Although the combination of “green” and product name has become a recognized symbol for low power consumption, the exact meaning of low power consumption is not often expressed.

The requirements of low power MCU will change with the application and the way of using MCU in the application. For example, in battery powered thermostat applications, low power consumption is mainly defined by the lowest power consumption mode that the device can drive the LCD display. In this case, reducing power consumption will extend the service life of the battery. In other low power applications (such as meters), low power consumption refers to the operating current consumed by the system during operation. The third kind of system is the system that needs to hold time, regardless of whether the main power supply of the system exists or not. Meters during blackouts are an example of the third type of system. Due to the different requirements of various applications, MCU with more flexible power consumption mode allows designers to further customize the system operation.

In the past, the working mode of MCU was used for device operation; idle and nap modes could reduce or eliminate the switching power of CPU while allowing peripherals to run; sleep mode allowed limited peripherals to run with the lowest power consumption. As today’s advanced MCU turns to more advanced silicon technology (which can minimize the system cost and reduce the operating current), some new low-power modes are increasing to improve the flexibility of MCU. We will explore the use of some new low-power modes in today’s advanced MCU in various applications.

We will use the ble software tool and 16 bit MCU to compare the various power consumption modes implemented in different applications. Microchip’s ble is a free software tool that allows designers to estimate the battery life of the system and determine which working mode is most suitable for their application. Pic24fj128ga310mcu series provides some new low power consumption modes, and its LCD display driver can play a good role in the following examples.

A variety of applications need low power MCU

Thermostats have become more complex and need to display more information and cover multiple areas. Therefore, a large number of on-chip flash program memories are usually required to store complex menus displayed in multiple languages.

Generally speaking, advanced technology is needed to produce large memory MCU with competitive price. With the development of semiconductor technology, it is a trend that the working current of transistor decreases and the leakage current increases. The increase of leakage current is most obvious in the current specification of low power consumption mode (such as sleep mode). The sleep current on advanced MCU is usually between 3 and 5? A while typical thermostat applications are mostly used to drive segmented LCD displays. Segmented LCD displays are usually driven in sleep mode, which allows some peripherals (in this case, LCD drivers) to run when the CPU and most peripherals power down. The thermostat must be periodically woken up and put into operation mode to read the temperature, update the display, and possibly signal the stove, fan, or air conditioner to turn on. However, only sleep mode is needed for more than 99% of the time. Because of a lot of time in sleep mode, improving sleep current can greatly increase the battery life of the system.

New low power consumption mode

In order to make the MCU have sub? Function? Class a power consumption mode, many suppliers have introduced a new low-power deep sleep mode. The typical deep sleep current is in the range of 10 to 50na, and these devices will increase the current by 400na when they run real-time clock calendar (RTCC). Very low current can be achieved by turning off the entire device while retaining only a small amount of memory, real-time clock (and perhaps watchdog timer). However, these deep sleep modes do not allow peripherals to run or hold data RAM on the device. When the device wakes up from deep sleep, if the ram content is lost, the device needs to execute a restart routine before resuming the program execution.

The new low-power mode (such as low-voltage sleep mode) can maintain the device’s data RAM with a typical base current of 330na, and allow the operation of additional low-power peripherals. This low-voltage sleep mode maintains the ram of the device and reduces the sleep current by reducing the output of the device’s on-chip voltage regulator. By reducing the power supply voltage to the device logic and limiting the working peripherals, the sleep current of MCU can be changed from 3.7? To 0.5? To 0? A decreased to 330na. In this type of MCU sleep mode, LCD drivers, timers, RTCC and other peripherals can still operate, and only a small increase in current. The time from low-voltage sleep mode to working state is less than half of the time from deep sleep wake-up. The device then executes the next instruction instead of the restart sequence normally required for deep sleep mode wakeup.

Figure 1: battery life estimator tool main screen

As shown in Figure 1, the main screen of the battery life estimator tool displays the MCU and its working voltage, battery and working mode. The battery life of the thermostat model is estimated to be 11 years and 88 days.

In addition, the ble tool also models the time that MCU will spend in each mode and the amount of power that MCU will consume in each mode. The output display of ble is shown in Figure 1, where you can set several key parameters of the system to get the life estimation and average system current. First, select the working voltage of MCU and system. This enables the battery life estimator to obtain the corresponding specification parameters. Then select the battery or battery pair – in this case, 2 AAA alkaline batteries. The expected operating voltage and temperature can also be selected to obtain the most suitable parameters for battery life estimation model. Finally, the working mode to be used in the system is defined. For our thermostat, two modes will be used.

