Solutions based on NEC Electronics' automotive instrument panel dedicated device series

　　Introduction:

　　As for the dedicated chip for car dashboard, the Chinese automotive electronics industry started late. Until around 2004, there were still almost no dedicated chips for dashboard in the market. The dial solution adopted by many customers is often composed of an ordinary chip, a stepper motor driver chip and an LCD driver chip. For automotive electronic products with high safety requirements, this situation is just a helpless choice in the absence of dedicated automotive-grade chips from the two aspects of extreme temperature and electromagnetic interference.

　　NEC Electronics has provided the market with a professional series of chips: CAN ASSPn since 1995. From the earliest μPD780852 to the later μPD780828 (according to customer needs, mask and flash versions of chips are provided respectively). This article takes the system structure of domestic mid- and low-end automotive dashboards as the main object, and introduces the system design based on the dedicated chip μPD78F0822 in the field of NEC Electronics automotive dashboards.

　　In the current Chinese market, the general configuration of mid- and low-end car dashboards is as follows:

　　• 4 stepper motor driven display meters: speedometer, tachometer, fuel gauge, water temperature meter → The 4 stepper motor drivers integrated in the 0822 chip are used to control the instrument pointer.

　　• LCD display: large and small mileage → The LCD driver integrated in the 0822 chip can achieve a maximum of 136 segments of display.

　　• Buzzer → Used to generate various alarm sounds such as: seat belt not fastened, door not locked, etc.

　　• Each function display light → Use the MCU's I/O port to control the alarm lights required by the customer.

　　In the system design, these four modules are the main objects to explain the function realization, resource utilization and control method of the solution.

　　System Design

　　In the solution designed by NEC Electronics (China) Co., Ltd. Shanghai Branch, the CAN bus is used to realize data transmission, drive the stepper motor and drive the signal light. Of course, according to the different needs of customers, wiring harness solutions can also be provided. Figure 1 is the system block diagram of CAN3+Demo and the CAN3+ microcontroller peripheral modules used in the system.



　　The single-chip microcomputer receives data sent by the CAN bus, mainly including vehicle speed, rotation speed, oil level, temperature, mileage, degree of damage and various indication signals. The single-chip microcomputer drives the stepper motor according to the actual situation of the received data so that the pointer reaches the specified position. If the damage to the car is greater than the set value, the buzzer will send an alarm signal. If the indication signal is received, the corresponding backlight indicator will be lit. To prevent mileage loss, the mileage value is saved in time during the program running.

　　The data received by the instrument panel from the CAN bus is provided by another Demo board, which sends the collected data related to the instrument panel to the instrument panel every 10ms.

　　Functional definition of four instruments

　　1. Speedometer: driven by a stepper motor; the maximum speed range is 200km/h, the minimum division is 5km/h, and the speed signal is transmitted from the CAN bus. The speedometer pointer responds in time. When the car speed changes steadily, the speedometer pointer moves smoothly. At any constant speed, the pointer swing range does not exceed 1km/h.

　　2. Tachometer: driven by a stepper motor; the maximum speed range is 8000r/m, the minimum division is 200r/m, and the signal is transmitted from the CAN bus. The tachometer pointer responds promptly. When the speed changes steadily, the tachometer pointer moves smoothly. At constant speed, at any constant speed, the pointer swing range is within 70r/m.

　　3. Fuel meter: driven by a stepper motor, and the signal is transmitted from the CAN bus. The pointer indicates the estimated amount of fuel in the fuel tank.
On the indication diagram: E-fuel tank empty; F-fuel tank full.

　　4. Water temperature meter: driven by a stepper motor, and the signal is transmitted from the CAN bus.

　　Instrument controller/driver

　　The instrument controller/driver can drive a stepper motor to control an external instrument or a cross coil. It has the following main functions:

　　• 8-bit pulse width can be set

　　• 8+1-bit pulse width with 1-bit additional function

　　• 4 360° type stepper motors can be driven simultaneously

Figure 2 is the structure diagram of the instrument controller/driver.



　　Specifications of stepper motors

　　The specifications of the stepper motors used are as follows:

　　• Operating voltage: 5V~10V

　　• Coil resistance: 280Ω

　　• Current consumption: 15mA

　　• Starting frequency: 160Hz

　　• Maximum drive frequency: 600Hz

　　In order to make the motor run more smoothly and reduce working noise, this design uses subdivision technology to drive the motor with a more precise and closer to sine wave pulse sequence, so that the motor can obtain small micro-stepping (as shown in Figure 3). [page]



　　LCD display

　　The LCD part mainly displays the mileage accumulation and mileage subtotal.

　　• Mileage accumulation

　　The mileage accumulation displays the total mileage (0~999999km). When it reaches the maximum reading (999999km), this value is maintained.
　　If the battery voltage is disconnected, in order to prevent the mileage accumulation value from being lost, the microcontroller updates the current mileage every two kilometers and saves it in the EEPROM. If the battery voltage is disconnected, the maximum cumulative kilometer loss value is +1km. If the value stored in the EEPROM is found to be incorrect, "Error" will replace the mileage accumulation display.

　　• Mileage subtotal

　　The mileage subtotal is displayed on the right side of the LCD. It shows the total mileage since the last mileage subtotal was cleared or the battery voltage was disconnected. Its working range is from 0 to 999.9km, with an accuracy of 0.1km, and the content is only retained in RAM.

　　Signal light control

　　In this design, the signal light is controlled by the port. A total of 24 signal lights are designed, including turn signals, door opening lights, parking lights, etc.

　　Software processing

　　The main program flow chart is shown in Figure 4.



　　The stepper motor control program is shown in Figure 5.



　　Functions of CAN controllers

　　Through the resources of 0822, data transmission can be achieved through the CAN interface or wiring harness and the control of motors, indicator lights, and LEDs can be completed.

　　Conclusion

　　NEC Electronics' dedicated chip for automotive instrument panels brings advantages to customers in the following two aspects:

　　1. The dedicated automotive-grade chip provides guarantees for extreme temperatures and anti-electromagnetic interference, thereby greatly improving the safety of the product.

　　2. The stepper motor drive and LCD drive are integrated in the chip, which provides convenience for customers: one chip integrates the functions of the past three chips, saving system costs for customers; it saves space when customers lay out the board, and reduces electromagnetic interference.

　　With the irreversible trend of automotive networking as automotive electronics, the instrument panel, as a node in the vehicle network, needs to communicate and exchange data with the vehicle network. NEC Electronics' μPD78F0822 chip with CAN channel enables the instrument panel as a node to send/receive commands through the vehicle network. At the same time, the intervention of the CAN channel can also make the instrument panel a node such as KWP2000, and thus be included in an item of automobile maintenance; CAN itself, as a serial port mode, provides an option for customers who need to use the self-programming mode to update the program.