With the rise of the global automotive electronics industry and people's increasing pursuit of information-based and intelligent automotive safety, Tire Pressure Monitoring Systems (TPMS) have rapidly developed into another emerging automotive safety industry after airbags and ABS. Currently, TPMS are mainly divided into indirect and direct types. Indirect TPMS uses ABS tire speed sensors to measure the rotational speed of each tire, thereby determining tire pressure. While this system has advantages such as not requiring batteries and high durability, its accuracy and reliability are poor. Direct TPMS systems are mainly used during vehicle operation to automatically monitor tire pressure in real time, providing warnings for low tire pressure due to tire leaks and tire blowouts due to high temperature and high tire pressure, ensuring driving safety, and thus gradually becoming the mainstream in the market. This article mainly introduces the design of a direct TPMS system based on the MPXY8020 sensor and analyzes the design challenges of direct TPMS. TPMS System Block Diagram and Working Principle: The TPMS system consists of several transmitting modules and one receiving module. Figure 1 shows the block diagram of the transmitting module. This module is installed inside the tire and measures the pressure and temperature of the gas inside the tire using pressure and temperature sensors. The central processing unit (CPU) at the transmitting end is responsible for data processing, framing the transmitter ID number, pressure, and temperature information, and then sending it to the RF transmitting circuit via Manchester encoding. Finally, the RF circuit modulates the data using FSK/ASK and transmits it through the transmitting antenna. The entire transmitting module is powered by a single battery. Figure 2 shows the block diagram of the receiving module. This module is installed at the front of the driver's cab in a location easily visible to the driver. The RF receiving circuit receives the wireless signal from the transmitting module via the receiving antenna, demodulates and decodes the received signal, and sends it to the CPU at the receiving end. After processing the data, the CPU displays the pressure/temperature values ​​of each tire on the corresponding position on the display according to the ID number, allowing the driver to monitor the pressure and temperature information of each tire at any time, ensuring driving safety. If any tire abnormality occurs, the receiving module will automatically issue an alarm to the driver. The receiving module can be powered by the vehicle's power supply or a battery. [align=center]Figure 2 Receiver Module[/align] Design of Transmitter Module Based on MPXY8020 Sensor The transmitter module design in the TPMS system adopts Motorola's MPXY8020 and 68HC908RF2. The former is a capacitive pressure and temperature sensing chip, and the latter is an integrated chip of MCU and RF transmitter circuit. Transmitter Module Hardware Circuit Design The MPXY8020 is a multi-functional, low-power sensing chip specifically designed for TPMS systems. In addition to pressure and temperature sensing circuits, it also has a digital interface circuit with internal wake-up function, as shown in Figure 3. The interface between the 8020 sensor and the MCU has 6 ports: S1 and S2 are the 8020's working mode control ports. Depending on the MCU's different logic state control of these two ports, the 8020 can work in low-power standby mode, pressure measurement mode, temperature measurement mode, and measurement data output mode, respectively; DATA and CLK are software-controlled serial interfaces for transmitting measurement data; the OUT port is a multiplexed port. In standby mode, the 8020 can internally wake up every 3 seconds and wake up the MCU via interrupt through the OUT port. When the 8020 is in measurement data output mode, OUT serves as the logic state output of the internal comparator; RST has the function of resetting the MCU after 52 minutes. [align=center]Figure 3 Transmitting Circuit[/align] It is worth noting that the A/D conversion of the 8020 sensor is a successive approximation feedback converter. Its principle is that the 8020 has an internal 8-bit shift register. The MCU first inputs 10,000,000 binary data through the DATA and CLK ports of the 8020. The analog quantity converted by the 8-bit register D/A converter is compared with the actual measured value of pressure or temperature, and the state of the OUT port is determined. If the OUT port is low, it means the approximation value is greater than the actual value, and the highest bit of the actual value can be determined to be 0; if the OUT port is high, it means the approximation value is less than the actual value, and the highest bit of the actual value can be determined to be 1. This process is repeated bit by bit from high to low, successively approximating the actual measured value, thus allowing the MCU to determine the final pressure and temperature measurement values. The 68HC908RF2's internal high-performance 8-bit processor is primarily responsible for data transmission with the sensor, data analysis and processing, and data transmission and control with the RF transmitting circuit. In the transmitting circuit shown in Figure 3, PTA1 and PTA2 serve as output ports controlling the 8020's operating mode; PTA3 and PTA4 act as outputs shifting the input measurement approximation value to the 8020; PTA5 is a multiplexed terminal. When the MCU reads the sensor's measurement value, PTA5 obtains the comparator's state through OUT. When the MCU is in power-saving mode, PTA5 serves as a keyboard interrupt input port, obtaining a 3s interrupt through OUT. Additionally, S1 is a speed switch; it can be turned on or off when the vehicle is running or stopped, allowing the MCU to effectively process the program based on the vehicle's operating status. The 68HC908RF2 also