The key requirements for a remote temperature-monitoring system include easy interrogation of the remote temperature monitor at any time, without interfering with the cargo or its environment; accurate temperature measurement; and low power consumption, to ensure long operation lifetime.

Such a temperature-monitoring system consists of two parts: the basestation and the remote monitor. In our case, a temperature monitor was implemented using an ADuC834 MicroConverter. The base unit consists of an RF transceiver, allowing wireless communication. The MicroConverter wakes up every 10 seconds to check whether it is time to take a temperature reading. If so, it will take the reading and will store the result in its own nonvolatile data memory.

After taking a temperature measurement, or if no temperature measurement is required, the monitor checks to see if the host is trying to communicate with it. If so, the monitor will stay awake to transmit and receive information as required by the host. Once finished communicating with the host, or if the host did not initiate communication with the temperature monitor within 10 ms of the temperature monitor's waking up, the monitor powers itself back down again.

The MicroConverter, operating at 1.57 MHz, consumes 1.5 mA, resulting in 7.5 mA being consumed over the 10-ms period. The power consumed during a temperature measurement every minute has a negligible effect on the average current. The data (up to 60 kbytes of temperature readings) is transmitted in packets at a rate of 57,600 bits/second. At this baud rate, the full 60 kbytes of data can be transmitted in just under 11 seconds.

Darragh Maxwell is applications engineer and Russell Williamson is Design Engineer for Analog Devices Inc. in Limerick, Ireland.