Flexible modern CPLD automotive digital dashboard
The car dashboard becomes a nerve center that gathers vehicle safety and manages all information, displaying various information for the driver. In today's digital age, vehicle instrumentation systems must be able to monitor all key functions, and the system is even personalized. The development of industry demand has led to the emergence of many semiconductor solutions, from ASSP to fully customized devices. These solutions may be solutions with fixed functions, which cannot flexibly develop products and cannot meet the requirements of designers. In contrast, the updatable solution supports multiple similar applications on one vehicle product line without any extra cost. This type of customized solution meets all needs at a very low cost.
This article briefly introduces an innovative CPLD architecture that completely avoids the use of microcontrollers and their drivers, thereby providing a low-cost, low-power combination digital dashboard solution. This analog dashboard solution (ADS) efficiently implements the digital automotive network and fully utilizes the advantages of digital technology.
Combo dashboard solution
Traditionally, the real-time output of meters such as driving range is obtained mechanically and displayed using an analog drive. However, with the digitization of these data inputs, stepper motors and LEDs have replaced meters and gauges. An expensive microcontroller is used to process and display the digital output. ASSPs later appeared, resulting in higher one-time cost expenditures (NRE), limiting product updates and improvements. Product life cycle and support for different product lines are also the main factors driving the adoption of inexpensive programmable alternative products.
A stepper motor is used on the pointer instrument display board to convert electrical pulses into discontinuous mechanical actions. When electrical control pulses are applied to the stepper motor in a certain order, the motor shaft rotates in discrete step increments. Combined digital instruments generally use stepper motors to imitate the performance and visual effects of analog panels and pointer display boards, while providing very precise position information required for digital settings. These motors need to be micro-stepped to achieve smooth and continuous pointer movement. In addition, the measured sample values ​​are broadcast from the vehicle sensors to the corresponding instrument parts, and the number of samples is limited by the bandwidth of the digital link. After a certain time interval, each measured sample value is displayed on the combination meter. On this type of combination meter, measures are urgently needed to overcome the difficulty of continuously displaying information. When the sensor does not output data to the meter, ensure that the pointer is in the correct position. To solve these problems, it is necessary to increase the processing capacity, thereby increasing the cost of the digital instrument panel system, and its low cost performance hinders its application in vehicles.
CPLD-based combination instrument panel controller
Using CPLD can easily overcome the limitations of this high-cost solution. With ADS, customers only need to update or modify programming files in the design to achieve product replacement, so it is very flexible. In addition, new functions or products can be added in the field, which not only improves the efficiency of technology implementation, but also meets the needs of special users and products. Using the same basic system, with a few changes, it is easy to use different devices in the new product line.
With CPLD-based ADS, product developers and manufacturers can choose from different devices as needed without worrying about the obsolescence of semiconductor components. It has a low sales price, supports high-end functions, and has a lot of room for expansion in the future. Its architecture uses Altera ’s MAX II CPLD and includes 6 modules: serial sensor data unit, main motion control and arithmetic unit, and 4 PWM generators. The serial sensor data unit receives input from the sensor, the main movement control and arithmetic unit completes the necessary calculations, and the PWM generator provides appropriate control signals for all phases of the stepper motor and receives commands from the main module (Figure 1).
Figure 1. Structure diagram of ADS based on CPLD
Before explaining this ADS architecture in detail, we must first understand the functions of the different modules in the narrowband vehicle data network and how the stepper motor driving the pointer instrument panel accomplishes microstepping control. Because all the display panels of the instrument cluster do not need continuous sensor data, when there is no data, the system keeps the pointer position unchanged. In order to achieve a smooth display, the movement command sent to the pointer is the current deflection function, and there will be no rapid step change of the pointer.
PWM generator
Four PWM generator modules drive the pointer instrument panel stepper motor to indicate data from different sensors (Figure 2).
