In principle, it’s simple: To get 20 years of life from a coin cell, a system needs to be asleep most of the time; it’s normal operating mode must be standby, with very short bursts of activity. In practice it requires some counterintuitive thinking: For example, the power that the system’s microcontroller consumes when it is not operating is often more important than its active processing power.
The basic idea behind “mostly asleep” systems is to minimize the active task, optimize the active code and increase the operating speed in active mode.
But, some other basics – “vectors” — also need attention: A lower supply voltage can help, as long as the component minimum voltage requirement is met; reducing the operating frequency may be prudent; transition times between active and low-power modes should be reduced, etc. Systems that use a combination of such optimizations, designed to minimize power consumption, are essential in remote sensing applications where battery replacement is difficult.
One such application: A system for physical presence detection that uses a Hall-effect sensor (to detect a magnetic field), married to an ultra-low-power microcontroller. When woken by a timer, the CPU powers up the sensor and if a magnet is detected, returns to a low power mode. If not, an alert is issued. Calculations show that the system, which consumes less than a microamp, can operate continuously for two decades from a common CR2032 coin battery.