Utilizing New Low-Power Modes of Advanced MCUs

The requirements for low-power microcontrollers (MCUs) vary depending on the application and how the MCU is used in the application. For example, in a battery-powered thermostat application, low power is primarily defined by the lowest power mode in which the device can drive an LCD display, in which case reducing power consumption will extend the life of the battery. In other low-power applications, such as electric meters, low power refers to the operating current consumed by the system during operation. A third category of systems are those that need to keep time regardless of whether the main power supply to the system is present. An electric meter during a power outage is an example of a third category of systems. Because the requirements of various applications vary, MCUs with more flexible power modes allow designers to further customize system operation.

In the past, MCU operating modes were used for device operation; Idle and Doze modes reduced or eliminated CPU switching power while allowing peripherals to operate; Sleep mode allowed limited peripherals to operate at the lowest power consumption. As today's advanced MCUs move to more advanced silicon processes (which minimize system cost and reduce operating current), some new low-power modes are being added to increase the flexibility of the MCU. We will explore some of the new low-power modes in today's advanced MCUs by observing their use in various applications.

We will compare various power modes implemented in different applications using the Battery Life Estimator (BLE) software tool and a 16-bit MCU. Microchip's BLE is a free software tool that allows designers to estimate the battery life of a system and determine which of the available operating modes is best for their application. The PIC24FJ128GA310 MCU family offers some new low-power modes, and its LCD display driver can play a good role in some of the following examples.

Thermostats have become more complex, needing to display more information and cover multiple regions. As a result, large amounts of on-chip Flash program memory are often required to store complex menus displayed in multiple languages.

Generally speaking, it takes advanced processes to produce a competitively priced, large memory MCU. As semiconductor processes advance, the trend is toward lower operating currents and higher leakage currents for transistors. The increased leakage current is most evident in the current specifications for low power modes, such as sleep mode. Sleep currents on advanced MCUs are typically in the 3 to 5 uA range, and a typical thermostat application spends most of its time simply driving a segmented LCD display. Segmented LCD displays are typically driven in sleep mode, which allows certain peripherals (in this case, the LCD driver) to run while the CPU and most peripherals are powered down. The thermostat must periodically wake up and enter operating mode to read the temperature, update the display, and perhaps signal the furnace, fan, or air conditioning unit to turn on. However, more than 99% of the time, sleep mode is all that is needed. Because of the large amount of time spent in sleep mode, improving sleep current can significantly increase the battery life of the system.

To enable MCUs with sub-uA power modes, many vendors have introduced new low-power deep sleep modes. Typical deep sleep currents are in the 10 to 50 nA range, and these devices add 400 nA when running a real-time clock calendar (RTCC). Extremely low currents are achieved by shutting down the entire device and only retaining a small amount of memory, the real-time clock, and perhaps a watchdog timer. However, these deep sleep modes do not allow peripherals to run or retain data RAM on the device. When the device wakes up from deep sleep, if the RAM contents are lost, the device needs to perform a restart routine before resuming program execution.

New low-power modes, such as low-voltage sleep mode, retain the device's data RAM with a typical base current of 330 nA and allow additional low-power peripherals to run. This low-voltage sleep mode retains the device's RAM and reduces the sleep current by reducing the device's on-chip regulator output. By reducing the supply voltage to the device logic and limiting the operating peripherals, the MCU's sleep current can be reduced from 3.7 uA to 330 nA. In this type of MCU sleep mode, peripherals such as LCD drivers, timers, and RTCC can still operate with only minimal current addition. The device resumes from low-voltage sleep mode to an operational state in less than half the time it takes to wake up from deep sleep. The device then begins executing from the next instruction instead of starting from the restart sequence that is usually required to wake up from deep sleep mode.

Figure 1: Battery Life Estimator Tool Main Screen

Description - As shown in Figure 1, the main screen of the Battery Life Estimator tool shows the MCU and its operating voltage, battery, and operating mode. The estimated battery life for the thermostat model is 11 years and 88 days.

In addition, the BLE tool models the amount of time the MCU will spend in each operating mode and the amount of power it will consume in each mode. Figure 1 shows the BLE output display where you can set several key parameters of the system to get lifetime estimates and average system current. First, select the operating voltage of the MCU and the system. This allows the battery life estimator to obtain the appropriate specifications. Then select the battery or battery pair - in this case, 2 AAA alkaline batteries. You can also select the expected system operating voltage and operating temperature to obtain the specifications that are best suited for the battery life estimation model. Finally, define the operating modes that will be used in the system. For our thermostat, two modes will be used.