Figure 2. Basic structure diagram of PWM generator
The micro-stepping control function is used to drive the motor and produce a smooth motor rotation. In microstepping control, the magnetic field generated by the motor is not parallel to the excitation coil, but at a certain angle. In this way, the holding torque can be generated at more positions, and the rotor can be held between the two pole shafts of the excitation coil. After the excitation coil is energized, the generated magnetic flux is proportional to the current flowing. If all the excitation coils are powered, the current direction and magnetic field can be obtained by the vector sum of the two winding currents. Therefore, if the current in the winding of the stepping motor gradually increases, then a set of equidistant positions of the excitation magnetic field can be generated to improve the stepping accuracy of the motor. Using this principle, the step size of the stepper motor is divided into micro-steps where the shaft actually rotates (Figure 3).
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Figure 3. Two sinusoidal current waveforms during microstep control
The 4 PWM modules provide a constant PWM pulse with a fixed duty cycle to the stepper motor, so that the pointer rotates at a certain speed, and the 4 stepper motors maintain a constant speed. They receive the movement commands and direction input from the movement control unit, input the appropriate voltage on the winding, and make each motor rotate a microstep in the required direction.
Mobile control and arithmetic unit
The delta generator is the main component of the mobile control and arithmetic unit. This module receives the target deflection signal from each of the 4 stepper motors of the sensor. After receiving the target deflection signal, the two signals are sent to their respective PWM modules. One signal, the trigger pulse, changes the current duty cycle value to the next value in the PWM module. Therefore, when the span range is larger, the pulse is sent at a higher frequency, and the PWM module quickly traverses each duty cycle value, thereby causing the stepper motor to rotate at a higher speed. When the pointer reaches the target deflection position, it stops sending trigger pulses to the PWM module, and the PWM module outputs pulses with a constant duty cycle to effectively maintain the position of the shaft. Another signal indicates to the PWM module, in which direction the pointer should turn.
The delta generator also has another important function, which periodically sends data representing the sampled value. If this information is directly added to the stepper motor, then the pointer will turn rapidly. To avoid this rotation, the delta generator uses the corrected value, expressed as delta δ. Each time a new target deflection value is received, the generator recalculates δ for this pointer value. Then, this incremental value δ is added to the current deflection of the pointer, and its processing rate is much faster than when a new target deflection value is received. Throughout the process, the current deflection of the unit is continuously monitored. When it changes, it sends a command to make the stepper motor step a microstep in the required direction. If the target deflection value is greater than the current deflection value, then it is clockwise, if it is less than the current deflection value, it is counterclockwise.
In this way, during the interval between receiving the two target deflection values, the pointer rotates at a faster rate and reaches the target value with a smaller step size, resulting in a smooth rotation. It infers that it can cover the appropriate value of this interval, and it can keep a smooth rotation no matter how the input changes.
Sensor data input unit
The sensor data input unit uses an SPI interface to support communication with slow peripherals and can periodically access these peripherals. The host / slave mode is used to communicate with the peripherals, and the host initiates the data frame. When the master generates a clock signal and selects the slave device, it can transfer data unidirectionally or bidirectionally. Implement the SPI slave module behind the CPLD for system input. The sensor data is imported into the arithmetic and movement control unit, which is the target deflection value of a certain sensor or new LED data. Sensor data is sent with the target address to distinguish its source.
It is easy to adjust the ADS based on CPLD on different platforms to improve its accuracy and achieve higher-end functions. It can provide different assembly layouts for different vehicle models, with only slight changes in procedures, almost no additional cost, and the development time decreases exponentially. ADS can be combined with a reliable public vehicle digital data network and integrated into the production process, and the flexible CPLD can easily implement built-in functions.
in conclusion
Using low-cost, low-logic-density CPLDs, complex ADS can be achieved, overcoming the shortcomings of traditional dashboard solutions. Because ADS has inherent programmability advantages, design reuse means that more and more IP and core libraries can be used to rapidly develop other solutions. It is easy to reconfigure and launch new products to users faster. Because the product life cycle is longer, it is easier to withdraw the NRE, and manufacturers can extend the life cycle of products that have been developed without new NRE investment.
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