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Embedded design is a vast project, today let’s talk about several considerations in hardware circuit design, first, let’s understand the hardware architecture of embedded systems.
We know that the CPU is the soul of this system, all peripheral configurations are related to it, which also highlights a characteristic of embedded design: hardware can be tailored. In embedded hardware design, the following points need attention.
The role of the power supply in an embedded system can be seen as the same as the role of air for humans, or even more important: the air we breathe contains oxygen, carbon dioxide, and nitrogen, but in stable amounts, which is equivalent to various noises in the power supply system, we hope to obtain a pure and stable power supply that meets requirements, but due to various constraints, it remains a dream. This requires attention to two aspects:
a. Voltage
Embedded systems require power supplies of various levels, such as the common 5V, 3.3V, 1.8V, etc. To minimize power supply ripple, LDO devices are used in embedded systems. If using DCDC, not only is it bulky, but its ripple is also a troublesome issue.
b. Current
The normal operation of an embedded system requires not only a stable and sufficient power supply but also sufficient current; therefore, when selecting power supply devices, the load must be considered, and I generally leave a 30% margin in my designs.
If it is a multilayer board, the power supply part must be split during layout; attention must be paid to the splitting path, and a certain amount of power supply should be placed together. If it is a double-sided board, the width of the traces needs to be considered, and they should be widened as much as possible within the board’s limits. Suitable decoupling capacitors should be placed as close to the power pins as possible.
The crystal oscillator is equivalent to the heart of the embedded system, and its stability directly affects its operating state and communication performance. Common oscillators include passive and active crystal oscillators. First, determine the oscillation frequency, and then determine the type of crystal oscillator.
a. Passive Crystal Oscillator
The selection of matching capacitors and matching resistors is generally based on the reference manual. In microcontroller design, plug-in crystal oscillators are often used with ceramic capacitors. In ARM, to save space and facilitate wiring, four-corner passive crystal oscillators are often used with surface-mount capacitors. Although we are familiar with the matching circuit of fixed crystal oscillators, to be foolproof, we still need to refer to the manual to determine the capacitor size and whether matching resistors are needed, etc.
b. Active Crystal Oscillator
It provides a better and more accurate clock signal, but it is generally more expensive than passive crystal oscillators, so cost is also a concern in hardware circuit design.
When designing circuit boards, care must be taken to keep the crystal oscillator traces close to the chip, and critical signals should be kept away from clock traces. If conditions permit, add a grounding protection ring. If it is a multilayer board, critical signals should also be kept away from the crystal oscillator traces.
During the embedded debugging phase, when pin resources are abundant, I usually reserve an IO port to connect an LED or a buzzer to lay the groundwork for the next step of software development. During the operation of the embedded system, the IO interface can be controlled appropriately to determine whether the system is operating normally.
An embedded system with a power supply, crystal oscillator, and CPU is what we are familiar with as a minimal system. If this embedded system needs to run a larger operating system, not only must the CPU have an MMU, but it also needs to connect to SDRAM and NAND FLASH. If the CPU has SDRAM and NAND FLASH controllers, then there is no need to consider the use of address lines too much in hardware design. If there are no relevant controllers, then attention must be paid to the use of address lines.
This part is a key point during layout, the reason being to make the relevant signal lines of equal length to ensure equal signal delays, and the clock and DQS differential signal lines must be routed carefully. Various wiring techniques need to be comprehensively used during routing, such as symmetrical distribution with the CPU, daisy chain wiring, T-type wiring, which all need to be selected based on the number of memory units; generally speaking, the more units, the more complex the wiring, but knowing the key points makes everything easier.
The most important aspect of an embedded system is to control peripheral modules through various interfaces to achieve the designer’s preset goals. Common interfaces include serial ports (which can be used to connect Bluetooth, Wi-Fi, 3G modules), USB interfaces, network interfaces, JTAG interfaces, audio and video interfaces, HDMI interfaces, etc. Since these interfaces connect with external modules, ensuring good electromagnetic compatibility design is an important task. In addition, attention should be paid to the use of differential lines during layout.
This function is listed separately because it is optional. If an embedded system is merely a connector to peripheral device modules, connecting to a computer host or directly to the network, then a screen is not necessary. However, if the product is a consumer product that interacts frequently with users, it must be mentioned.
Capacitive screens are the first choice for embedded screens. In circuit design, attention must be paid to the layout of touch screen connection lines and display screen connection lines. During routing, the lines should be kept as short as possible and close to the main control CPU, while ensuring paired signals follow differential line routing, and RGB control signals maintain equal lengths. The spacing between various signal lines should follow the 3W rule to avoid mutual interference. In the design of the screen, it is essential to ensure power and prevent interference to avoid flickering and distortion of the screen.
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