Taking advantage of new low-power modes on advanced microcontrollers
To get a better feel for the new low-voltage sleep mode and how an operational mode is implemented in the BLE tool, we will look at its Add/Modify Mode screen, as shown in Figure 2. From this screen, a designer can adjust the settings for the duration, which is currently set to 29.5 seconds. By using the Additional System Current entry box, designers can add an estimated current consumption for the currents that surround the MCU. In this case, 4 µA of system current has been added to represent the current consumed by the LCD display, and an additional 1 µA of current has been added to represent the current required for the internal LCD bias resistors. Next, the power mode is selected, in this case low-voltage sleep, and the required peripherals. To provide an accurate model of the system current, the LCD Drive, BOR, WDT and RTCC have been selected. The total system current consumed by the MCU itself is 1.88 µA, which is added to our 5 µA system current to arrive at the 6.88 µA required by the system while in low-voltage sleep mode.
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Figure 2: The Battery Life Estimator tool’s mode-edit screen allows a designer to name and specify the conditions of each power mode used.
The BLE main screen shows that an average 6.88 µA is consumed while the device is in low-voltage sleep mode and just over 327 µA is consumed during the short time the device is in an active state, for an average current of less than 6.9 µA. The estimated battery life for the system is almost 12 years, or almost 5 years beyond the shelf life of the batteries. A similar analysis using the sleep mode rather than the low-voltage sleep mode is shown in Figure 3, and results in an average current of approximately 10.5 µA and a three-year reduction in the battery life.
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Figure 3: A battery life estimate based upon the use of a sleep mode shows a three-year reduction in battery life, using a standard sleep mode.
At the opposite extreme for an MCU is a system that would spend most of the time in an active mode, such as an electricity meter. Today’s electricity meters spend all of their time in one of two states. The normal operating mode occurs when electric power is available. In this mode, the MCU is active and is constantly measuring the voltage and current and calculating the power being sourced through the meter. The meter may also be monitoring for potential tampering, driving an LCD display, and communicating with the meter-reading infrastructure.
While the electricity meter is running, it may seem that power is abundant. In reality, the power is the product being supplied by the electric utility - the end customer of the meter manufacturer. The electric utility company is supplying millions of customers with power, and even a small power drain is costly to the power companies’ business. In fact, most meters must operate under a 10 VA power budget, established by the IEC. When the possible line variations, component tolerances, and system-design margins are taken into consideration, the end result is a current budget of about 10 mA for the system MCU, when a capacitive power supply is being used.
Some of today’s low-cost electricity meters utilize 8-bit MCUs that typically consume over 10 mA when operating at their full speed in an active mode. In order to stay within the system power budget, designers are often required to operate the MCU at a reduced frequency. Many of today’s 16-bit MCUs take advantage of advanced processes and design techniques to provide typical operating currents as low as 150 µA/MHz, and can operate at a full 16 MIPS while consuming a maximum of 6.9 mA. The reduced operating current provides the designer with the choice of either reducing the MCU operating speed of the MCU to lower the system power consumption, or adding additional functions while keeping the system within the allotted power budget.
While electricity meters spend the vast majority of their time in an active state, they are also an example of an application that can take advantage of one of the lowest power modes—Vbat. Vbat functionality provides a dedicated pin that is supplied with a backup power source, such as a LTC battery, or a super capacitor. When the primary power to the system fails, as it would during a power failure, the power for the RTCC automatically transitions to the backup Vbat pin. The RTCC is important in a power meter during a power outage, as time-of-use billing is becoming increasingly popular. While operating through Vbat, the RTCC allows an LTC (lithium thionyl chloride) battery to last for tens of years, allowing an almost indefinite power back-up operation. The use of the Vbat functionality with RTCC is not limited to power meters. Many applications, including the thermostat discussed above, can utilize the RTCC to maintain the time during a power failure or battery change. Vbat, with a capacitor or battery, can go a long way toward eliminating the annoying blinking lights that result from a power failure.
The low-power evolution of MCUs in a power-conscious world has led to extremely flexible general-purpose MCUs. Advances in process technology and design techniques have lead to 16-bit MCUs with active currents as low as 150 µA/MHz. New low-power modes, such as low-voltage sleep and Vbat, have added flexibility to the power-management chain, and are now allowing general-purpose MCUs to operate in a wider variety of applications. The end result is a powerful and adaptable microcontroller that will allow more energy-efficient and customer-friendly end applications.
Donald Schneider is a product marketing manager with Microchip’s Advanced Microcontroller Architecture Division, working on the Company’s 16-bit PIC24F product line. Since joining Microchip in 2005, Don has worked to define and promote Microchips high-performance products, with a particular focus on the PIC24F family. Prior to joining Microchip, Don spent 16 years at Toshiba, eight years as an Applications Engineer for automotive semiconductors and another eight years in various Microcontroller marketing roles. Don’s responsibilities included tactical and strategic marketing for 8- and 16-bit microcontrollers. He earned his B.S.E.E. degree from Lawrence Technological University (Southfield, MI) in 1989.
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