Friday, 19 May 2017

i.mx6sx - One Wire Digital Temperature gauge using DS18B20 + UDOO Neo + LCD

The challenge here was to implement the Dallas one wire protocol so that two DS18B20 sensors could be wired to the cortex M4 to form the basis for a temperature gauge. I recycled the LCD display from the previous post to display temperature readings using 'ring meter' widgets. The end result is that the M4 is used for reading DS18D20 sensors and updating the 'ring meter' widgets. In the video we have one sensor in a 3 pin TO-92 package reading room temperature and the other as a waterproof probe dipped into hot/cold water.

The DS18D20 one wire protocol simply requires a gpio pin that can be toggled between input and output modes albeit using precise timings to the microsecond (between 1 and 480 microseconds). The protocol and timings are described in the DS18B20 datasheet. To achieve the necessary level of time precision we utilised the imx6 Enhanced Periodic Interrupt Timer (EPIT) timers sourced from a 24Mhz clock. Since there are two DS18B20 sensors on the one wire bus we first query for the address of the sensors and then in turn poll for the temperature from each which takes approximately 480 microseconds.

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