How to extend the lithium battery life of microcontroller equipment

　　Author ：Iflowpower – Portable Power Station Supplier

Battery power supply equipment, whether it is electric toothbrush, razor, mobile phone, personal digital assistant (PDA), MP3 player, or remote control equipment that is unable to go, becomes part of everyday life. Therefore, power management is a considerable thing to today's embedded design engineers. Universally existing microcontrollers supply a large number of management power requirements for design engineers in many equipment applications.

Different kinds of MCU themselves have a range of current consumption and many energy-saving features. However, in the design based on the microcontroller, the management of the power supply is not just as simple as the correct microcontroller. Power management also needs to use MCU itself to reduce current consumption and energy-saving development strategies.

On the system level, even if the MCU you choose is independent, you can also use many policies to further extend the battery life of your application. Application example: Wireless bicycle mileage Next, we will display valid power management as an example with wireless bicycle miles. The mileage consists of three parts modules: a control panel on the car, a speed sensor located in the wheel and a display on the rider helmet.

The speed sensor feeds back the speed of the bike to the control panel, calculates such as: driving speed, driving mileage, driving time, energy consumption, and communicates the calculated information to the display. Below Figure 1 is a block diagram of a bicycle milestometer control panel. Figure 1: Wireless bicycle miles Control panel block diagram showing today's MCU continuously enhanced power management features.

The new MCU geometry of the low power mode is minimized to reduce the chip area, which causes the transistor to fail to withstand direct use of 3V or 3V above voltage. Therefore, it is necessary to use a voltage regulator in internal logic to reduce the voltage. Unfortunately, these voltage regulators will increase the current consumption of MCUs.

However, since the power size is equal to the voltage multiplied by current, the system power consumption of 1.8V to 3V with the adjuster is still lower than the system power consumption of 5V without the adjuster. The MCU relies on power management mode, which can still support adjustment power and speed clock speed while reducing overall working currents.

The new MCU can supply many low power modes to meet these requirements while maintaining system flexibility. Freescale's MC9S08GB60MCU has four low-power mode: depth stop state (STOP1), moderate stop state (STOP2), mild stop state (STOP3) and launch mode. In the waiting mode, the power consumption is reduced by turning off the CPU clock, but the system clock is supported by other MCU peripherals, such as: Modes (A-D) converter, timer, or serial communication module.

This mode is used to reduce power consumption in the case of peripherals, but the CPU cannot work before performing tasks peripheral. In our example, the waiting mode is used in the serial peripheral interface (SPI) for communication with RF (RF) transceiver. To further reduce power consumption, use three stop modes.

STOP1, STOP2, STOP3 supply different levels of reduced power consumption. STOP3 is the strongest function in three stop modes. In STOP3 mode, the on-chip voltage adjuster is in power saving mode, but it still supplies minimum adjustments to retain the content of the random memory (RAM) and input / output (I / O) registers.

Several interrupt sources and reset can wake up the MCU from STOP3 mode. STOP3 is the only mode in the three stop modes and the mode that can still work in the three stop modes. In our example, in a period of time between the speed sensor read the speed value, the MCU is in the waiting state, and the STOP3 mode can be used.

The real-time interface (RTI) function of working in STOP3 mode can be used to wake up the MCU in time for the next reading. STOP2 is functional than STOP3, but its power is lower. In STOP2 mode, voltage regulator is in power saving (PoweredDown).

However, the RAM content is still saved. The I / O register is also in power-saving state, and it is necessary to reconfigure when it is awakened from the stop mode. In STOP2, it is possible to wake up less in the MCU, but still has RTI functions.

Back to our example, STOP2 can replace STOP3 to further reduce power consumption. Since the RTI function and the RAM are still working, the time between speed reading can still be measured. STOP1 is the lowest mode of power consumption in the MCU.

In this mode, voltage regulators and all peripherals, CPU, RAM, and I / O are completely entering power saving state. Only reset and IRQ interrupt feet can wake up MCU. When the MCU can enter the power-saving state, but in external excitation, if you still need to make a response when the button is pressed, STOP1 mode is available.

In this example in the bicycle, you can enter the STOP1 mode when the mileage table is in power saving state. The STOP1 mode in the power saving state is the smallest mode that may exist in the MCU without cutting off the power from the chip. Why don't you cut out the power supply from the chip? Because you cut off the power from the chip to use a more expensive toggle switch.

Similarly, the MCU can use a button switch connected to the interrupt foot to achieve many different purposes. These different uses depends on the current state of the system. Therefore, the STOP1 mode can maintain simple design, low cost, and almost no current consumption, is perfect.

Clock Management Many designers will work with low power and low clock frequencies. In fact, according to the different operations and MCUs on behalf of the MCU, it is actually able to reduce power consumption at the highest speed. If the MCU has a valid low power mode, it is possible to minimize power consumption in the longest time to minimize power consumption.

Therefore, if the CPU is executed before returning sleep mode, the code execution is completed with the possible highest speed, then return low power mode than the current consumes low speed. Let's take a look at the example of the bicycle miles, assume that the control panel updates the speed once every second, and the 16,000 bus cycle is looped to calculate the data and display it on the display. Work by a typical 32kHz crystal, and assumes that there is a common one-to-two bus clock, we can have 16kHz bus, in which case, use a second to complete the calculation.

Now, if we can use 8MHz bus clock, you can only cost 2 milliseconds to complete the calculation, and the remaining 998 milliseconds can be in low power mode. Of course, each task that is not MCU must be remembered from high-speed performance. In our example, if the data speed is quite slow, the time required for wireless communication may not be 8MHz bus rate.

Therefore, in this case, we should minimize power consumption, we should run the MCU as soon as possible until the end of wireless communication. Therefore, we want a clock, flexible MCU, such as Freescale's MC9S08GB60MCU. With this device, you can use high frequency crystals, low frequency crystals or internal oscillators.

With any such clock source, you can use the on-chip frequency lock ring (FLL) to increase or decrease the bus speed to meet the task requirements and minimize power consumption. Figure 2 is a change in power consumption in different operating modes in a bicycle mileage. Figure 2: In the bicycle mileage example, how to perform power management through the conversion between highly active short pulses and longer non-active low power mode.

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