The instrument panel is the interface between the driver and the car, providing the driver with necessary information such as vehicle operating parameters, faults, and mileage. It is an indispensable component of every car. The intuitiveness and aesthetics of the instrument panel display make driving not only a necessity for transportation but also a part of a comfortable life. The accuracy and reliability of parameter transmission are directly related to driving safety. The most prominent feature of modern instrument panels is their modularity. Typically, assembling an instrument panel involves combining several functional modules on a custom PCB to obtain a complete system. In economy cars like Changan, the analog instrument panels are generally independent modules, with warning/alarm functions implemented by small bulbs on the PCB. In the SANTANA series, the pointers are driven by cross coils on the printed circuit board, and all warning/alarm lights are LEDs. The main data, sources, and corresponding display methods displayed on the dashboard of a car are shown in the table below (based on SANTANA2000): [align=center]Table 1 [img=208,156]http://www.e-works.net.cn/images/128002109448437500.GIF[/img][/align] It can be seen that current car dashboards are mainly mechanical, and communication methods are mainly based on wiring harnesses. The biggest drawback of this model is its over-reliance on wiring harnesses, leading to system complexity. With the rapid development of the automotive industry, the automotive electronics market, which still has great potential, is increasingly attracting the attention of major car manufacturers and electronics suppliers. This includes clean energy vehicles that are still in the development and experimental stage, in-vehicle audio equipment currently dominated by *** in the world market, promising in-vehicle computer information terminal equipment, and various in-vehicle electronic control units, such as engine electronic control units, ABS, bus systems, and new electronic instruments. For car dashboards, the advantages of using electronic dashboards over traditional dashboards are:

[＊] Software-based hardware functionality

With the development of microelectronics technology, microprocessors are becoming increasingly faster. Some functions with high real-time requirements, which were originally performed by hardware, can now be implemented entirely through microprocessor programming.

[＊] The system integration level is greatly improved.

