Sunday 27 October 2013

Magnetometer progress report: link indicator LED

I recently added a new feature to the magnetometer remote sensor unit firmware. To help indicate when it is within radio communication range of the base unit an LED is turned on at the start of a message communication. When the sensor unit receives the acknowledgement from the base unit of successful message receipt the LED is switched off. In normal operation the LED should blink briefly every 30 seconds, after each sampling interval. If the LED remains on it indicates a problem with the radio link. For the initial installation the sampling time can be reduced to 5 seconds for to obtain faster feedback of whether communication is successful or not.

Improvements for battery-powered operation

Indicating link errors by using an LED is convenient during installation but the power wasted after installation is not compatible with battery-powered operation. To save power the LED is used only the first 15 minutes of operation. Only user-initiated reset actions (power on or reset switch pressed) cause the LED to be used, resets from by the watchdog timer or brown-out detector are ignored. The cause of the reset is detected by examining the microcontroller's status register (MCUSR).

Communication timeout feature added

I've also added a timeout which detects when communication has been lost for an extended period. The microcontroller system is rebooted in the hope that the error is recoverable. Loss of communication, along with low battery status, can also be sensed and reported by running the latest version of the data plotting software on the Raspberry Pi. If either error is detected a message can be sent via email, Twitter or Facebook.

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