In order to model the time when the thermostat only displays LCD screen, a working mode named “display LCD” is created. The “display LCD” operation mode uses low voltage sleep mode to provide the lowest power consumption mode for driving the LCD. The battery life estimator tool is modeled to set the device’s duty cycle to 30 seconds, of which 29.5 seconds is used for low voltage sleep mode. The second working mode is to update temperature and LCD, which is used to model the time required by MCU to monitor temperature, update LCD screen and communicate with HVAC device.

In order to better understand the new low voltage sleep mode and how to realize the working mode in ble tool, you can view the add / modify mode screen, as shown in Figure 2. In this screen, the designer can adjust the duration setting, which is currently 29.5 seconds. By using the additional system current input box, the designer can add an estimate of the current consumption of the peripheral circuits of the MCU. In this case, 4? The system current of a represents the current consumed by the LCD display, and 1? A is added? A represents the current required by the internal LCD Bias Resistor. Next, select the power consumption mode (low voltage sleep in this case) and the required peripherals. In order to provide an accurate system current model, LCD driver, bor, WDT and RTCC have been selected. The current consumed by MCU is 1.88? A. This current is similar to our 5? A system current is added to achieve the required 6.88? A current in low voltage sleep mode? A。

Figure 2: the battery life estimator tool mode edit screen, which allows the designer to specify and name the conditions for each power consumption mode used.

The main screen of ble shows that the average current consumed by the device in low-voltage sleep mode is 6.88ua, and the current consumed by the device in short-term working state is slightly higher than 327ua, so the total average current is less than 6.9ua. The estimated battery life of the system is about 12 years, which is about 5 years longer than the storage life of the battery. Figure 3 shows a similar analysis, but using sleep mode instead of low voltage sleep mode, which results in an average current of about 10.5ua and a three-year reduction in battery life.

Based on the three-year battery life estimation tool, the three-year battery life is estimated.

For MCU, most of the time in the working mode of the system is the other extreme, such as electricity meters. Today’s meters will only be in two states in the whole working cycle. It is in normal operation mode when powered on. In this “normal” working mode, MCU is in active state, continuously measuring voltage and current, and calculating power provided by ammeter. Meters may also monitor potential tampering, drive LCD displays, and may communicate with meter reading facilities.

When the meter is running, it may look like there is enough power. In fact, electricity is a product provided by the electricity company, the ultimate customer of the meter manufacturer. Power companies provide power to millions of customers, even a small loss of power for the business of power companies have to pay a high price. In fact, most meters have to work under the 10 VA power budget set by IEC. If considering the possible line variation, component tolerance and system design margin, the final result of system MCU current budget is about 10mA when using capacitive power supply.

Nowadays, some low-cost meters use 8-bit MCU, which consumes more than 10mA current at full speed in working mode. In order to keep within the system power budget, designers usually need to run MCU at a lower frequency. At present, many 16 bit MCU use advanced process and design technology to provide as low as 150? A / MHz typical working current, and can run at 16mips at full speed, and the current consumption is not more than 6.9ma. The reduced operating current provides designers with two options: reducing the operating speed of MCU to reduce the system power consumption; adding additional functions while keeping the system power consumption within the allocated budget.

Although the meter will spend most of its time in the working state, it is also an application example of using one of the lowest power consumption modes (Vbat). The Vbat function provides a dedicated pin that provides backup power, such as LTC batteries or supercapacitors. If the main power supply of the system fails (such as during power failure), the power supply of RTCC will automatically switch to the standby Vbat pin. RTCC in meters is very important during power outage because billing by time of use is becoming more and more popular. When working through Vbat, RTCC allows LTC batteries to be used continuously for decades, which is almost unlimited standby power supply. The use of Vbat function with RTCC is not limited to meters. Many applications, including the thermostats mentioned above, can use RTCC to hold time during power failure or battery replacement. Vbat with capacitor or battery also helps to eliminate annoying flash caused by power failure.

In the environment of high attention to power consumption, the development of low-power MCU has led to a very flexible general-purpose MCU. With the development of technology and design, the working current of 16 bit MCU can be as low as 150ua / MHz. New low-power modes (such as low-voltage sleep and Vbat) add flexibility to the power management chain, allowing general-purpose MCU to work in a wider range of applications. The final result is the emergence of a powerful and adaptable MCU to achieve customer-friendly and energy-efficient terminal applications.
Editor in charge: Tzh

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