TPMS design based on Infineon smart sensor SP12/SP30

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TPMS design based on Infineon smart sensor SP12/SP30 [Copy link]

The automobile tire pressure monitoring system (TPMS) mainly uses the pressure sensor installed in each tire to directly measure the tire pressure. Through radio frequency wireless transmission, it automatically monitors the tire pressure in real time when the car is driving, and alarms for tire leakage, low pressure and high pressure to ensure driving safety.

Infineon's SP12/SP30 sensor for TPMS applications integrates silicon micro-machined pressure and acceleration sensors, temperature sensors and a battery voltage monitor, providing four-in-one sensing functions, and is equipped with a CMOS large-scale integrated circuit that can complete measurement, signal compensation and adjustment, and SPI serial communication interface. The SP30 has a built-in 8-bit Harvard structure RISC MCU and a low-frequency (LF) interface of a 2D channel, and consumes only 0.4uA of current.

Based on Infineon's sensor SP12/SP30, Star Semiconductor Co., Ltd. provides ICs and solutions involved in the entire TPMS system to ensure the stability and reliability of the system.

TPMS composition

1. Transmitter module

The transmitter module consists of a pressure sensor, MCU, RF transmitter chip, battery and antenna. The module collects data on tire pressure, temperature, battery voltage and acceleration, and transmits the data wirelessly. The

transmitter module has two solutions based on SP12/SP12T and SP30: Solution one is SP12/SP12T+MCU+TDK5100F (see Figure 1), in which the pressure sensor SP12 (100~450kPa)/SP12T (0~1,400kPa) and the RF transmitter chip TDK5100F (434MHz ASK/FSK transmitter) are both from Infineon. Figure 1




: TPMS transmitter

module based on SP12 Solution two is SP30+TDK5100F (Figure 2).




Figure 2: Block diagram of application solution based on SP30

The pressure sensor SP30 (100~900kPa) has a built-in 8-bit Harvard structure RISC MCU and a 2D channel LF interface. The RF transmitter IC uses Infineon's TDK5100F (434MHz ASK/FSK transmitter). The system can directly receive a 125kHz low-frequency wake-up signal to control the transmitter module to work in different modes.

2. Receiving module

The receiving module consists of TDA5210, XC866/XC886, LCD module and antenna (see Figure 3).




Figure 3: Block diagram of the receiving module of TDA5210+XC866/XC886+LCD module+antenna

The receiving module demodulates and decodes the information sent by the TPMS transmitting module, and outputs the received data through the LCD display. The MCU and RF receiving chip of this module use Infineon's XC866/XC886 and TDA5210 respectively. Among them, XC866/XC886 is an 8-bit MCU designed for automotive electronics, with a CAN/LIN controller, which can quickly transfer TPMS functions to secondary tasks.

SP30_TPMS transmitter module principle

The TPMS transmitter system is actually a timed monitoring wireless system. The core issues of the entire system design are mainly reflected in the low power consumption of the system and the RF receiving sensitivity and noise suppression when the car rotates at high speed. For the Infineon SP30+TDK5100F transmitter system, the following is a detailed explanation of these two core issues

First, the low power consumption of the system is based on hardware and is achieved in combination with software programs, so the selection of static low-power hardware is a prerequisite; secondly, the system should try to stay in the PWDN mode with the lowest power consumption; thirdly, the hardware wake-up of the system from PWDN to RUN mode should be combined with the software value comparison; finally, when the system enters RUN mode, when some units independent of the RISC core are working, the system can be put into idle mode to wait, which will also save power consumption. The measured static current of the Startech DEMO board is 3uA. Figure 4 is the program flow of the SP30+TDK5100F transmitter system.



Figure 4: Program flow chart of the SP30+TDK5100F transmitter system



Figure 5: SP30 internal function block diagram There are some things to note in the software architecture:

1. The entire software architecture is mainly composed of three modules: initialization module, sensor data measurement module (Measure_Data), and RF transmission module (SSI_Datagram). These three modules should be made into function packages as much as possible to facilitate transplantation and function upgrades.

