Design of low-power active RFID tag based on MSP430 microcontroller

“Radio frequency identification (RFID) technology has developed rapidly at home and abroad in recent years. For portable systems that require battery power, power consumption is also a growing concern. This article will specifically describe the hardware and software implementation method of the bidirectional active label based on the low power consumption design concept of MSP430F2012 and CC1100.

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Radio frequency identification (RFID) technology has developed rapidly at home and abroad in recent years. For portable systems that require battery power, power consumption is also a growing concern. This article will specifically describe the hardware and software implementation method of the bidirectional active label based on the low power consumption design concept of MSP430F2012 and CC1100.

low power design

Low Power 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 design. In actual research, it can be divided into:

(1) Transient power optimization: The goal is to reduce peak power consumption and solve circuit reliability problems.

(2) Average power consumption optimization: The goal is to reduce the energy consumption in a given time, mainly for battery-powered portable Electronic devices, in order to prolong the battery life or reduce the weight of the device.

Physical source 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 charge and discharge of the capacitor and the short-circuit current. The static power consumption is mainly leakage current, including PN junction reverse current and subthreshold current, and penetration current. If the working sequence and software algorithm design are defective, it will reduce the working efficiency of the system, prolong the working time, and directly increase the energy consumption of the system.

Low Power Design Strategies

Algorithm-level power consumption optimization: At the beginning of circuit design, algorithm selection is necessary, and an algorithm with high power consumption efficiency should be selected as much as possible. First of all, from the perspective of the size of the logic required to implement the algorithm, the less 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 the algorithm is applied. In the actual design, it is necessary to balance the overall performance and power consumption 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 cooperative modules, and the smaller the power consumption required to implement the algorithm. In addition, there are few temporary variables in the algorithm, the effective time of temporary variables is short, and the rational 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 practice is that when it is in an idle state, it operates in a sleep state, and only some devices are working; when an interrupt is generated, other devices are woken up by this interrupt. In fact, this part requires the support of hardware, such as: low power consumption technology of the power system; the division of system software and hardware is to determine which functional modules are implemented by software with low power consumption, and which functional modules are implemented by hardware with low power consumption ; Selection of low-power processors.

System Hardware Design

Considering the source of system power consumption and the low power consumption design strategy comprehensively, the hardware design selects single chip microcomputer and RF transceiver chip with low power consumption characteristics, and simplifies the circuit as much as possible to reduce power consumption expenditure.

Selection of main chips

The structure of MSP430 series microcontrollers is completely based on the low power consumption of the system. The power supply adopts a low voltage of 1.8~3.6V, the power consumption of the active mode is 250μA/MIPS, and the power consumption of the RAM data retention mode is only 0.1μA. Since the system is in sleep or low power consumption state more than 90% of the time, leakage current becomes another important factor affecting system power consumption. The maximum leakage current of its I/O input port is only 50nA. Plus there is 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 clocks required by the CPU and each functional module are generated by the clock system, and these clocks can be turned on or off under the control of instructions, so as to control 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 is one active mode (AM) and five low-power modes (LPM0~LPM4) in the system. In addition, MSP430 series MCU adopts vector interrupt, supports more than ten interrupt sources, and can be nested arbitrarily. It only takes 6μs to wake up the CPU with an interrupt request. Through reasonable programming, the system power consumption can be reduced, and the external request can be quickly responded.

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 is small in size and low in 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., Four bands, 433MHz and 315MHz, can also be configured by program to provide -30~10 dBm output power in all bands Built-in address decoder, FIFO stack area, modulation processor, clock processor, GFSK filter, low noise amplifier, Frequency synthesizer, power amplifier and other functional modules. It has two low-power operating modes: shutdown mode and idle mode, and the operating current is less than 200nA in shutdown mode. In this paper, CC1100 works on the frequency of 433MHz, adopts FSK modulation mode, the data rate is 100kbps, and the channel interval is 200kHz.

circuit design

In order to simplify the system structure, this system only consists of the necessary microprocessor unit, radio frequency transceiver unit, antenna and battery unit. The voltage regulator circuit between the battery and the device is omitted, and the system is directly powered by the battery. The quiescent current consumption caused by the voltage regulator circuit is saved, and the battery life is further extended. In order to prevent the transient reduction of the battery voltage caused by the large current in the emission state, a large-capacity capacitor is used in parallel with the battery. The zero-power under-voltage reset (BOR) protection function integrated in MSP430F2012 can perform a complete reset when the voltage is lower than the safe operating range, which solves the problem of random error operation caused by incomplete reset of the microcontroller.

software design

Replacing hardware with software as much as possible is also a measure often taken in low-power system design. This program development comprehensively considers two aspects of timing scheduling and work efficiency to reduce the power consumption of the system.

Reasonable design of work schedule

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, and it is the key to software design to reduce the power consumption of the single-chip microcomputer system in the idle mode or the power-down mode for a long time. The flow chart of program operation is shown in Figure 2 (a) and (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 will the microcontroller wake up and enter the working mode. Complete the processing of information or data in a short period of time, and return to the low-power mode immediately when the processing ends to 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 into the stack (RAM), and when it leaves, it will pop the CPU register off the stack, which will bring at least two operations to the RAM, so reading RAM will be more difficult than reading Flash. greater power consumption. Using macro definitions to replace subroutine calls will undoubtedly reduce the power consumption of the system.

Minimize the computing workload of the CPU, pre-calculate the results of some operations, put them in Flash, and use the table lookup method to replace real-time computing, reduce the computing workload of the CPU, and effectively reduce the power consumption of the CPU; unavoidable real-time computing Calculation ends when the precision is enough; try to use short data types: for example, try to use 8-bit character data instead of 16-bit integer data, try to use fractional operations instead of floating-point operations, etc.

Let the I/O modules run intermittently, and the I/O modules that are not in use or the I/O modules that are used intermittently should be turned off in time to save power. Unused I/O pins should be set as output or input, and pulled up with a pull-up resistor. If the pins are not initialized, the leakage current of the microcontroller may increase.

in conclusion

This paper introduces the design of low-power active RFID tag based on MSP430 microcontroller in detail, and makes reasonable use of MSP430 microcontroller’s interrupt, timing, operation and other functions. With the help of software advantages, the CC1100 module with low energy consumption is limited in energy. The measures improve the battery life and increase the reliable operation time of the system. The power consumption comparison with other designs is shown in Figure 3. The design of this active RFID tag improves the performance of RFID, which largely solves the identification problem of long-distance, large-flow, anti-interference, and high-speed moving markers. The designed RFID tag and the matching reader can form a person or item identification and positioning system, which is widely used in mining, industrial production, road transportation, logistics transportation, medical treatment, medicine, national defense and security and many other fields.