Design of low power consumption system for wireless sensor network nodes

1.1 Portable module node hardware low power design

(1) Processor selection

The ATmega324p is a powerful microcontroller that provides a flexible and low-cost solution for many embedded control applications:

①TQFP (Thin Quad Flat Package), small size and high integration;

②6 power saving modes selectable through software;

③The maximum throughput rate reaches 20MIPs (at 20 MHz).

(2) Low power consumption design of interface circuit

The low power design of the interface circuit is often an easily overlooked link. In this link, we must first select low-power peripheral chips, and then the fundamental method is to make the interface circuit in a low-power state. In addition, the following two factors must be considered:

① Pull-up resistor/pull-down resistor selection. If the subsequent stage can be driven normally, choose a larger resistance value as much as possible. In addition, when the signal is low in most cases, you can also consider using a pull-down resistor to reduce power consumption.

②Handling of floating pins. The input impedance of the floating input of CMOS is extremely high, which may induce some charge and cause the device to be broken down by high voltage. It will also cause the input signal level to change randomly, causing the CPU to be constantly awakened when in sleep mode, thus failing to enter sleep mode, or causing other inexplicable failures. Therefore, the correct way is to connect the unused input to the power supply VCC or ground.

(3) Communication chip selection

IA4421 is an integrated RF transceiver chip launched by Integration Associates, which operates in the 433/868/915 MHz frequency band. The chip has an operating voltage of 2.2 to 5.4 V, uses a low power mode, a standby current of 0.3 μA, uses FSK modulation mode, a transmit power of 5 to 8 dBm, and a receive sensitivity of -109 dBm.

IA4421 has a high data transmission rate, the digital signal transmission rate can reach 115.2 kbps, the analog signal transmission rate can reach 256 kbps.

1.2 Low-power software design for portable controllers

(1) Various power consumption mode conversions

The portable controller consists of ATmega324p, IA4421, Samsung LCD and peripheral circuits. Taking the portable controller as a whole, four different working modes are defined, as listed in Table 1. Different working modes are determined by the working mode combination of related functional chips on the portable controller.

①Reasons for ATmega 324p to choose Power-save mode: In Power-er-save sleep mode, in addition to all the functions in Power-down mode, Timer/Counter2 can work normally, so in Power_save sleep mode, the system's real-time clock system can operate normally, which also provides conditions for timed wireless transmission and reception in system power consumption test.

② Wireless transceiver working process based on power mode conversion: When the portable controller has no receiving and sending tasks, it enters sleep mode, that is, LCD is turned off, ATmega324p is in power-save mode, and IA4421 is in sleep mode. In practical applications, the portable controller should be in sleep mode for the longest time.

If the user has a requirement to transmit data, the portable controller can wake up the controller through buttons, asynchronous timer 2 (real-time clock) and external interrupt signal (INT2) generated after receiving the host signal, and perform related operations of sending and receiving. After the task is completed, it enters sleep mode again.

(2) Low power keyboard software design

ATmega 324p's PortA, PortB, PortC, and PortD have a total of 32 I/O ports, each of which is an external interrupt source. When a level jump is detected on the port, an external interrupt (PCINT) can be generated. This function makes the number of external interrupt ports of the controller no longer limited. The 6 interfaces of the 3×3 keyboard can be connected to the ordinary I/O ports to realize the interrupt key. The interrupt key has the following advantages in this system:

① The interrupt key program does not require the controller to be in the scanning running state all the time, which greatly reduces the power consumption compared with the keyboard scanning program in Polling mode.

② The interrupt key program can wake up the controller by generating an interrupt by pressing a key, which facilitates the system to switch between various modes without affecting the system function. The detailed keyboard system software design process is shown in Figure 1.

2 Low power consumption experiment and result analysis

2.1 Power consumption experiment of ATmega324p small system

The ATmega324p mini-system includes the ATmega324p microcontroller, Samsung's S6B0741 LCD module, and a power supply (5 V, 2.5 A power adapter). The wireless communication chip is not included in the power consumption experiment system of the minimum system.
(1) Theoretical current consumption

The theoretical current consumption values ​​when ATmega324p operates at 8 MHz and LCD (S680741) operates in display on (backlight off), sleep mode and LCD (S680741) off are listed in Table 2.