2. After the interval timer (IT) wakes up the system, use the software to set several value zones to determine parameter measurement and RF transmission to save power as much as possible. For example, use acceleration measurement to determine the start/stop state of the car to determine whether other parameters need to be measured, determine the approximate speed of the car to determine the RF transmission frequency, etc.; use undervoltage and high voltage level areas to determine the RF transmission alarm frequency, etc. These need to be improved by customers according to actual conditions. It should be noted that all the values should not be set to a fixed value, but to a certain area. Of course, the setting of this area should be repeatedly measured. If it is too small, it will affect the stability of the system and is not conducive to saving power consumption. If it is too large, the measurement sensitivity is poor.

3. Use IT and LT (LF timer) to enable and disable LF interface detection, so that the LF interface does not need to be always open and can ensure real-time detection of host commands, saving power to a greater extent.




Figure 6: TDK5100F internal block diagram




Figure 7: TDA5210 internal block diagram

Secondly, the RF receiving sensitivity and noise suppression when the car rotates at high speed. The transmitting module is built into the tire. In addition to the impact of the ambient temperature in the tire and the RF signal shielding on the RF, the impact of the high-speed rotation of the car on the RF performance needs to be solved. It should be emphasized that the RF devices selected by the TPMS system are basically integrated circuits, and the performance of the RF IC directly affects the performance of the entire system. What the design engineer needs to do is to select some peripheral devices of this IC (crystal oscillator, antenna), match the parameters of the power amplifier and antenna, and layout the RF. Doing these aspects well can basically solve the above problems. Infineon RF TX chip TDK5100F is a low-speed (120Kbaud) low-power RF IC designed for automotive level, and the matching RX chip TDA5210 has a receiving sensitivity of -107dBm.

The following issues need to be paid attention to in RF design:

1. The selection of crystal oscillator. TDK5100F/TDA5210 is a narrowband RF IC. The crystal frequency difference caused by temperature and the inconsistency of crystal load capacitance will lead to differences in receiving sensitivity, so the selection of crystal characteristics is very important.

2. Antenna selection. The installation requirements of the TPMS transmission system are relatively high. Of course, in addition to the antenna performance, there are also equally high requirements for its appearance structure. Considering the compromise between the two, the spiral antenna is currently more commonly used. Although the PCB loop antenna has the best structure and cost, it is not widely used in TPMS because its resonant center frequency and equivalent impedance need to be calibrated by a network analyzer, and the antenna loss caused by its own PCB material. The performance of the single-stage antenna can be done very well, but the structure does not have good installation performance, and it is not used much. 3. TD5100F layout

points. The crystal layout should be far away from the antenna, the matching components should be arranged at right angles to each other, and the antenna should not be laid on the ground or run other signal lines.

4. Under the premise that there is no need to make major changes to the PCB, use a network analyzer to make the final determination of the antenna parameter matching, and measure the transmission power and receiving sensitivity.

The receiving module

of this system is composed of TDA5210+XC866. In fact, XC866 only needs one I/O to receive the demodulated data of TDA5210. The TPMS system to be considered belongs to the automotive electronic safety system. In order to upgrade and expand the system tasks, the selection of MCU should also meet the requirements of the automotive environment as much as possible. Infineon XC866/XC886 is an 8-bit MCU designed for automotive electronics with a CAN/LIN controller, which can quickly transfer the TPMS function to secondary tasks. If the TPMS function exists as a node at this time, the MCU can be released to perform other tasks. In automotive electronics, such tasks are quite a lot of motor control, which uses the powerful peripheral functions of XC866 (motor control unit and PWM capture comparison unit). Therefore, the selection of the receiving end MCU is not only related to the automotive-level MCU, but also should have some advanced awareness.

jacky.lu

In fact, when it comes to solutions, everyone has an idea in mind. It can also be said that it is just a selection process in the early stage, and the concept of combination is different. In the final analysis, the real core technology of TPMS is not in the selection of these combinations, but the key lies in the EMC interference problem in the system design process. For example, circuit board layout design, component selection, antenna parameters, shape, material, etc., and the most important is the EMC filtering algorithm of the software. Of course, now that many manufacturers have formed a vehicle body network, how to diagnose is also a very important thing. The vehicle manufacturer's network protocol is not connected, and the corresponding system diagnosis protocol is also different.