Embedded design is a vast field. Today, we will discuss several considerations in hardware circuit design, starting with an understanding of the embedded hardware architecture.
We know that the CPU is the soul of this system, and all peripheral configurations are associated with it. This highlights a characteristic of embedded design: hardware can be tailored. In embedded hardware design, the following points need attention.
1
Power Supply Determination
The role of the power supply in an embedded system can be likened to the role of air for the human body, or even more critical: the air we breathe contains oxygen, carbon dioxide, and nitrogen, but in stable proportions. This is analogous to the various noise in the power supply system; we hope to obtain a pure and stable power supply that meets requirements, but due to various constraints, this remains a dream. Two aspects need attention:
-
Voltage
Embedded systems require various levels of power supply, such as the common 5V, 3.3V, and 1.8V. To minimize power supply ripple, LDO devices are used in embedded systems. If using DC-DC converters, not only are they larger, but their ripple can also be a significant issue.
-
Current
For the normal operation of embedded systems, not only is a stable and sufficient power supply required, but also adequate current. Therefore, when selecting power supply devices, their load capacity must be considered. I generally leave a 30% margin in my designs.
If it is a multilayer board, the power supply section should be split during layout. Care should be taken with the splitting paths, and a certain amount of power supply should be placed together. If it is a double-sided board, the trace width needs attention; it should be widened as much as possible within the board’s limits. Suitable decoupling capacitors should be placed as close as possible to the power pins.
2
Crystal Oscillator Determination
The crystal oscillator is akin to the heart of the embedded system, and its stability directly affects its operational state and communication performance. Common oscillators include passive and active crystal oscillators. First, the oscillation frequency must be determined, followed by the type of crystal oscillator.
-
Passive Crystal Oscillator
The selection of matching capacitors and resistors is generally based on the reference manual. In microcontroller designs, plug-in crystal oscillators are often used in conjunction with ceramic capacitors. In ARM designs, to save space and facilitate routing, four-corner passive crystal oscillators are often used with surface-mount capacitors. Although we are familiar with the matching circuits for fixed crystal oscillators, to ensure reliability, it is still necessary to refer to the manual to determine capacitor sizes and whether matching resistors are needed.
-
Active Crystal Oscillator
Active crystal oscillators provide better and more accurate clock signals, but they are generally more expensive than passive crystal oscillators. Therefore, cost is also a consideration in hardware circuit design. When designing circuit boards, it is important to route the crystal oscillator traces as close to the chip as possible, keeping critical signals away from clock traces. If conditions permit, add grounding protection rings. In multilayer boards, critical signals should also be kept away from the crystal oscillator traces.
3
Reserve Test IO Ports
During the embedded debugging phase, when pin resources are abundant, I usually reserve an IO port to connect an LED or a buzzer, laying the groundwork for the next step in software development. During the operation of the embedded system, this IO interface can be controlled appropriately to determine whether the system is functioning normally.
4
External Storage Devices
If an embedded system has a power supply, crystal oscillator, and CPU, then it constitutes the most basic system we are familiar with. If this embedded system needs to run a larger operating system, not only must the CPU have an MMU, but it also needs external 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 focus during layout because it is essential to ensure that related signal lines are of equal length to guarantee equal signal delays, especially for clock and DQS differential signal lines. Various routing techniques need to be comprehensively applied, such as symmetrical distribution with respect to the CPU, daisy chain routing, and T-shaped routing, which should be chosen based on the number of memory units. Generally, the more units there are, the more complex the routing becomes, but knowing the key points makes everything manageable.
5
Functional Interfaces
The most important aspect of an embedded system is to control peripheral modules through various interfaces to achieve the designer’s intended goals. Common interfaces include serial ports (which can connect to Bluetooth, Wi-Fi, 3G modules, etc.), USB interfaces, network interfaces, JTAG interfaces, audio/video interfaces, HDMI interfaces, and so on. Since these interfaces connect to external modules, ensuring good electromagnetic compatibility design is a crucial task. Additionally, attention should be paid to the use of differential pairs during layout.
6
Screen
This function is listed separately because it is optional. If an embedded system is merely a connector for peripheral device modules, connecting to a computer host or directly to a network, then a screen is unnecessary. However, if the product is a consumer device that frequently interacts with users, it must be addressed.
Capacitive screens are the preferred choice for embedded displays. In circuit design, attention must be paid to the layout of touch screen connection lines and display screen connection lines. During routing, these should be kept as short as possible and close to the main control CPU, while ensuring paired signals use differential pairs, and RGB control signals are routed with equal lengths. The spacing between various signal traces should follow the 3W rule to avoid mutual interference. In screen design, it is essential to ensure power and prevent interference to avoid flickering and distortion of the screen.
Screenshots of Electronic Books in the Resource Collection
[Complete Set of Hardware Learning Materials Collection]
