TPMS design based on intelligent sensor MPXY8300

Abstract: This paper introduces the design method of the current tire pressure monitoring system, proposes the design scheme of the tire pressure monitoring module based on the MPXY8300 intelligent sensor and the receiving module based on the MC33596 receiver, and gives the hardware circuit and software program flow. The system can measure the actual temperature and instantaneous pressure inside each tire at any time, determine the faulty tire and issue a warning in real time, effectively avoiding the occurrence of tire blowout accidents.
Keywords: tire pressure monitoring system; MPXY8300; MC33596

1 Overview
When the tire is under pressure, the tire temperature will rise, causing damage to the tire; under pressure, it will increase fuel consumption and even affect the maneuverability and braking performance of the car. The automobile tire pressure monitoring system (TPMS) is mainly used for real-time automatic monitoring of tire pressure when the car is driving, and alarming for tire leakage and low pressure, which can effectively extend the tire life, save fuel, and improve the safety of the car. In terms of the TPMS system structure, the temperature and pressure data collected need to be sent and received wirelessly, and the transceiver circuit must be installed in the tire. This requires that the chips that make up the circuits must have the characteristics of high temperature resistance, low power consumption, and small size.
At present, there are three popular TPMS solutions from Infineon, Freescale, and General Motors, all of which are developed with their own sensors as the core. Infineon's TPMS sensors mainly include SPl2, SPl2T, SP30, etc. Freescale's TPMS system is mainly composed of MPXY80x0 sensors and MC68HC908RF2 microprocessors. General TPMS sensors use NPXI and NPXII. NPXI integrates silicon pressure sensors, voltage sensors, temperature sensors, 8-bit RISC microprocessors, large-capacity memory, and an LF input stage. All measurement signals are output as digital signals for direct customer calls; NPXII has all the functions of NPX and also integrates an acceleration sensor. This type of TPMS design is shown in Figure 1.

The common defect of the above schemes is that the integration of the tire monitoring module is not high enough. Even for the general TPMS solution, it is necessary to connect the wireless RF transmission module outside the NPX module, which inevitably increases the system power consumption and module volume. Based on the intelligent sensor MPXY8300 and the MC33596 receiver, this paper proposes a design method for a highly integrated TPMS system.

2 TPMS hardware circuit design
The highly integrated TPMS system based on the intelligent sensor MPXY8300 is mainly composed of TPMS sensors, microcontrollers and wireless RF receiving modules. The key point in the design of the TPMS system is the data transmission part. The entire data transmission part consists of two parts: the wireless receiving part in the cab and the wireless transmitting part in the tire. The accuracy and stability of the data transmission of these two parts are important manifestations of the excellent performance of the system.
2.1 Data acquisition and transmission circuit design
The data acquisition and transmission circuit is based on Freescale's highly intelligent sensor MPXY8300, as shown in Figure 2. The MPXY8300 series integrates Freescale's low-power S08 core, contains 512 bytes of RAM and 16 KBFlash (8 KB of which is firmware, i.e. some low-level drivers, test programs and calibration data, etc.); at the same time, it integrates a temperature sensor and a single-channel LF low-frequency input function. The RF transmission of the MPXY8300 series supports two carrier frequencies of 315MHz and 434 MHz, and can be configured as ASK or FSK modulation through registers. It also integrates a charge pump function, which can increase the power supply voltage of the RF transmission part when the battery voltage is low, so that it can still achieve a certain RF transmission intensity.