With the rapid development of large-scale integrated circuit technology, integrated circuits are becoming increasingly dense, smaller, more complex in internal structure, and more powerful in function. As some functions of instrumentation hardware are increasingly replaced by software, the integration level of the entire system is also increasing accordingly. This design is a car instrument cluster based on a Freescale microcontroller. It targets the mainstream traditional automobiles currently on the market and uses the instrument cluster of Changan cars as a template, aiming to implement a low-end solution for a car electronic instrument cluster. 1. Design Overview The main objects of this solution include vehicle speed, engine speed, pressure, temperature, mileage, time, and some indicator/alarm light signals, respectively displayed on the speedometer, engine tachometer, oil pressure gauge, water temperature gauge (pointer), and odometer. The odometer periodically records the vehicle speed signal, integrates the values ​​over the driving time to obtain the mileage, and stores it as the mileage in the user's Flash memory. The mileage data is accumulated periodically (1 second). Since this design targets mass-market traditional automobiles, cost is the most important criterion for component selection. For the microcontroller selection, we did not consider the promising but not yet widely adopted CAN bus, but instead chose the LJ12 8-bit microcontroller with a built-in LCD driver. The MC68HC908LJ12 is a member of the Freescale 908 family of microcontrollers, and its main parameters and functional modules are as follows: • Maximum bus frequency of 8MHz; • 12k user Flash; • 512b RAM; • Two independent dual-channel timer modules; • Internal real-time clock, capable of recording year, month, day, hour, minute, and second; • SCI and SPI communication modules; • 6-channel 10-bit A/D converter; • LCD module, capable of driving 27×3 or 26×4 segment LCD modules. The stepper motor driver chip MC33970 is a member of the Freescale analog family, containing 6 registers, with command status words written via SPI: configuration/calibration register, pointer speed register, pointer 0/1 position register, pointer return-to-zero register, and return-to-zero setting register. Features of the MC33970: • Compatible with MMT-certified two-phase stepper motors; • Pointer can stop at 4095 stable positions; • Maximum rotation angle 340°; • Maximum acceleration 4500 deg/s²; • Maximum speed 400 deg/s; • Analog microstepping (12 steps equal 1°); • Pointer calibration and homing; • SPI control, 16-bit command word; • Internal calibration clock; • Low current sleep mode. 2. Hardware Description [align=center][img=327,239]http://www.e-works.net.cn/images/128002110303906250.GIF[/img] Figure 1[/align] The LJ12 chip has abundant internal hardware resources, and we have configured it as follows: • PLL: External crystal oscillator 32.768k; internal bus frequency multiplied to 4M; • SCI: Used for host computer communication, bit rate 9600, interrupt mode; • SPI: 1M clock, used as control of MC33970, using two general-purpose pins as SPI chip select signals; • LCD: Uses 26×4 mode, i.e. 26 front poles, 4 back poles, dynamically driven; because a 6-bit LCD module is used in the design, FP1～FP12 and BP0～BP3 are sufficient; • Flash: Uses resident memory functions (ROM-Resident Routine) to store mileage data in the user flash memory in the form of 32-bit data. The stepper motor produced by SWITEC was selected as the actuator. A six-digit segment dynamic drive LCD module was used to display mileage, single mileage or time and date. Due to habit, the car instrument needs to be kept powered on at all times without resetting the time parameters. For the convenience of demonstration, two buttons were used to switch the LCD display mode. Six LEDs were used to display braking, danger, oil pressure, battery, engine and neutral information respectively. MC1413 was used as the LED driver. MC33970 was directly driven by the microcontroller's SPI; vehicle speed and engine speed were pulse width modulation signals or from serial port input. The frequency was obtained through the input capture function of the timer module and the data was calculated. (1) 7805 was used for the power supply and power failure protection input. In order to save the mileage data in time when the power is off, a 1000uF or two 470uF electrolytic capacitors were added to the power ground input. When the power is disconnected, an interrupt signal will be generated on the IRQ pin. The large capacitor can maintain the microcontroller power supply for a long enough time so that the microcontroller can complete the external interrupt service program. (2) Conditioning circuit In order to improve the waveform, a conditioning circuit was added outside the timer pin. (3) Overall design This design is aimed at the mid-to-low-end cars on the market. In order to express this intention, the PCB of the instrument is designed according to the instrument shell and internal dimensions of the Changan car, and the layout is rearranged. [align=center][img=337,137]http://www.e-works.net.cn/images/128002110882656250.GIF[/img] Figure 2 System block diagram[/align] 3. Software description (1) Initialization function The internal bus frequency is multiplied to 4M; the timer module channel is set to capture function; SCI baud rate 9600, receive interrupt enabled; SPI clock frequency 1M; LCD module 26×4 mode; Real Time Clock module enabled; MC33970 and pointer initialization; Read the previously recorded mileage data from the user Flash and display it according to the ODO/TRIP signal. (2) Interrupt Service Routine • External Interrupt: After power failure, the pointer returns to zero by discharging the large capacitor for a short time. • SCI Interrupt: Follows a certain host computer protocol, accepts commands, changes the LCD display content, and changes the vehicle speed and RPM data. • RTC Interrupt: Records mileage data once per second; calculates mileage data based on the vehicle speed signal. (3) The main function reads mileage data periodically and displays it on the LCD; restarts WATCHDOG at the end of each main loop; reacts by changing the pointer position when the vehicle speed or RPM signal changes. [align=center][img=500,387]http://www.e-works.net.cn/images/128002111134218750.GIF[/img] Figure 3 Main Program Flowchart[/align] The following is a simple comparison between the stepper motor driver chip x15.089 and MC33970 produced by SWITEC, and then draws a conclusion.

[＊] Superior performance

The table below compares the performance of the two, with the MC33970's biggest advantage being its superior real-time response: Table 2 Comparison of the performance of stepper motor driver chips x15.089 and MC33970 [img=500,126]http://www.e-works.net.cn/images/128002111715781250.GIF[/img]

[＊] Compact structure

In simple terms, the x15.089 works by using two pins of a microcontroller to control one stepper motor. One pin outputs a direction control signal, and the other outputs a pulse. Therefore, controlling each motor requires two microcontroller pins, and controlling four motors requires eight general-purpose I/O ports. The MC33970 is different. It can control two stepper motors via SPI command input; when four stepper motors are needed, two MC33970s are used, but only one chip select signal is added. This way, only an SPI module and two general-purpose I/O ports are required, thus saving microcontroller resources.

[＊] Competitive cost (data from www.freescale.com)

The LJ12 microcontroller, with a unit price of RMB 20 for orders over 10,000 units, eliminates the need for an external LCD driver like the SED1335 due to its built-in LCD control driver. The MC33970, with a unit price of $1.3 (RMB 13) for orders over 10,000 units, offers high cost-effectiveness. In comparison, the SWITEC 312.017 stepper motor costs around RMB 18, and the x15.089 driver chip costs around RMB 16. Below is a simplified cost estimate table: [align=center]Table 3 Simplified Cost Estimate Table[img=494,172]http://www.e-works.net.cn/images/128002112262500000.GIF[/img][/align] As can be seen, in mass production, as a fully digital system, its total cost is below RMB 150, approximately between RMB 120 and 140, making it quite competitive. 4. Conclusion This solution is a low-end automotive electronic instrument panel based on Freescale microcontrollers, targeting various economy cars. It features complete functions, compact structure, and low price, and has broad market prospects.