Flash in battery-powered applications

One advantage Freescale microcontrollers have over many of our competitors is that our flash memories can be programmed and erased at low voltages. On most low voltage 8-bit S08 and 32-bit ColdFire® MCUs (the ones specified to run from 1.8V to 3.6V), the flash memories can be written and erased across the entire operating voltage range.

This provides several advantages for battery-powered applications:
· No worries about battery level for user-programmable applications,
· No worries about battery level for firmware updates
· Ability to save the current RAM/register states when the battery gets close to its end of life.

User-programmable applications
It is fairly easy to see the advantage of programmability across battery-life in a user programmable application. A programmable remote control is a good example of such an application. With no limitations on battery voltage, users can add or change functions on their remotes whenever needed.

If the MCU places limitations on programming voltage, then the user has to change to fresh batteries when adding the new DVD player to the master remote. Alternatively, the remote has to maintain the settings in RAM, meaning the user has a limited amount of time to replace batteries when they are drained.

Firmware updates
Let’s take the example of a monitored home security system that includes battery-powered wireless sensors throughout the house. Suppose the manufacturer of the system has a software upgrade to improve the security of the wireless transmissions. If the MCUs used in the battery-powered sensors cannot re-write the flash across the entire battery voltage range, then the company that installed the system either has to contact the homeowners to instruct them to install fresh batteries in every sensor or send a technician out each home to perform the update.

Using MCU’s that can rewrite flash across the entire battery voltage range, the upgrade can be made without concern for the current voltage output of the batteries. The manufacturer can therefore provide a remote way to upgrade the firmware through the monitoring system without having to contact the homeowner, making the process much more efficient and reliable.

Saving the current state
The last advantage I want to discuss is saving the current state of the system. Some smoke detectors and carbon monoxide detectors will keep a running count of their sensor readings in RAM to keep track of the highest readings for the last hour, day, week, etc.

With a voltage-limited flash, these values can only be saved into flash when the detectors have relatively fresh batteries or when AC line powered. If the power goes out or the batteries aren’t replaced before going dead, the RAM contents will be lost.

With a flash that programs across operating voltage, the RAM contents can be saved as the batteries get near end-of-life or if the power goes out and backup power is available from a capacitive storage.

-Scott