Design of Low Power Consumption Active RFID Tag Based on MSP430 Microcontroller
Low power design
Low power consumption overview
Power consumption is basically defined as the rate of energy consumption, which can be divided into two categories: transient power consumption and average power consumption. The two have different meanings, have different application backgrounds and optimization strategies, and are generally summarized as low-power designs. The actual research can be divided into:
(1) Transient power optimization: The goal is to reduce peak power consumption and solve circuit reliability issues.
(2) Average power consumption optimization: The goal is to reduce energy consumption within a given time, mainly for battery-powered portable electronic devices, to extend battery life or reduce device weight.
Physical sources of power consumption
The power consumption of the chip circuit mainly comes from two aspects: dynamic power consumption and static power consumption. The dynamic power consumption is mainly the charging and discharging of the capacitor and the short-circuit current. Static power consumption is mainly leakage current, including PN junction reverse current and sub-threshold current, and penetration current. If the working timing and software algorithm design are defective, it will reduce the system's working efficiency, extend working time, and directly increase the system's energy consumption.
Low power design strategy
Algorithm-level power optimization: At the beginning of the circuit design, it is necessary to choose the algorithm, you should try to choose an algorithm with high power consumption efficiency. First of all, from the perspective of the size of the logic required to implement the algorithm, the fewer the number of operations, the required bandwidth, storage operations, and port operations in the algorithm, the lower the power consumption of the circuit to which this algorithm is applied. In actual design, the overall performance and power consumption need to be balanced according to the requirements of the application. At the same time, the co-processing required in the algorithm must be considered. The simpler the co-processing required by the algorithm, the fewer collaboration modules, and the lower the power consumption required to implement the algorithm. In addition, fewer temporary variables in the algorithm, short time for temporary variables to be effective, and reasonable use of loops will reduce the power consumption required by the algorithm.
System-level power design and management: System-level power management is mainly dynamic power management. The usual approach is to operate in a sleep state when in an idle state, and only some devices are in operation; when an interrupt is generated, other devices are awakened by this interrupt. In fact, this part needs the support of hardware, such as the low power consumption technology of the power system; the division of system software and hardware is to decide which functional modules are realized by software with lower power consumption and which functional modules are realized by hardware with lower power consumption ; The choice of low-power processors.
System hardware design
Comprehensively considering the source of system power consumption and low-power design strategies, the hardware design selects single-chip computers and RF transceiver chips with low power consumption characteristics, and simplifies the circuit as much as possible to reduce power consumption expenses.
Selection of main chips
The structure of MSP430 series single-chip microcomputer is completely based on the low-power operation of the system. The power supply uses a low voltage of 1.8 ~ 3.6V, the active mode consumes 250μA / MIPS, and the power consumption in RAM data retention mode is only 0.1μA. Because the system is in sleep or low power consumption state for more than 90% of the time, the leakage current becomes another important factor affecting the system power consumption. The maximum leakage current of its I / O input port is only 50nA. Coupled with a unique clock system design, including two different clock systems: basic clock system and frequency-locked loop (FLL and FLL +) clock system or DCO digital oscillator clock system. The clock system generates the clocks required by the CPU and each functional module, and these clocks can be turned on or off under the control of instructions, thereby achieving control of the overall power consumption. Due to the different modules used when the system is running, that is, using different working modes, the power consumption of the chip will be significantly different. There are one active mode (AM) and five low power consumption modes (LPM0 ~ LPM4) in the system. In addition, MSP430 series MCUs use vectored interrupts, support more than ten interrupt sources, and can be arbitrarily nested. It only takes 6μs to wake up the CPU with an interrupt request. Through proper programming, it can not only reduce system power consumption, but also make a quick response to external requests.