In order to model the time that the thermostat displays only the LCD screen, an operating mode called "Display LCD" was created. The "Display LCD" operating mode uses a low-voltage sleep mode to provide the lowest power mode for driving the LCD. The Battery Life Estimator Tool models the device as having an operating cycle of 30 seconds, with 29.5 seconds in low-voltage sleep mode. The second operating mode, Update Temperature and LCD, models the time required for the MCU to monitor the temperature, update the LCD screen, and communicate with the HVAC unit. [page]

To better appreciate the new low voltage sleep mode and how to implement the operating mode in the BLE tool, the Add/Modify Mode screen can be viewed, as shown in Figure 2. In this screen, the designer can adjust the Duration setting, which is currently set to 29.5 seconds. Using the Additional System Current input box, the designer can add an estimate of the current consumption of the MCU's peripheral circuits. In this case, 4 uA of system current is added to represent the current consumed by the LCD display, and 1 uA of additional current is added to represent the current required by the internal LCD bias resistors. Next, the power mode is selected (low voltage sleep in this case) and the required peripherals. In order to provide an accurate system current model, the LCD driver, BOR, WDT, and RTCC have been selected. The MCU itself consumes 1.88 uA, which is added to our 5 uA system current to reach the 6.88 uA required by the system in low voltage sleep mode.

Figure 2: Battery Life Estimator Tool Mode Edit Screen

Description: The mode edit screen of the Battery Life Estimator tool allows the designer to specify the conditions for each power mode used and give it a name.

The BLE main screen shows that the device consumes an average current of 6.88 uA in low voltage sleep mode, and a little over 327 uA when the device is in short-term active state, so the total average current is less than 6.9 uA. The estimated value of the system battery life is about 12 years, which is about 5 years longer than the shelf life of the battery. Figure 3 shows a similar analysis, but using sleep mode instead of low voltage sleep mode, which gives an average current of about 10.5 uA, which reduces the battery life by three years.

Figure 3: Battery Life Estimator Tool

Note: The estimated battery life based on Hibernation mode is three years less than the estimated battery life based on Standard Hibernation mode.

At the other extreme for an MCU is a system that spends most of its time in active mode, such as an electricity meter. Today’s electricity meters are in only two states throughout their operating cycle. They are in normal operating mode when powered on. In this “normal” operating mode, the MCU is active, constantly measuring voltage and current, and calculating the power being delivered through the meter. The meter may also monitor for potential tampering, drive an LCD display, and possibly communicate with meter reading facilities.

When the meter is operating, it may appear that there is plenty of power. In reality, power is a product provided by the meter manufacturer’s end customer, the utility. Utilities provide power to millions of customers, and even small power losses can be costly to the utility’s business. In reality, most meters must operate within the 10 VA power budget established by the IEC. If possible line variations, component tolerances, and system design margins are taken into account, the final result is a system MCU current budget of approximately 10 mA when using a capacitive power supply.

Some of today's low-cost electricity meters use 8-bit MCUs that typically consume more than 10 mA when running at full speed in active mode. In order to stay within the system power budget, designers are often required to run the MCU at a lower frequency. Many current 16-bit MCUs use advanced process and design techniques to provide typical operating currents as low as 150 uA/MHz and can run at full speed at 16 MIPS while consuming no more than 6.9 mA. The reduced operating current provides designers with two options: reduce the operating speed of the MCU to reduce system power consumption; or add additional functionality while keeping the system power consumption within the allocated budget.

Although an electricity meter spends the vast majority of its time in an operating state, it is also an example of an application that takes advantage of one of the lowest power modes (Vbat). The Vbat feature provides a dedicated pin that can provide a backup power source, such as an LTC battery or supercapacitor. If the main power source to the system is lost (such as during a power outage), the power source for the RTCC automatically switches to the backup Vbat pin. RTCCs in electricity meters are very important during power outages, as time-of-use billing is becoming more and more popular. When operating from Vbat, the RTCC allows the LTC battery to last for decades, providing virtually unlimited backup operating power. Using the Vbat feature with an RTCC is not limited to electricity meters. Many applications, including the thermostat mentioned above, can take advantage of the RTCC to keep time during a power outage or battery replacement. Vbat with a capacitor or battery can also help eliminate annoying flashes caused by power outages.

In an environment where power consumption is of great concern, the development of low-power MCUs has led to the emergence of highly flexible general-purpose MCUs. Advances in process technology and design techniques have enabled 16-bit MCUs to operate at currents as low as 150 uA/MHz. New low-power modes (such as low-voltage sleep and Vbat) add flexibility to the power management chain, allowing general-purpose MCUs to work in a wider range of applications. The end result is a powerful and adaptable microcontroller that enables customer-friendly, energy-efficient terminal applications.