Occupational division in the embedded industry

技术小白521

Occupational division in the embedded industry [Copy link]

Embedded design is a huge project. Today, let's talk about some precautions in hardware circuit design. First, let's understand the embedded hardware architecture. We know that the CPU is the soul of this system, and all peripheral configurations are related to it. This also highlights a feature of embedded design: hardware can be tailored. In embedded hardware design, the following points need to be paid attention to. First, power supply determination The role of power supply in embedded systems can be regarded as the role of air in human body, or even more important: there are oxygen, carbon dioxide and nitrogen in the air people breathe, but the content is stable, which is equivalent to various clutters in the power supply system. We hope to get a pure and stable power supply that meets the requirements, but due to various factors, it is just our dream. There are two aspects to pay attention to: a. Voltage Embedded systems require power supplies of various magnitudes, such as the common 5v, 3.3v, 1.8v, etc. In order to minimize the ripple of the power supply, LDO devices are used in embedded systems. If DCDC is used, it is not only large in size, but its ripple is also a headache. b. Current The normal operation of the embedded system requires not only a stable and sufficient power supply, but also sufficient current. Therefore, when selecting the power supply device, its load needs to be considered. I generally leave a 30% margin when designing. If it is a multi-layer board, the power supply needs to be divided during layout. At this time, you need to pay attention to the division path and try to place a certain amount of power together. If it is a double-sided board, the trace width needs to be paid attention to and it should be widened as much as possible if the board allows. The appropriate decoupling capacitor should be as close to the power pin as possible. Second, determine the crystal oscillator. The crystal oscillator is equivalent to the heart of the embedded system. Its stability is directly related to its operating status and communication performance. Common oscillators include passive crystal oscillators and active crystal oscillators. First, its oscillation frequency must be determined, and then the type of crystal oscillator must be determined. a. The selection of matching capacitors and matching resistors for passive crystal oscillators is generally based on the reference manual. In the design of single-chip microcomputers, plug-in crystal oscillators are often used in combination with ceramic capacitors. In ARM, in order to reduce space and facilitate wiring, four-corner passive crystal oscillators are often used in combination with chip capacitors. Although we are familiar with the matching circuit of fixed crystal oscillators, in order to be foolproof, we still need to refer to the reference manual to determine the size of the capacitor, whether matching resistors are required, and other details. b. Active crystal oscillators have better and more accurate clock signals, but in comparison, they are more expensive than passive crystal oscillators, so this is also a cost that needs to be paid attention to in hardware circuit design. When designing a circuit board, it is necessary to pay attention to the crystal oscillator routing as close to the chip as possible, and the key signals away from the clock routing. Add a ground protection ring when conditions permit. If it is a multi-layer board, the key signals should also be kept away from the crystal oscillator routing. Third, reserve a test IO port. In the embedded debugging stage, when the pin resources are abundant, I usually reserve an IO port to connect the LED or speaker to pave the way for the next software writing. During the operation of the embedded system, the IO interface is properly controlled to determine whether the system is running normally. Fourth, external storage device If an embedded system has a power supply, crystal oscillator and CPU, then this is the smallest system we are familiar with. If the embedded system needs to run a larger operating system, then not only does the CPU need to have an MMU, but the CPU also needs to be connected to SDRAM and NANDFLASH. If the CPU has SDRAM and NANDFLASH controllers, then there is no need to consider the use of address lines too much in hardware design. If there is no relevant controller, then you need to pay attention to the use of address lines. This part is a key point in LAYOUT. The reason is that the relevant signal lines should be equal in length to ensure equal signal delays and the differential signal lines of the clock and DQS. When wiring, various wiring techniques need to be used in combination, such as symmetrical distribution with the CPU, daisy chain wiring, and T-type wiring. This needs to be selected based on the number of memories. Generally speaking, the more the number, the more complicated the wiring, but knowing the key points, everything will be solved. Fifth, functional interface The most important thing for an embedded system is to control peripheral modules through various interfaces to achieve the designer's preset purpose. Commonly used interfaces include serial ports (which can be used to connect Bluetooth, WiFi, and 3G modules), USB interfaces, network interfaces, JTAG interfaces, audio and video interfaces, HDMI interfaces, etc. Since these interfaces are connected to external modules, it is an important task to do a good job of electromagnetic compatibility design. In addition, pay attention to the use of differential lines when laying out. Sixth, the screen function is listed separately because it is dispensable. If an embedded system is only used as a connector to connect peripheral device modules, connected to a computer host through relevant interfaces or directly hung on the network, then the screen is not needed. But if the product is a consumer product that interacts frequently with users, this has to be said. Capacitive screens are the first choice for embedded screens. In circuit design, attention should be paid to the layout of touch screen cables and display screen cables. In the process of routing, try to keep the cables as close to the main control CPU as possible. At the same time, pay attention to the differential lines of paired signals and the equal length of RGB control signals. The spacing between various signal routings follows the 3W rule to avoid mutual interference. In the design of the screen, it is necessary to ensure power and prevent interference to prevent screen flickering and screen distortion.