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(2) Determination of the actual current value of the minimum system in different working modes Download the C program in the system and measure the current consumption of the system in different combination modes. The test environment is in the laboratory, the temperature is about 20℃; use a multimeter and a 100 Ω resistor, ICCAVR development environment, and STK500 downloader.

A 100 Ω resistor is connected in series to the main power interface of the portable controller, and the voltage value on the resistor is measured in different system modes, and then the current value is calculated. The comparison results between the test value and the theoretical value are listed in Table 3.

(3) Experimental results analysis

①The LCD module mainly includes the control chip and LCM (display). In the theoretical value, the theoretical value of the current of LCD (S6B0741) does not include the current consumed by LCM (display). When the LCD is turned on, the ATmega324p is in idle mode and normal working mode, the difference between the theoretical value and the actual measured value is about 3 mA. It can be concluded that the current of 3 mA is the approximate current consumed by LCM (display).

②The actual measured current value is larger than the theoretical value. This difference in energy consumption is mainly consumed in the peripheral circuit of the portable controller module. The theoretical current consumption value of each electronic component in the peripheral circuit is difficult to find and is not included in the calculation.

③ Although the measurement method is very simple and only measures the static value of the system current, the measured current value can generally reflect the power consumption trend of the system under different working conditions, and has certain significance and application value for the low power consumption research of the system.

④ From the comparison of the actual test results under various working modes, it can be seen that the difference between the minimum energy consumption and the maximum energy consumption of the minimum system is about 10 mA. Therefore, in low-power design, the conversion of different working modes under different functional requirements is very meaningful.

⑤ The difference in current consumed when the backlight of the LCD module is turned on and off is about 6 mA, which shows that the LCD backlight is a very energy-consuming device in the system. Therefore, from the perspective of energy saving, the LCD backlight is generally not turned on under normal circumstances.
2.2 Portable controller low power consumption test experiment

In order to verify the power consumption performance of the portable controller, after taking the above-mentioned software and hardware low-power consumption measures, the following experiment was conducted on the power consumption performance of the portable module. The verification results show that the designed module basically meets the application requirements of the system in terms of power consumption.

(1) Experimental content

① Use the timer 2 of ATmega324.p to perform regular transmission and reception. Send and receive data once every 2.5 hours, and send data 9 times a day.

② During the period when there is no sending or receiving task, ATmega324p is in a low-power sleep state Power-save, the LCD module is turned off, and IA4421 works in sleep mode. From the above small system power consumption experiment, it can be seen that the energy consumption of the entire portable module is the lowest under this working state. The software flow of the test is shown in Figure 2.

③ The wireless communication parameters of IA4421 are: working frequency band 433 MHz, data transmission rate 9.6 kbps, relative transmission power 0 dBm, receiving sensitivity -109 dBm. Such parameter selection, while satisfying the normal transceiver function of the system, tries to use low frequency band and low transmission rate to meet the long-distance transmission and minimize the power consumption of transmission and reception.

④Powered by 3 No. 7 Nanfu alkaline high-energy batteries, the battery voltage is 4.86 V at the beginning of the test.

(2) Experimental results and significance

The test system is designed for point-to-point communication between the portable controller and the host. The experimental results are listed in Table 4.

The communication frequency is once every 2.5 hours, which is a good simulation of the frequency of users using this system. The experimental results show that the battery life is about 5.5 months, and the result is obtained based on 10 communications per day. This battery life indicator basically meets the requirements of system design, and also proves that the above-mentioned software and hardware measures are appropriate and effective.

3 Conclusion

This article analyzes the low-power software and hardware design methods in detail. Selecting different working modes under different work tasks is of great significance to reducing power consumption. After using appropriate software and hardware measures, the battery life of the designed portable controller module reached about half a year, meeting the application needs of the wireless sensor network system. The low-power design methods and ideas in this article have certain reference value for the development of actual products.