Low consumption is often considered a fundamental aspect of a green product, but the nature of low consumption is rarely detailed or quantified.
Low power microcontroller requirements will vary depending on the application and how the microcontroller will be used in the application. Its use can be classified into three main areas:
• Low Power Mode – Will be used in applications such as battery powered thermostats. Low power mode defines the lowest level of power available to drive the LCD display. This reduction in power consumption allows for longer battery life.
• Active current consumed – For applications such as an electricity meter, the level and nature of low consumption refers to the active current consumed by the system during its operation.
• Time Driven Applications – There are systems that are required to maintain the date and time, regardless of whether there is a main power source to the system, such as a power meter if there is a power failure.
As application requirements diversify, designers look for microcontrollers with more flexible power modes to accommodate system operation as closely as possible.
In the past, microcontrollers had an active mode for device operation; Idle or Doze modes to reduce or eliminate CPU power on CPU switching while allowing peripherals to function; and Sleep mode (asleep) that allows limited operation of peripherals with minimal power consumption. New low power modes have been added to increase flexibility to advanced microcontrollers in a trend toward more advanced silicon processes that minimize cost and reduce active current. To demonstrate some of the operating modes available on today's advanced microcontrollers, this article will examine how these new low-power operating modes are used in various applications.
The examples are created using a Battery Life Estimator (BLE) software tool and a 16-bit microcontroller to provide a comparison between various power modes, when implemented in different applications . Microchip's BLE is a free software tool that allows a designer to estimate battery life in their system and determine which operating mode is best suited for their application. The functionality of the PIC24FJ128GA310 family includes new low power modes and an LCD display controller, as the following examples demonstrate.
Thermostats have become more complex, need to display more information, and cover diverse geographic areas. As a result, significant onboard Flash program memory capacity is required to store the complex multilingual menus.
In general, advanced processes are needed to produce microcontrollers with large memories at competitive prices. With the advancement of semiconductor processes, there are trends for a reduction in operating (active) currents and an increase in transistor leakage current. The increase in leakage current is most visible in the current specifications for low consumption modes such as Sleep mode. Sleep mode currents of advanced microcontrollers are typically on the order of 3-5 µA, whereas a typical application for a thermostat is just driving an LCD display most of the time. The segmented LCD display is typically a controller in Sleep mode which allows operation of the peripherals (in this case, the LCD controller) while the CPU and most peripherals are powered off. The thermostat will periodically wake up and go into active mode – reading temperature, updating the display and perhaps signaling the start of heater, fan or AC units. Since only Sleep mode is used 99% of the time, the current in this mode is one area where improvements can greatly benefit the system's battery life.
To provide microcontrollers that draw current less than 1 µA, many vendors have introduced new low-power Deep Sleep modes. Typical values of current in deep sleep mode are 10 to 15 nA and these devices can drive a Real Time Clock Calendar (RTCC) with an added current of 400 nA. One option is to turn off the entire device – with the exception of a small amount of memory, a real-time clock, and possibly a supervisory timer – to achieve extremely low currents. However, these deep sleep modes do not allow peripherals to function or maintain data RAM on the device. Loss of RAM content requires the device to run a reboot routine before resuming program execution when awakened from deep sleep mode.
An alternative can be found in new low-power modes, such as Low-Voltage Sleep mode, which maintains RAM data at a typical current of 330 nA and allows other low-power peripherals to operate. This low voltage sleep mode maintains device RAM and decreases sleep current by reducing the output of the integrated regulator. By lowering the supply voltage to the device logic and limiting active peripherals, the microcontroller's sleep current can be reduced from 3,7 µA to 330 nA. Within the microcontroller's sleep mode, peripherals such as LCD controllers, timers, and the RTCC can operate with minimal additional current. Low voltage sleep mode allows the device to return to an active state in less than half the time it takes to wake up from deep sleep mode. The device then begins executing the next instruction instead of initiating the usual reboot sequence upon waking up from deep sleep mode.
As shown in Figure 1, the main screen of the BLE tool indicates the microcontroller and its operating voltage, battery, and operating modes. The result of the Thermostat model is an estimated life of 11 years and 88 days.
The BLE tool models how much time a microcontroller will spend in each operating mode and how much power the device will consume in each mode. Figure 1 visualizes the BLE, which is used to adjust several key system parameters and to provide the result of the estimated life and average current of the system. First the microcontroller and the operating voltage of the system are selected. This allows BLE to pick up the appropriate specification parameters. A battery or pair of batteries is selected, in this case 2 AAA alkaline batteries. The system's expected operating voltage and operating temperature can also be selected to obtain the most appropriate specification used by the model estimating battery life. Finally, the operating modes that the system will use are defined. In the case of our thermostat, two modes will be used.