The MPXY8300 TPMS is a product that integrates a pressure sensor, an 8-bit MCU, an RF transceiver, and a dual-axis (XZ) accelerometer into one package. The MPXY8300 TPMS has the following features:
① Accurate pressure measurement. The pressure cell (P-cell) of the low-power surface micromachined capacitive pressure sensor can measure a pressure range of 100 to 800 kPa, and also provides a high-pressure range pressure cell for truck tires (100 to 1400 kPa), and provides optional low-precision calibration for low-cost applications. Capacitive surface microelectromechanical system (MEMS) pressure sensing technology has advantages over piezoresistive bulk MEMS in terms of power usage. Freescale's capacitive surface microelectromechanical system provides 0.14 μA current (3 V, 30 kHz), while piezoresistive large-scale MEMS provides 600 to 10,000 μA. The former has a minimum charge of 0.9 nAs (nano-amp-second) per reading, while the latter is 60 to 1000 nAs.
② Fully integrated. The fully integrated MPXY8300TPMS module provides independent pressure measurement for each individual tire (including spare tire). Each module integrates an RF transceiver based on 315/434 MHz PLL, which can maintain continuous communication even after the tire is rotated or replaced. The integrated motion sensor can be programmed to transmit measurement data at a specific speed (tire rotation), including data of tires that are not rotated.
③ Extended battery life. Some regulations require that the battery life of TPMS solutions be 10 years. The MPXY8300 combines a series of low-power technologies to ensure long-term stable operation using minimal battery resources. The low-frequency vibration that drives the low-power wake-up timer, the regular reset driver, and the specific power management technology of TPMS can extend the battery life and achieve more convenient and economical operation.
2.2 Wireless receiving interface circuit design
The receiving module of the TPMS system is mainly composed of an antenna, a radio frequency receiving circuit, a main control chip MCU, a keyboard, and a display. It is used to receive the tire temperature and pressure data transmitted by each transmitting module, display the ID identification code and measurement data of each tire, and sound and light alarm when abnormal conditions occur. Since the receiving module is installed in the car compartment, the requirements for the selection of devices are not high, and industrial grade is sufficient.
MC33596 is a high-temperature integrated UHF superheterodyne radio receiving module from Motorola. Its interface circuit is shown in Figure 3. MC33596 uses LQFP-24 package, operates in the 300-450 MHz frequency band, and has a voltage range of 4.5-5.5 V; the receiving sensitivity is as high as -103 dBm. The biggest feature of the chip is that it has a serial peripheral interface SPI. Through SPI, it allows the CPU to communicate and exchange information with various peripheral interface devices in serial mode. The SPI interface uses 4 lines; serial clock line (SCLK), host input/slave output data line MISO, host output/slave input data line MOSI, and low-level effective slave selection line CONFB.

The main control chip uses NXP's wireless receiving ARM7 microcontroller LPC2292, which is connected to MC33596 through the SPI interface. LPC2292 contains multiple 32-bit timers, 4 10-bit ADCs, 2 CANs, and up to 9 external interrupts, etc. It is particularly suitable for automobiles, industrial control applications, medical systems, and fault-tolerant maintenance buses; the internally integrated 2-way CAN controller complies with the CAN specification 2.0BISOll 898-1; 32-bit registers and RAM can be accessed; the data rate of each bus is 1 MB/s; the global acceptance filter can identify the 11-bit and 29-bit Rx identifiers of all buses; the acceptance filter
provides a FulICAN-style automatic reception function for the selected standard identifier.
2.3 CAN bus interface circuit design
The TPMS system designed in this paper has CAN bus function and can be connected to the CAN communication port of the car dashboard to directly display the tire pressure, temperature and other data of each tire on the dashboard display. The normal operation of CAN requires a CAN controller and a CAN bus driver. The former can realize the functions of the data link layer and the physical layer in the network hierarchy, and the latter provides the interface between the CAN controller and the physical bus and the differential transmission and reception functions of the CAN bus.
The LPC2292 microcontroller contains two CAN controllers, with a data transmission rate of up to 1 Mb/s on a single bus, 32-bit registers and RAM access, and global filters and acceptance filters. This system uses a dual-channel isolated CAN transceiver CTM8251D, which can connect at least 110 nodes. By expanding the CAN bus interface, the choice of serial communication methods is more diverse. When the in-vehicle instruments also have a CAN bus interface, they can directly use this interface to communicate with the recorder. The CAN bus interface circuit is shown in Figure 4.

3 Software Design
3.1 Data Transmission Module Programming
The tire monitoring module sends data in the form of data packets (frames). When the MPXY8300 in the tire module decides to send data (temperature and pressure data collected by the sensor), it wakes up the receiving module by sending the leading bit of the data frame and then sends the data frame. The data frame format is as follows:

The transmission program flow is shown in Figure 5. After the monitoring module is awakened, it first performs a power detection. If the difference between the pressure value P1 and the pressure threshold value P2 stored in the ROM exceeds the set pressure difference, it means that the tire pressure exceeds the limit and an alarm is required. At this time, in order to enhance the reliability of the receiver receiving data, 16 frames are sent continuously.

3.2 Data receiving module program design
After the power is turned on, the receiver initializes itself, configures the transmitter related parameters, the indicator light flashes, and the module enters the working state. After receiving a data frame, the data is checked and verified to see if it is wrong, and the corresponding indicator light is lit according to the device ID of the received data to indicate an alarm, and a voice alarm can also be realized. At the same time, the data frame is transmitted to the instrument panel through the CAN bus interface to complete the display alarm of the information. The receiving program flow is shown in Figure 6.

Conclusion
This paper uses Freescale's highly integrated MPXY8300 chip, receiving chip MC33596, and microcontroller chip LPC2292 to obtain a relatively complete TPMS system design solution. The system has obvious advantages in power consumption, volume, transmission and reception distance, reliability, and safety.