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Smart embedded MCUs help attain energy efficiency

Posted: 04 Jan 2013 ?? ?Print Version ?Bookmark and Share

Keywords:microcontrollers? battery? low-power mode?

Unfortunately, most applications involve events of the less predictable type, and require the MCU to respond in real timeor at least almost real time. Examples include threshold/alarm conditions, power interruptions, and wireless protocols that can transmit and receive at random intervals. In such situations, failure to wake within an appropriate timeframe may mean losing critical data or failing to respond to a user command. In the worst case, there may be a risk of system damage if an alarm condition is not dealt with in a timely fashion.

The task for the designer is therefore to find the lowest-power sleep mode that provides adequate response to anticipated (if unscheduled) events. Energy Micro's EFM32 Gecko family of energy-optimised 32bit MCUs provides a good example of the options available. Although the Gecko architecture is optimised to perform beyond the basic requirements of each mode, most MCUs used for these types of applications have similar operating states and characteristics (figure 1).

Figure 1: The EFM32 family's energy mode transitions (most alternative architectures combine EM2 and EM3 in a single 'deep sleep' mode).

At the lowest level, most MCUs have a mode designated as "off" or shut-down mode, that preserves the minimum functionality needed to trigger wake-up from an external stimulus: in our example of the Gecko series, this is designated EM4. In this mode, the entire device is powered down, other than the interrupt monitoring circuitry on the reset pin and GPIO pin wake up. The EFM32 draws around 20nA of current in this mode, although a typical 32bit MCU would require nearer 1.5?A.

Restarting from such an "off" mode is essentially a device reset, a process that for the EFM32 takes around 160?s. Main memory contents will have been lost and must be re-loaded. Some processors include a small (512byte in the case of the EFM32) block of memory whose content is preserved for use on start-up.

A kind of 'mezzanine' state above EM4 preserves a few more critical functions, in particular the real-time clock and 512B of backup memory. Drawing only 400nA, this state would consume the capacity of a pair of AA cells in roughly seven years. Although intended primarily for use in the event of power supply failure, it can be an excellent alternative when RTC and a faster wake-up time are desirable.

With more functionality still, the 'stop' mode (EM3 in figure 1) enables a limited degree of autonomous peripheral activity and faster wake-up. Here, the high- and low-frequency oscillators are disabled, but the MCU's full RAM and configuration register states are preserved. In addition to the elements active in EM4, the power-on reset and brown-out detector are active, and the CPU can be woken up by an asynchronous external interrupt or via a number of internal sources, such as the device's analogue comparators (ACMP) and pulse counter (PCNT).

For a designer, the key specifications to note in this mode are the time to wake up C which should be of the order of a few microseconds C and the absolute power consumption (10 to 30?A for a typical MCU, 590nA for a best-in-class device such as the EFM32). Perhaps the most important point for the designer to remember is that some MCUs do not include full internal memory retention in this mode.

The deep sleep mode (EM2 in figure 1) leaves all of the MCU's critical elements active, while disabling high frequency system clocks and other non-essential loads. In addition to the EM3 functionality, the 32kHz oscillator used to clock on-chip peripherals remains active. This allows selected low-energy functions including the RTC, watchdog timer, and some external interfaces to remain active.

As in EM3, the designer needs to take care, since not every MCU series provides full register and RAM retention. Choosing a device with this capability allows the device to return to active state and resume program execution quickly. Best-in-class figures for current draw in this mode can be as low as 900nA (with RTC running from a precise clock source) [see footnote], with time to return to active mode of as little as 2?s.

Finally, in sleep/standby mode (EM1), the MCU's pointer and configuration register states are fully preserved, eliminating the need to save them on power-down, and restore them on power-up. This typically saves hundreds to thousands of instruction cycles for each wake-up. The high-frequency oscillator remains active with the CPU clock tree disconnected, allowing the device to resume execution on the next clock cycle after a wake-up event. High-frequency peripherals (for instance direct memory access (DMA), analogue/digital and digital/analogue converters (ADC/DAC) and hardware encryption) remain active. Sleep mode current draw ranges from 45?A/MHz to 200?A/MHz, depending on the choice of MCU.

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