integrates a multi-band FSK/OOK modulation circuit, whose operation is determined by the logic state of the digital control terminals (BAND and MODE). BAND is the operating frequency band selection port. Setting BAND high selects crystal oscillator Y1 at 13.56MHz, which generates an RF signal with a carrier frequency of 434MHz after 32-fold frequency multiplication. MODE is the FSK/OOK modulation mode selection port. Setting MODE high enables the RF circuit to operate in FSK mode. Although the 68HC908RF2 integrates the MCU and RF circuit, its interface circuit still requires external connection, as shown in Figure 3. PTB1 is the enable terminal for output control of the RF circuit; PTB2 and PTB3 are serial ports connected to RFDATA and DATACLK of the RF circuit. Information such as pressure and temperature, after Manchester encoding, is transmitted to the transmitting circuit as a binary data stream, and the transmitting circuit then transmits it in FSK mode. The working principle of its FSK is relatively simple. When the RFDATA input is "1" or "0", it causes a change in the output impedance of the CFSK, thereby switching the two load capacitors C1 and C2 of the crystal oscillator Y1. The change in load capacitance causes a small shift in the resonant frequency of the crystal oscillator. After frequency multiplication, the FSK signal can be generated. Transmitter Module Firmware Program Design The firmware program of the transmitter module is relatively simple in function, but considering the reliability and lifespan of the system, it places high demands on the safety, economy, and effectiveness of the program design. Especially since the TPMS transmitter, relying on a single 500mAh lithium battery, needs to operate for more than 8 years, in addition to excellent hardware design, the economical and effective control of the various circuits of the transmitter module by the firmware program is particularly important. Figure 4 shows a simplified program flowchart. Considering power saving, the entire transmitter module is generally in power-saving mode. When the MPXY80203s interrupt wakes up the MCU, the MCU immediately controls the 8020 to perform pressure and temperature detection and obtain the measured values. The MCU then judges the measured data to see if the tire pressure and temperature are normal: if the tire pressure and temperature are normal, it then judges the timed data transmission time. If the timed time has not yet arrived, it enters power-saving mode; if the timed time has arrived, it performs framing, Manchester encoding, and RF data transmission, and finally enters power-saving mode again. Conversely, if the tire pressure and temperature are abnormal, it directly enters the data transmission program. [align=center] Figure 4 Transmission Flowchart[/align] Receiver Module Design The receiver of this system uses Motorola's MC33594 receiver chip and 68HC908GT16 central processing unit, and the display uses an LCD screen. Receiver Module Hardware Circuit Design The MC33594 is a high-sensitivity OOK/FSK demodulation chip with automatic gain control, which includes circuits such as mixer, intermediate frequency amplification, phase-locked loop, demodulation, data management, and SPI interface. The MCU can configure the internal registers of the MC33594 via the SPI interface, thereby setting information such as the modulation type, data reception code rate, and RF carrier frequency of the receiver chip. Figure 5 shows the TPMS receiver circuit. The MC33594 receives the RF signal transmitted by the transmitter through the receiving antenna, demodulates the RF signal, and transmits it to the 68HC908GT16 (MCU) via the SPI interface in interrupt mode. The MCU is responsible for processing and displaying the data, and activating the alarm circuit when necessary. [align=center] Figure 5 Receiver Circuit[/align] Receiver Module Firmware Program Design The receiver program is similar to the transmitter program. Although the function is simple, from a reliability perspective, especially when a receiver needs to receive data from four or more transmitters simultaneously, the effectiveness and timeliness of the data processing in the receiver program are more important. Figure 6 shows the receiver program flowchart. Considering the effectiveness of the received data, we designed the data to be received via SPI interrupt mode. After receiving a data frame, the MCU parses the transmitter ID number, pressure value, temperature value, and other information, and then judges whether the transmitter ID is consistent with the ID stored in the receiver. If they are inconsistent, the data is discarded and the system enters a power-saving state. If the IDs match, the data is processed and the pressure and temperature data of the corresponding tire are displayed according to the ID. A timely and accurate alarm is triggered when the pressure or temperature exceeds the normal range. [align=center] Figure 6 Receiver Flowchart[/align] Wireless Communication and Protocol The design of wireless communication in the TPMS system is crucial to the reliability of data transmission throughout the system. The MC33594 receiver chip provides flexible hardware and software communication resources. By programming the internal registers CR1, CR2, and CR3 of the MC33594 through software, the carrier frequency of this TPMS system is set to 434MHz, and the wireless data transmission rate is set to 9600bps. Data transmission between the transmitting and receiving modules uses a fixed frame length, with the following format: frame header (2 bytes) + transmitter ID (4 bytes) + pressure data (1 byte) + temperature data (1 byte) + status information (1 byte) + checksum (1 byte) + frame tail (1 byte). The frame header includes a synchronization header, preset ID information (for RF signal identification), and a header flag (in binary Manchester encoding 0110). The synchronization header is