Portable, battery-powered devices are sweeping through society like wildfire. Mobile computing and sensor devices are springing up everywhere, providing engineers with not only a plethora of data but also applications. Requirements often dictate constraints on size and weight that limit how much capacity the battery can carry. The number of features on devices in addition to the time between charges make it very challenging, to near-impossible, to meet the requirements.
Selecting a low-power microcontroller is an obvious first step, but there are a number of software and hardware tips that can be followed to ensure that every last milliampere-hour of charge is put to good use.
Tip #1: Create a battery budget
Early in the design cycle it is highly recommended that you put together a battery budget. Current requirements for each device on the board can be tallied to obtain a rough idea of how much battery current is going to be needed and whether the selected battery is up to the job. Device data sheets have become fairly good at providing minimum, typical, and maximum current data.
Taking a very conservative approach, a battery budget could be based solely on the maximum current values for the devices. An Excel worksheet is easy to duplicate, however, and creating a budget for both typical and maximum current data will produce a good ballpark range.
If more battery is needed than is available, please don’t just move forward on the project. Make the necessary changes up front to spare weeks or months of heartache down the road. Figure 1 shows an example battery-budget template that can be downloaded from http://bit.ly/17XMtfe.
Figure 1 Early in the design cycle, it is highly recommended that you put together a battery budget.
Tip #2: Set unused MCU I/O to lowest power state
It is easy to overlook what should be done with an input/output pin that is not being used. This oversight, however, can