Application design of active RFID tag based on MSP430F2012

This article gives a detailed introduction to the design of active RFID tags based on MSP430F2012 and nRF24L01. Analyzed the low-power performance of the two chips and put forward their own low-power design scheme; combined with the characteristics of RFID positioning, introduced the design method that is different from the general label with identification as the main purpose, and analyzed its Software design process; in view of the characteristics of many identified targets in general space and often in a moving state, the system’s anti-collision capabilities are introduced.

Introduction: This article gives a detailed introduction to the design of active RFID tags based on MSP430F2012 and nRF24L01. Analyzed the low-power performance of the two chips and put forward their own low-power design scheme; combined with the characteristics of RFID positioning, introduced the design method that is different from the general label with identification as the main purpose, and analyzed its Software design process; in view of the characteristics of many identified targets in general space and often in a moving state, the system’s anti-collision capabilities are introduced.

1 Introduction

Radio frequency identification (RFID) technology uses radio frequency to achieve two-way data exchange and identification. RFID positioning uses this identification feature and uses the communication signal strength between the reader and the tag for spatial positioning.

RFID tags are divided into two types: active and passive according to the power supply mode[1], Passive tags obtain energy by capturing the electromagnetic waves emitted by the reader, and have the advantages of low cost and small size; active tags usually use battery power, which has the advantages of long communication distance, fast reading speed, and good reliability.[2], But in order to meet the underground positioning of coal mines, low-power design needs to be considered to enhance battery life. Starting from the design concept of active tags, this article designs the RFID tags based on the principles of low power consumption and high efficiency for the needs of small-scale RFID positioning, and expounds its hardware composition, software flow and anti-collision capabilities.

2. System hardware design

2.1 System structure

In addition to low-cost and miniaturization in the design of active tags, the most important thing is to adopt a low-power design.

From the overall structure of the RFID tag, it usually includes two parts: the control end and the radio frequency end. Therefore, it is necessary to give priority to its low power consumption performance when selecting the control chip and the radio frequency chip. On this basis, this article chooses the MSP430F2012 control chip and nRF24L01 radio frequency chip; the antenna uses the PCB single-ended antenna of Nordic; the label uses a 3V-500mAh button battery to supply power. The system works in the 2.4GHz frequency band. The system structure block diagram is shown as in Fig. 1.

Application design of active RFID tag based on MSP430F2012

2.2 Chip selection and low-power design

TI’s MSP430 series microcontroller is a 16-bit Flash type RISC instruction set microcontroller[3], Well-known in the industry for its ultra-low power consumption.

The working voltage of the MSP430F2012 chip is only 1.8~3.6V, the current consumption is 0.1μA in the power-down mode, and the current consumption is only 0.5μA in the standby mode. In this design, the MSP430F2012 is placed in the standby mode for a long time and wakes up through interrupts. The way to make it enter the working state for a short time, in order to save energy. MSP430F2012 has 3 groups of independent clock sources: on-chip VLO, off-chip crystal oscillator, DCO. Among them, the off-chip clock is based on an external crystal oscillator; DCO is generated on-chip, and the frequency is adjustable. Obviously, the main system clock frequency determines the power consumption of the system, especially when a high-speed off-chip crystal oscillator is selected. Therefore, MSP430F2012 provides the function of switching between different clock sources. In the actual design, the basic clock control register is reconfigured in real time to realize the switch between the main system clock and the auxiliary system clock, which not only saves performance but also saves energy consumption.

MSP430F2012 has five low-power modes, LPM0~LPM4, and reasonable use of these five preset modes is the key to reducing MCU power consumption. In this design, MSP430F2012 will directly enter LPM3 mode after power-on configuration, and enable interrupts at the same time , Waiting for external interrupt signal. In addition, because MSP430F2012 is a multifunctional general-purpose single-chip microcomputer with many functional modules integrated on the chip, stopping all unused functional modules during power-on configuration can also achieve the purpose of reducing system power consumption.

nRF24L01 is a 2.4GHz ultra-low power single-chip wireless transceiver chip developed by Nordic. The chip has 125 frequency points, which can realize point-to-point and point-to-multipoint wireless communication. The maximum transmission rate can reach 2Mbps, and the working voltage is 1.9~3.6 V[4]In order to highlight its low power consumption performance, the chip is preset with two standby modes and a power-down mode.More worth mentioning is the ShockBurstTM mode and enhanced ShockBurstTM mode of nRF24L01[4], Realized low-speed input and high-speed output, that is, the MCU sends data into the nRF24L01 on-chip FIFO at low speed, but transmits it at a high speed of 1Mbps or 2Mbps. This design utilizes the enhanced ShockBurstTM mode, which enables the MSP430F2012 to transmit data at high speed through the radio frequency even under the 32768Hz low-speed crystal oscillator, which not only reduces power consumption, but also improves efficiency, and enhances the system’s anti-collision and coping with mobile Target ability.

