The CM3-Home full edition includes a YARM D21 module. It gives to the CM3-Home the ability to act as a gateway between the 868MHz free communication band and all the other protocols available on the board.
As shown on the schematic diagram four of the YARM's Microchip SAM D21 MCU GPIOs are accessible from outside. Two of them can be used as analog inputs, other two as an I2C bus additional to the one directly attached to the CM3 module.
This article explains how to program the SAM D21 MCU and how it can be driven by the CM3-Home main processor through a dedicated serial line.
The analog inputs are pulled-up to the 3.3V with a 3.3kΩ resistor. Therefore these ports, acting as a resistor divider, can be used to measure an external resistor.
A typical usage for this kind of interface is to control a line against tampering. A sensors line for an antitheft alarm is usually balanced, meaning that it has a specific resistance in the idle condition. This resistance changes if the line is tampered, both through shortening or cutting it. Measuring the difference from the idle condition we can know if the line has be cut, shortened or if an alarm condition occurs. The resistor used for a single balanced line is commonly imposed by the manufacturer. For double balancing lines it's a little bit more complicated and the idle value resistance is often auto-calibrated during the initial setup.
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