【正文】 chip peripherals, brownout detection and protection, pin leakage current, and processing efficiency. Average Current ConsumptionIn lowpower designs, the average current consumption determines battery life. For example, if an application uses a battery rated at 400mAh, the application must draw less than 400mAh/8760h = of average current to provide one year of battery life. PowerDown ModesPowerdown modes are among the most important features enabling the MCU to meet the currentbudget. Lowpower MCUs have powerdown modes providing different levels of functionality. For example, the MSP430 provides five powerdown modes. Lowpower mode zero (LPM0) turns off the CPU and leaves everything else functional. Modes LPM1 and LPM2 add various clocking functions to the list of disabled functions. LPM3 is the most used lowpower mode, leaving only a lowfrequency clockoscillator running, and any peripheral that uses that clock. LPM3 is often called the realtime clock mode because it allows a timer to operate from a lowpower 32768Hz clock source, consuming1uA, and periodically wake the system for activity. Finally, LPM4 turns off all clocks on the device, thus turning off any peripheral that uses clocks automatically. Analog peripherals may still be active, but if none are, LPM4 current consumption is only 100 nanoamps, including RAMretention. Clocking SystemsThe clocking system is critical to MCU power consumption. Applications may enter and exit various lowpower modes several times or several hundred times a second. The ability to get into and out of the lowpower modes, and process data quickly, is crucial because current is wasted by the CPU waiting for the clock to bee stable. Most lowpower MCUs have “instanton” clocks that are ready for the CPU in less than 1020us. But it is important to understand which clocks are instanton and which are not. Some MCUs have a twostage clock wakeup providing a lowfrequency (usually 32768Hz) clock to the CPU while a highfrequency clock is being stabilized, which can take up to a millisecond or longer. On these devices, the CPU may be operational in about 15us, but running at a low, inefficient frequency or an incorrect highspeed frequency.The MSP430 provides a stable, highspeed clock to the CPU in less than 6 microseconds (often faster), requiring only about 9us for the same 25 instructions (6us wake + 25 instructions at .125us instruction rate) and enabling instanton highspeed serial munication and instant access to a timingaccurate clock source. InterruptsEventdriven capability goes handinhand with clock system flexibility. Interrupts bring an MCU out of a lowpower mode, so the more interrupts an MCU has, the more flexibility to prevent currentwasting CPU polling and reduce the power consumption. Polling can mean the difference between making the power budget and not, because it wastes CPU bandwidth and requires excess current while waiting for an event to occur. A good lowpower MCU will have extensive interrupt capability, providing interrupts for all its peripherals and many external interrupts for external events. The MSP430 provides interrupts for 16 I/O pins and all peripherals. Some peripherals, like Timer_A, Timer_B and ADC12 have multiple interrupts for total flexibility. PeripheralsPeripheral powerconsumption and powermanagement should also be considered when choosing a lowpower MCU. Some lowpower MCUs are retrofitted versions of 2030 yearold architectures that were not designed for low power. The MSP430 was designed for low power from the start and has lowpower features built into the peripherals. One thing to look for is the ability to individually enable or disable the peripherals when needed, or more importantly the automatic enabling or disabling of the peripherals. The MSP430 ADC12 is an example of such intelligent peripherals. If it is not actively converting, it draws no current. It automatically disables some its internal oscillator and digital circuitry when it is not converting, effectively turning itself off automatically. The require circuitry is then reenabled, or turned on, automatically if a conversion is triggered.Another key benefit peripherals can provide is interoperability. Brownout ProtectionMany MCUs have integrated brownout protection, resetting the MCU when the power supply dips below the normal operating range. For most, the brownout reset circuit requires an additional 10 – 70 uA of current for operation. The ability to enable or disable the brownout protection to save power consumption is often given, but brownout protection must be enabled 100% of the time to be useful because brownout cannot be predicted. This adds the brownout current consumption directly to the bottom line of the power budget of the system. Pin LeakageLeakage current is sometimes overlooked when choosing a lowpower MCU, but it must be considered for the most demanding lowpower applications. Most lowpower MCUs have specified input leakage currents of 1uA. This could consume up to 20uA for a 20input device! The MSP430 specifies pin leakage currents of 50nA – 1uA max for the same 20input device. Processing EfficiencyFinally, MCU processing efficiency is often misunderstood. It is often thought that 16bit MCUs require twice the memory of 8bit MCUs, but a 16bit architecture can actually require less code than an 8bit architecture and 16bit MCUs usually execute tasks more quickly. For example 8bit MCUs require CPU overhead to manage data in applications that have 10bit A/D conversion data or applications that require 16bit math. SummaryChoosing a lowpower MCU requires investigation beyond datasheet bullet points. But the effort pays off when battery life is extended, or the currentbudget is met.