used to wake up the internal circuitry of the MC33594 and lock the RF carrier frequency via a PLL. The preset ID information is used to identify system information matching. If the preset ID information matches, the data manager is activated, and then it checks whether the header has arrived. After receiving the header, it officially receives the data. Analysis of Challenges in TPMS System Design Signal Reliability Requirements: TPMS is a wireless transceiver system that measures tire pressure, temperature, and other safety information. Signal reliability is a constant consideration in its design. This reliability includes two aspects: data reception rate and bit error rate. Data reception rate refers to whether the receiver can reliably receive each frame of data transmitted by the transmitter. This involves not only the receiver's sensitivity and the transmitter's transmission power, but also a crucial factor: when the transmitter is mounted on a tire and the receiver is placed inside the vehicle, the vehicle itself acts as a shielding box, significantly attenuating the signal. Combined with data conflicts from multiple transmitters and interference from the surrounding environment, this results in a low data reception rate. Bit error rate (BER) refers to the rate at which the receiver receives incorrect data due to interference from the external environment during transmission, leading to reduced system reliability. In the MPXY8020-based TPMS design, we employ a high-gain transmitting antenna and matching circuit to achieve the ideal transmission power design requirements. Particularly at the receiver, in addition to the matching circuit design, we utilize a dual-antenna receiving mode based on research and analysis of radio transmission paths in automobiles. Furthermore, considering the randomness and conflict of signals transmitted by multiple transmitters, we employ time-gap data transmission. The receiver software design also incorporates an interrupt-reception loop processing mode, resulting in a TPMS signal reception rate exceeding 98%. Additionally, the software design combines multiple verification methods to significantly reduce the system's BER. While frequent data transmission increases the probability of data reception and improves system reliability, it also significantly reduces battery life. Environmental requirements: As a product used in automobiles, TPMS requires extremely stringent environmental adaptability, especially the transmitter. Besides a wide temperature range, it must also meet numerous requirements such as waterproofing, salt spray resistance, vibration resistance, shock resistance, and electromagnetic compatibility. This places high demands on raw materials. For example, the battery's operating temperature must be between -40°C and 125°C; the transmitter casing must be made of high-strength, high-toughness, and high- and low-temperature resistant materials. The manufacturing process of the TPMS transmitter also requires high precision, employing processes such as sealing and potting, and incorporating numerous environmental tests during production. One of the key components, the battery, uses TADIRAN's TLH2450 battery to meet the design temperature requirements. Combining the environmental requirements of the transmitter casing with an analysis of the properties of various plastic materials, the casing is designed using synthetic materials such as nylon and glass fiber, achieving vibration resistance, impact resistance, and high- and low-temperature resistance. Furthermore, the transmitter is manufactured using a potting and sealing method, allowing it to transmit data normally even when completely submerged in water. Size and weight requirements: The TPMS transmitter module needs to be housed within a tire, so its size cannot be too large, as this would cause significant problems during tire installation and removal. Secondly, the transmitter module must be very lightweight, otherwise it would affect the tire's dynamic balance. Therefore, the design of the TPMS strictly limits the size and weight of the transmitter module, achieving a small size and light weight. The lifespan requirement for TPMS systems is generally 8-10 years, which presents a significant challenge for transmitters operating on a single 500mAh battery (larger batteries would increase the size and weight of the transmitter module). Besides the need for extremely low power consumption in the hardware circuitry, the firmware design of the TPMS transmitter is crucial. While ensuring system reliability, the number of sensor measurements and RF data transmissions must be strictly controlled. Therefore, designers often have to optimize the trade-off between system reliability and lifespan requirements. In the TPMS design based on the 8020 sensor, considering that the transmitter operates by waking the MCU with a 3-second interrupt, the transmitter module spends most of its time in power-saving mode. Therefore, the power consumption in power-saving mode is designed to be below 0.6uA. Furthermore, based on statistical analysis of the number of transmitter transmissions under extreme battery life testing, the theoretical lifespan of this TPMS system is calculated to be over 10 years. Conclusion Direct TPMS systems face many challenges in product design. As automotive safety products, more attention should be paid to system failure analysis. With the continuous development of TPMS technology, new TPMS technology solutions are becoming increasingly integrated. In particular, by integrating sensors, MCUs, and transmitter circuits into one unit, the transmitter module is not only smaller and lighter, but its performance and functions are also greatly improved. For example, functions such as LF, LVD, and acceleration testing have been added. The TPMS system can even be connected to the vehicle's electronic system through the CAN bus to achieve information sharing.