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How microcontrollers add to battery life

Posted: 02 May 2005 ?? ?Print Version ?Bookmark and Share

Keywords:mcu? consumer device? adc? battery? stop1?

How many battery-powered devices have you used since waking up today? Whether they are toothbrushes, shavers, cellphones, PDAs, MP3 players and a remote control for anything not in arm's reach, battery-powered devices are part of everyday life. As such, power management is an issue for designers choosing an MCU for these applications, so it is important to take a look at some enhanced features finding their way onto the latest MCUs.

To demonstrate the effectiveness of these features, consider a wireless bicycle computer. This computer is composed of three modules: a control panel on the handlebars, a speed sensor on a wheel and a display on the rider's helmet. The speed sensor tells the control panel how fast it is turning. The control panel calculates information such as speed, distance covered, elapsed time and calories burned, and sends this data to the display. power consumption is a sum of all the components in the system. However, we will focus on the current used by the MCU.

Many designers equate low power with slow clock frequencies. However, depending on what the MCU is doing and what low-power modes are available on it, running at top speed can actually save power.

If the MCU has an efficient low-power mode, you will save the most power by spending the most time in this mode. So if the CPU needs to execute code before returning to sleep, running at the fastest possible speed to complete code execution and returning to low-power mode can burn less current than running constantly at slow speeds.

Of course, not every task the MCU has to perform will benefit from top-speed performance. In our bike computer example, the time required for the wireless communication would probably not require an 8MHz bus rate if the data is fairly slow. So in this case, to minimize power consumption, we want to run the MCU as slowly as possible until the wireless communication is complete. We need an MCU with flexible clocks.

More low-power modes

MCUs are moving into smaller geometries to reduce die size, which results in transistors that cannot tolerate direct application of three or more volts. Hence, voltage regulators are used to drop the voltage to the internal logic. Unfortunately, these regulators add to the MCU's current draw. But since power is equal to voltage times current, a 1.8V to 3V system with a regulator may still be lower in power than a 5V system without one.

MCUs rely heavily on power-management modes to keep the overall operating current down while still supporting regulated power supplies and increased clock speeds. New MCUs are providing multiple low-power modes to address these needs and maintain system flexibility. An MCU could have four modes, labeled as stop1, stop2, stop3 and wait.

In wait mode, power is reduced by turning off the CPU clock, but leaving the clocks enabled to other MCU peripherals such as ADCs, timers or serial communication modules. This mode is useful for saving power when these peripherals need to function, but the CPU has nothing to do until the peripheral completes its task. Wait mode could be used while communicating with the RF transceiver.

To reduce power consumption further, three stop modes can be used, each providing different levels of operation that will reduce power consumption.

Stop3 can provide the most functionality of the three by putting the on-chip voltage regulator into a power-saving mode that still provides minimum regulation for retaining RAM and I/O register contents. Several interrupt sources and the reset pin can wake the MCU from stop3. The crystal oscillator can remain enabled.

Stop2 provides slightly less functionality, but reduces power consumption further. In stop2, the voltage regulator is powered down. However, the RAM contents are retained.

Stop1 is the lowest-power mode of the MCU. It powers the voltage regulator down completely along with all peripherals, the CPU, RAM and I/O. Only the reset or IRQ pins can awaken the MCU. Stop1 is used when the MCU can be put into a powered-down state, but still needs to respond to an external stimulus such as the press of a button.

In the bike computer example, stop1 would be entered when the computer is powered down. Powering down with stop1 mode puts the MCU in the lowest possible power mode without actually removing power from the chip. Why not do that? Removing power from the chip requires a more expensive toggle switch to disable power.

Also, using a pushbutton switch tied to an interrupt pin allows the MCU to use the switch for multiple purposes, depending on the current state of the system. Stop1 mode is perfect for keeping the design simple and inexpensive, and yet consumes almost no current.

- Scott Pape

Systems Engineer

Freescale Semiconductor Inc.

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