Battery-powered micro-power design for single-chip microcomputers

For most single-chip microcomputer systems, due to the high speed of single-chip microcomputers, single-chip microcomputers have a lot of idle waiting time during operation. In some cases, the waiting time of the system can even reach more than 95% of the total working time. During the waiting process, the single-chip microcomputer does not do any work, but just waits in steps, or judges whether there are new external requests in a loop. In this process, most of the circuits inside the single-chip microcomputer can be operated in a dormant state, which can greatly reduce the power consumption of the single-chip microcomputer. At the same time, the relevant external circuits can also be operated in a dormant state, which greatly reduces the power supply of the entire product. This non-continuous working characteristic of the product is the basic idea of ​​micro-power design. In addition, more design details should be considered according to the characteristics of the product.
Choosing a suitable CPU chip is the key to micro-power design .
There are many types of single-chip microcomputers at present, and most of them are aimed at a specific application. The appropriate single-chip microcomputer can be selected according to the specific application situation. In applications that require micro-power design, you can choose according to the following rules:
1. Choose a single-chip microcomputer that minimizes external circuits as much as possible. With the rapid development of integrated circuit process technology, truly single-chip single-chip microcomputer systems have gradually become mainstream products.
2. Pay attention to comparing the working current and the static current. Due to different processes, the internal working current and static current of the microcontroller are not the same, and some are even very different. When choosing a microcontroller, not only its working current should be considered, but also its static current in the sleep state should be carefully considered.
3. Through comparison, it can be seen that the use of a dedicated low-power microcontroller can control its power consumption more flexibly, and make it work in the most power-saving mode as much as possible under the premise of meeting the design requirements.
4. Choose the right ROM and RAM. Generally speaking, the larger the memory, the greater the power consumption. Under the condition of meeting the design requirements, use the ROM and RAM inside the microcontroller as much as possible.
5. Choose the right working clock frequency. At a lower clock frequency, the power consumption of the microcontroller is also lower. Taking MSP430F1121 as an example, when working at a main frequency of 1MHz, the typical current consumption is 300uA; and when working at a main frequency of 4096Hz, its current is only 3uA.
6. Choose the right number of IO pins, and the right IO drive capability and display drive capability. The more IO pins driven by the microcontroller, the greater its power consumption.
7. Selecting a suitable MCU to achieve true monolithicization can save a lot of hardware development and debugging work, improve work efficiency, and significantly improve the reliability and anti-interference ability of the system. At the same time, it reduces the cost of the system, making it more suitable for miniaturization and portability, and plays a decisive role in reducing system power consumption.
Low-power design strategy
a. Make the internal circuit work selectively
Generally, not all the internal circuits of the MCU are used in the design, and those circuits that are not used will generate additional power consumption. In applications that require micro-power design, different functional modules can be selected by programming the internal special function registers, and the unused functional modules can be stopped to reduce the invalid power consumption of the system.
b. The low-voltage design of the product can reduce the power consumption of the product
Generally, the higher the working voltage of the MCU, the longer the internal transistor works in the amplification area, and the greater the power consumption of the MCU. Due to the use of advanced chip production technology, the voltage range of the MCU is generally very wide, such as being able to work normally within the power supply voltage range of 1.8V to 5V. In order to reduce system power consumption, low-voltage design can be used as much as possible.
The relaxation of the microcontroller power supply voltage range can further broaden the application field of microcontrollers, especially in portable or handheld devices, where batteries can be used as power sources without worrying about whether the voltage curve during the discharge process is balanced or whether it will affect the normal operation of the microcontroller under low voltage. There is no need to add a voltage stabilization circuit specifically for battery power supply, which can reduce a lot of power consumption.
c. Use a low-speed clock signal in idle state
The power consumption of a microcontroller is proportional to its operating frequency. The higher the system operating frequency, the greater the power consumption. Figure 1 shows the relationship between the current on the Vcc of Philips' 80C31 microcontroller and the main clock frequency. It can be seen that as the main clock frequency of the microcontroller increases, the current on its Vcc also increases linearly, and its power consumption also increases with the increase of the main clock frequency.
In order to better reduce power consumption, two independent clock systems are integrated inside many microcontrollers, namely a high-speed main clock and a low-speed sub-clock. When high-speed operation is not required, a low-speed sub-clock can be selected to maintain the basic internal timing requirements. The main clock of some microcontrollers can also be reset through the function register. When the functional requirements are met, the main clock frequency can be reduced by a certain proportion to reduce power consumption. During the program running, the clock frequency can be changed online by assigning values ​​to the special function registers through software, or the main clock and sub-clock can be switched.
d. Work in sleep mode as much as possible
. In order to reduce power consumption, microcontrollers usually provide multiple working modes. When they are idle, they enter the sleep mode. When an event raises an interrupt request, they can quickly return to the normal operation mode. This can ensure system power saving without affecting normal operation.
Different microcontrollers have different working modes. For example, the 51 series microcontrollers have idle mode and power-down mode. In different working modes, some functional modules in the microcontroller core will be set to sleep state. For example, the MSP430 series microcontrollers have 6 different working modes. Except for one normal operation mode (active mode), the other five are low-power modes. In these modes, the CPU, internal clock, internal bus, and even the internal crystal oscillator can be turned off respectively to minimize the power consumption of the microcontroller. Only when an interrupt request or reset occurs, the system is awakened and enters the normal operation mode.
Micro-power design of external circuits
The micro-power design of the microcontroller peripheral circuit is very complex and is also very important for the overall power consumption of the product. Complex and large peripheral circuits will bring great power consumption. Therefore, external circuits should be used as little as possible, and the internal resources of the microcontroller should be used as much as possible.
As a battery-powered device, its static power consumption is preferably a few microamperes to tens of microamperes. Since this part of the current is added to the device in the standby state, it is a constant power supply current, which will cause a lot of power waste when the system is not working. Therefore, in the design, the external circuit should be minimized, and the part of the external circuit that needs to be powered in the static state should be reduced. At the same time, the following issues need to be considered:
1. Other devices other than the microcontroller in the system should use devices with low static power consumption as much as possible, such as CMOS chips as much as possible, and use less bipolar transistor gate circuits, because bipolar circuits require a constant maintenance current, which increases the static power consumption of the circuit.
2. According to the requirements of the chip, connect unused pins to ground or high level, and floating input pins will increase the static current of the chip.
3. Use as few pull-up or pull-down resistors as possible on IO pins, as these resistors will consume a certain amount of static current.
4. The design of the analog part of data acquisition can use a rail-to-rail BiCMOS operational amplifier. For example, when LMV824 is used to replace LM324, the power supply can be as low as 2.5V, the unit bandwidth can be up to 5MHz, and it is only 250μA/channel.
5. Design the power control circuit of external devices to shut down the power supply of external devices or equipment when they are not working, reducing invalid power consumption. The price of low-power devices is generally slightly higher. If the price allows, corresponding low-voltage, low-power alternatives can usually be found.
6. Use more voltage drive circuits and less current drive circuits. For example, to display the operation results, current status or control information, there are usually two options: LCD display and LED display. LCD output generally only has a few microamperes of current; while LED output has tens of milliamperes of current.