The current TPMS (TIre pressure monitoring system) is mainly divided into two types, one is indirect and the other is direct. The direct TPMS system has been affected by its poor accuracy of alarms and the interference of shielded electromagnetic waves and magnetic fields of the hub during wireless transmission. Based on the RF chip produced by Nordic, this paper introduces the circuit design and solution of the wireless transceiver system of the automotive TPMS system, and improves the above-mentioned direct TPMS system.
In recent years, wireless data transmission technology has become a major component of small industrial monitoring systems. Compared with wireless data transmission and existing wireless data transmission, wireless data transmission in industrial monitoring mainly uses radio frequency. (RF) technology to send and receive data packets. The entire system is dedicated to the purpose of wireless data transmission, so in the software design of the entire system, the wireless data transmission part (RF module) is the most important part. Radio frequency is the RF current. It is an abbreviation of high-frequency AC-changing electromagnetic wave. The alternating current that changes less than 1000 times per second is called low-frequency current, and the high-frequency current is more than 10,000 times. The radio frequency is such a high-frequency current. Radio frequency technology has a wide and irreplaceable role in the field of wireless communications.
1 TPMS system design
TPMS system mentioned herein is done by prompting the transmission and reception of RF-related information information. In the tire part, there are mainly a pressure detecting module and a radio frequency transmitting module, and in the driver part, there are mainly a radio frequency receiving module and a display alarm module.
The pressure detection module mainly detects the pressure data of the measured tire in real time, and stores the corresponding changes of the data on the central processor, waiting for the radio frequency transmitting module to transmit. After receiving the data, the RF receiving module processes the data accordingly and displays the data on the accessory LCD, and judges the security of the data. After the tire pressure is found to exceed the acceptable range, the alarm is sent through the buzzer. Remind the driver. The specific working principle diagram is shown in Figure 1.
2 Comparison and selection of wireless transceiver chipsDue to the variety and number of wireless transceiver chips, the choice of wireless transceiver chips is crucial in the design. The correct choice can reduce the development difficulty, shorten the development cycle, reduce the cost, and bring the products to market faster. When selecting a wireless transceiver chip, you should consider several factors: power consumption, transmit power, receiving sensitivity, number of peripheral components required for transceiver chip, chip cost, and whether Manchester data encoding is required for data transmission.
Several important indicators used to evaluate wireless data transmission and reception are: receiving sensitivity, dynamic range, selectivity, receiving frequency stability, transmission ratio power, efficiency, transmission frequency range, power consumption and other factors. The following is a comparison of the performance of the three popular wireless transceiver chips at the current stage in the market, as listed in Table 1.
According to the expressions of Table 1 and the technical indicators, for receiving and transmitting data, the larger the range of the frequency band is satisfied, the higher the sensitivity, the smaller the receiving current under the same condition, the larger the emission current, and the fewer peripheral devices required. The transceiver chip application is more extensive and can be adapted to a variety of situations.
Based on the above considerations, the superiority of nRF905 is obvious, so choosing nRF905 in this design is the most suitable.
3 Introduction to nRF905The wireless transceiver chip nRF905 is a monolithic wireless transceiver chip introduced by Nordic Corporation of Norway. The nRF905 is a single-chip RF transceiver operating in the 433/868/915 band. Because of the international ISM band, there is no so-called band limitation. The nRF905 consists of a fully integrated power management, frequency synthesizer, modulation receiver, power amplifier, crystal oscillator and regulator. The nRF905 features ShockBurst, which automatically processes the preamble and CRC checksum in the packet. The nRF905 configuration operation can be easily programmed via the SPI interface. The nRF905 consumes very little power. In the transmit mode, the output power consumption is only 11 mA at an output power of -10 dBm; the same power consumes 12.5 mA in the receive mode. In the Power DOWN power-down mode, it can save more power.
3.1 nRF905 control modeThe nRF905 has two active modes and two power saving modes.
The activation modes include the ShockBurst RX receive mode and the ShockBurst TX transmit mode. Power-saving modes include PowerDown an DSP I-programming power-down and SPI programming modes and Standby and SPI-programming standby and SPI programming modes.
The settings of TRX_CE, TX_EN and PWR determine the control of the mode. The specific control is listed in Table 2.
3.2 Connection between MCU and nRF905The nRF905 communicates with the outside world via an SPI interface, and the rate is determined by the interface speed set by the microcontroller. In RX mode, the address match (AM) and data ready (DR) signals inform the MCU that a valid address and data packet have been received, and the microcontroller can read the received data via the SPI. In the TX mode, the wireless communication module automatically generates a preamble and a CRC check code, and the data ready signal informs the MCU that the data transmission has been completed.
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