The radio frequency chip is the core part of the entire RFID card, which is directly related to the reading and writing distance and reliability of the tag, and also directly affects the power consumption of the entire system. CC1100 is a single-chip UHF wireless transmitter chip launched by Chipcon. It has small size and low power consumption. The data rate supports programmable control of 1.2 ~ 500kbps. Its operating voltage range is 1.9 ~ 3.6V, and it can work at 915MHz., 868MHz., 433MHz and 315MHz four bands, can also be configured to provide -30 ~ 10 dBm output power in all frequency bands through the program. Built-in address decoder, first-in first-out stack area, modulation processor, clock processor, GFSK filter, low noise amplifier, Function modules such as frequency synthesizer and power amplifier. It has two low-power operating modes: shutdown mode and idle mode, the operating current is less than 200nA in shutdown mode. In this paper, CC1100 works at a frequency of 433MHz, adopts FSK modulation, the data rate is 100kbps, and the channel interval is 200kHz.
Circuit design
In order to simplify the system structure, the system consists of only the necessary microprocessor unit, radio frequency transceiver unit, antenna and battery unit. The voltage stabilizing circuit between the battery and the device is omitted, and the system is directly powered by the battery. It saves the quiescent current consumption brought by the voltage stabilizing circuit, and further extends the battery life. In order to prevent the battery current transient decrease caused by a large current in the emission state, a larger capacity capacitor is used in parallel with the battery. MSP430F2012 integrates a zero-power brown-out reset (BOR) protection function, which can perform a full reset when the voltage is below the safe operating range, which solves the problem of random erroneous operation caused by incomplete reset of the microcontroller.
software design
Trying to replace hardware with software is also a common measure for low-power system design. This program development comprehensively considers the two aspects of timing scheduling and work efficiency to reduce system power consumption.
Reasonable design work sequence
Since the running time of the CPU has a great influence on the power consumption of the system, its working time should be shortened as much as possible. Being in idle mode or power-down mode for a long time is the key to reducing the power consumption of the microcontroller system by software design. The program operation flow chart is shown in Figure 2 (a), (b). When the system is powered on and completes the initialization operation, it immediately enters the low-power mode. Only when the system receives the correct information and generates an interrupt, it will wake up the microcontroller to enter the working mode. Complete the processing of information or data within a short period of time, and return to the low-power mode immediately when the processing is over and wait for the next interrupt.
Improve work efficiency
Use macro definitions instead of subroutine calls. Because when the CPU enters the subroutine, it will first push the current CPU register onto the stack (RAM), and when it leaves, it will pop the CPU register off the stack, which brings at least two operations to the RAM, so reading RAM will bring more than reading Flash. Greater power consumption. Replacing subprogram calls with macro definitions will undoubtedly reduce the power consumption of the system.
Minimize the amount of CPU calculations, calculate the results of some calculations in advance, put them in Flash, replace the real-time calculations with table look-ups, reduce the amount of CPU calculations, and effectively reduce CPU power consumption; The calculation ends with sufficient precision; try to use short data types: try to use character 8-bit data instead of 16-bit integer data, and use fractional operations to avoid floating-point operations.
Let the I / O modules run intermittently. I / O modules that are not used or intermittently used should be turned off in time to save power. Unused I / O pins should be set to output or set to input, and pulled high with a pull-up resistor. If the pin is not initialized, it may increase the leakage current of the microcontroller.
in conclusion
This article describes in detail the design of low-power active RFID tags based on MSP430 MCU, reasonable use of MSP430 MCU's interrupt, timing, calculation and other functions, with the help of software advantages, to adopt energy-limited work for CC1100 modules with lower energy consumption Measures to improve the life of the battery and increase the reliable operation time of the system, compared with other design power consumption shown in Figure 3. The design of this active RFID tag has improved the performance of RFID. It has largely solved the problem of identifying long-distance, large-flow, anti-interference, high-speed moving markers. The completed RFID tag and matching reader can form a person or item identification and positioning system, which is widely used in many fields such as mining, industrial production, road traffic, logistics, medical treatment, medicine, national defense and security.
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