t values a 32bit int value and select only the valuable parts of 72 information from that. The MAX31855 datasheet explains exactly which bits are 73 useful, we will just do the same as the adafruit code, first checking the error 74 bit and then scrapping everything but the 14bit of thermocouple data that are 75 useful to us and converting it to a double. 76 77 @snippet src/max31855/max31855.cxx conversion 78 79 ### Finalizing 80 81 Our final example, very easy to use API! 82 83 @snippet examples/c++/max31855.cxx Interesting 84 85 ### Building 86 87 The we need to add it to the examples/CMakeLists.txt. Only three lines are required 88 89 ~~~~~~~~~~~ 90 add_executable (max31855-example max31855.cxx) 91 include_directories (${PROJECT_SOURCE_DIR}/src/max31855) 92 target_link_libraries (max31855-example max31855 ${CMAKE_THREAD_LIBS_INIT}) 93 ~~~~~~~~~~~ 94 95 Note you don't have to rebuild everything, cmake keeps target lists so if you 96 named your example target modulename-example you can simply do make 97 max31855-example and both the library & example will build. 98
