Battery Life Calculator for Freescale Microcontrolers

We get many questions from customers asking what the IDD of a particular MCU is at a specific voltage/frequency/temperature combination. Obviously we cannot take data on all of MCUs across all possible combinations of operating conditions. So to help customers answer questions about power consumption under various conditions we created a battery life calculator.

 

How does it work?

This battery life calculator is a small executable program that allows the user to enter the operating conditions (voltage/frequency/temperature) along with battery capacity, the percentage of time the application spends in each of the MCUs modes (run, wait, stop3, stop2 and stop1), and a periodic wakeup interval. The calculator takes this information and determines the average current the MCU is consuming and an estimation of the resulting battery life.

 

MC9S08GB/GT Battery-life Calculator. Click to enlarge.

 

How does it calculate battery life?

The calculator was created by measuring currents on a few typical samples under many different conditions. The data results were analyzed and equations created for each mode of the MCU (i.e. run, wait, stop3, etc.) that fit the data as much as possible. The results of each equation are averaged together based on the percentages and wakeup interval the user provides. This resulting average current is then used to calculate the battery life based on the provided values of the battery capacity and an estimate of the battery self-discharge current for the type of battery. Capacities and self-discharge currents are provided for some common alkaline and lithium batteries.

 

Which devices are covered?

Currently, we only have a calculator for the MC9S08GB/GT 8-bit MCU family, but we are working on versions for the new QE128 Flexis Duo. The MC9S08GB/GT can be downloaded free of charge from:

http://www.freescale.com/files/microcontrollers/software/app_software/HCS08BATTCALC.zip?fpsp=1

 

-Scott

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