Low-power RFID positioning design scheme based on MSP430F2012 and nRF24L01
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1. System hardware design
1.1 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.
1.2 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 an interrupt The way to make it enter the working state for a short time to save power. 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. 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], Realizing 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 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.
1.3 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.
2. System software design
2.1 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” command broadcast by the reader, and wakes 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 verifications like ordinary RFID tags. This system adopts 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 timing 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.
2.2 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, which requires 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. Assuming that 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 very small compared to the entire timing period of 500ms, but it is still possible to send a saturated state. 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.
3. 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 7200 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 receiving sensitivity of the reader-side antenna can also significantly improve the system performance.
4 Conclusion
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 recognized 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 and low power consumption. The communication distance can reach several tens of meters through a well-matched antenna, which can meet the positioning requirements in the general small-scale space underground in the coal mine industry.
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