To model the time in which the thermostat only displays the LCD screen, an operating mode called "LCD Display" is created. The LCD Display operating mode uses a low voltage sleep mode to provide the lowest power mode from which the LCD can be controlled. The BLE tool is set to model low-voltage sleep mode for 29,5 seconds in the 30-second loop used to model device lifetime. A second operating mode “Update Temp and LCD” is also used to model the time that the microcontroller will take to monitor the temperature, update the LCD screen and communicate with the air conditioning units.
Figure 2 shows the new low voltage sleep mode and the implementation of an operational mode in the BLE tool ("Add/Modify Mode" screen). From this screen, a designer can adjust the duration setting, which is now set to 29,5 seconds. In the “Additional System Current” entry, designers can add an estimated current draw for the currents surrounding the microcontroller. In this case a system current of 4 µA has been added to represent the current consumed by the LCD display and a current of
1 µA to represent the current required by the internal bias resistors of the LCD. The power mode is then selected, in this case the low voltage sleep mode, as well as the necessary peripherals. LCD controller, BOR, WDT and RTCC have been selected to provide an accurate model of the system current. The total system current drawn by the microcontroller is 1,88 µA, which is added to our system current of 5 µA to get up to the 6,88 µA needed by the system in low voltage sleep mode.
As Figure 2 shows, the BLE tool's mode edit screen allows the designer to name and specify conditions for each power mode used. The main BLE screen indicates that an average of 6,88 µA is consumed. while the device is in low voltage sleep mode and just above 327 µA during the short time the device is in the active state, for an average current of less than 6,9 µA. The estimated battery life for the system is almost 12 years, which is almost 5 years longer than the average battery life. A similar analysis using sleep mode instead of low voltage sleep mode is shown in Figure 3; the result is an average current of approximately 10,5 µA and a reduction of three for battery life. As Figure 3 shows, the estimate of battery life based on the use of sleep mode offers a reduction of three years on battery life with standard sleep mode.
An application that allows comparisons with a microcontroller is a system that would spend most of its time in active mode, such as an electricity meter. Today's electricity meters spend all their time in one state or another. The normal operating mode is used when electrical power is available. In this "normal" operating mode, the microcontroller is active and constantly measures the voltage and current, and then calculates the power going through the meter. The meter must also monitor potential tampering, controls an LCD display, and may communicate with meter reading infrastructure.
While the electricity meter is running, it may appear that power is plentiful. In reality, energy is the product supplied by the electricity company, which is the final customer of the meter manufacturer. The electric company supplies power to millions of subscribers and the smallest amount of lost power affects your business.
In fact, most meters are required to work below a 10 VA rating, as established by the IEC.
When possible line variations, component tolerances, and system design allowances are taken into account, the end result is a current draw of about 10 mA for the system microcontroller when using a capacitive power supply.
Some of the current low-cost electricity meters use 8-bit microcontrollers that usually consume more than 10 mA when working at full speed in active mode. To accommodate the power allocated to the system, designers are often required to run the microcontroller at a lower frequency. Many of today's 16-bit microcontrollers take advantage of advanced processes and design techniques to provide typical operating current starting at 150 µA/MHz and can run at a maximum of 16 MIPS consuming a maximum of 6,9 mA. The lower operating current allows the designer to choose between reducing the operating speed of the microcontroller or adding other functions while keeping the system within its rated power.
Since electricity meters are in an active state most of the time, they are also an example of an application that can take advantage of the lowest consumption modes (Vbat).
Vbat functionality offers a dedicated pin that is supplied with a backup power source such as an LTC battery or super capacitor.
When the system's primary supply drops, such as during a utility failure, RTCC power is automatically switched to the standby Vbat pin. The RTCC is important in an energy meter if there is a supply cut since the billing for the time of use is more and more widespread. While powered by Vbat, RTCC allows an LTC battery to last for tens of years, thus allowing for almost indefinite operation from standby power.
The use of a Vbat functionality with RTCC is not limited to energy meters. Many applications, like the thermostat shown above, can use the RTCC to keep time if the power fails or when changing the battery. Vbat, with a capacitor or battery, can go a long way towards eliminating those annoying flickering lights that arise as a result of a power failure.
The evolution of low power microcontrollers in an increasingly energy conscious world leads to general purpose microcontrollers characterized by high flexibility. Advances in process technology and design techniques have made it possible to achieve 16-bit microcontrollers with active currents starting at 150 µA/MHz. Flexibility has been added to the power management chain through new low power modes such as low voltage sleep mode and Vbat, which allow general purpose microcontrollers to operate in an ever-widening range of applications.
As a result, designers can access powerful and adaptable microcontrollers that will enable greater energy efficiency and more user-friendly end applications.