2.3 Circuit design

This system is mainly used in RFID positioning. In addition to simple identification, the focus is on the reader’s measurement of tag signal strength. Therefore, there will be no frequent read and write operations with large amounts of data between the reader and the tag, which can be omitted during circuit design. Off-chip EEPROM. At the same time, voltage regulator circuits can be omitted to save quiescent current consumption. The hardware schematic diagram is shown as in Fig. 2.

Application design of active RFID tag based on MSP430F2012

3. System software design

3.1 Software process

This system is a two-way communication system, the tag is in the monitoring state before sending data, the receiving function of nRF24L01 is turned on, and the MSP430F2012 is in LPM3 mode until it receives the “start” instruction broadcast by the reader, and wakes up the MSP430F2012 through an interrupt. After the MSP430F2012 is awakened by an interrupt, it starts to judge whether the instruction is correct. If it is correct, it enters the normal sending cycle, otherwise it returns to the LPM3 mode.

Taking into account the need for real-time positioning, the system cannot perform only limited verification like a general RFID tag. This system uses a mode of continuous transmission at equal intervals to facilitate the reader to monitor the target position in real time. The normal transmission period set by the system is 500ms. Timer_A timing of MSP430F2012, after the 500ms timer starts, the tag ID is sent to the FIFO via SPI, nRF24L01 adopts the enhanced ShockBurstTM mode, the transmission will continue to be retransmitted if the transmission fails, after the tag ID is sent, MSP430F2012 judges whether the timer has timed out, and once it times out, Enter the next sending cycle, otherwise it will be in a waiting state until it times out. When the reader stops broadcasting the “start” command, MSP430F2012 re-enters the LPM3 mode to reduce power consumption.

The complete process of the system is shown in Figure 3.

Application design of active RFID tag based on MSP430F2012

3.2 Anti-collision design

nRF24L01 has its own carrier detection function. Before sending data, switch to receiving mode to monitor, confirm that the frequency channel to be transmitted is not occupied before sending data. This function can realize simple hardware anti-collision.

Taking into account that this system uses a uniform transmission interval of 500ms, identification conflicts may occur when there are many targets to be located, so it is necessary to reasonably increase the anti-collision algorithm in the program. The ALOHA algorithm is mainly used for active tags. Its principle is that once a data packet collision occurs in the source, the source is randomly delayed before sending the data again. Considering that the complexity of the program will inevitably lead to an increase in processing time and additional energy consumption, this system uses a simpler pure ALOHA algorithm, that is, the tag ID is sent randomly within each 500ms timing period. A random delay is inserted into the program, and the selection of the delay time is realized by a random value function, and the random delay range is 0~300ms. This simple anti-collision algorithm not only simplifies the instruction, but also greatly reduces the probability of conflict.

In addition, the transmission rate of n RF 2 4 L 0 1 is 1 M bps or 2 Mbps, and a single data packet is sent at a time. The maximum single data packet is 32 bytes. If the tag ID is 32 bytes, the signal width (transmission time) of the ID is sent once at a rate of 2 Mbps. It is about 100~150μs, which is insignificant relative to the entire timing period of 500ms, but there may still be a state of transmission saturation. At this time, the timing period can be appropriately extended to increase the channel capacity. A faster transmission rate is helpful for the identification and positioning of moving targets, and a shorter data length can also significantly improve the tag’s anti-collision capabilities based on random delays. Therefore, as far as possible, the length of the tag ID is limited to 32 bytes.

4. Test results

For RFID systems, the most important parameter is the reading distance[5]And effective read rate. The test equipment in this experiment is 3 tags, one reader and one PC. The reader is based on MSP430F149 and nRF24L01 chip design, and communicates with PC through RS232 serial port. In the test, three tags were placed 15m, 30m, and 45m away from the reader. The note IDs were AABBCCDDFFFFFF01, AABBCCDDFFFFFF02, AABBCCDDFFFFFF03, and each tag was read continuously for one hour (about 7,200 times).

From the test results in Table 1, it can be seen that within 30m is the normal reading distance of the tag, which can meet general indoor applications. When the distance is 45m, the reading rate drops significantly.Because the design of the antenna has a greater impact on system performance[6], By improving the antenna of the tag to obtain a larger output power, improving the antenna receiving sensitivity of the reader can also significantly improve the system performance.

Application design of active RFID tag based on MSP430F2012

5. Concluding remarks

This article gives a detailed introduction to the design of active RFID tags based on MSP430F2012 and nRF24L01. Analyzed the low-power performance of the two chips and put forward their own low-power design scheme; combined with the characteristics of RFID positioning, introduced the design method that is different from the general label with identification as the main purpose, and analyzed its Software design process; in view of the characteristics of many identified targets in general space and often in a moving state, the system’s anti-collision capabilities are introduced. The whole system has simple circuit, small size, low power consumption, and the communication distance can reach tens of meters through a well-matched antenna, which can meet the positioning requirements of the general small-scale space underground in the coal mine industry.

The Links:   LM171WX3-TLC2 NL6448BC33-71

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