PCB News - How to ensure the safety of PCB boards in high-speed DSP systems

How to ensure the safety of PCB boards in high-speed DSP systems

With the rapid development of microelectronics technology, the design of its PCB printed board exhibits completely different behavior characteristics from low-speed design, that is, signal integrity problems, aggravated interference problems, electromagnetic compatibility problems, and so on.

The method to solve these problems mainly depends on the circuit design. Therefore, the design quality of the PCB printed board is very important, and it is the only way to transform the optimal design concept into reality. The following discusses several issues that should be paid attention to in the reliability design of PCB boards in high-speed DSP systems.

The first thing that needs to be considered in the PCB board design of a high-speed DSP system is the power supply design. In power supply design, the following methods are usually used to solve signal integrity problems.

Consider the decoupling of power and ground

With the increase of DSP operating frequency, DSP and other IC components tend to be miniaturized and packaged densely. Usually, multi-layer boards are considered in circuit design. It is recommended that both power and ground can use a dedicated layer, and for multiple power sources, For example, the DSP I/O power supply voltage is different from the core power supply voltage, and two different power supply layers can be used. If the high processing cost of the multilayer board is considered, a special layer can be used for more wiring or relatively critical power supplies. The power supply can be routed the same as the signal line, but the width of the line must be sufficient.

Regardless of whether the circuit board has a dedicated ground layer and power layer, a certain and reasonably distributed capacitance must be added between the power supply and the ground. In order to save space and reduce the number of through holes, it is recommended to use more chip capacitors. The chip capacitor can be placed on the back of the PCB board, that is, the soldering surface. The chip capacitor is connected to the through hole with a wide wire and connected to the power supply and the ground through the through hole.

Separate analog and digital power planes
High-speed and high-precision analog components are sensitive to digital signals. For example, the amplifier will amplify the switching noise to make it close to the pulse signal, so the analog and digital parts of the board, the power layer is generally required to be separated.

Isolate sensitive signals
Some sensitive signals (such as high-frequency clocks) are particularly sensitive to noise interference, and high-level isolation measures must be taken for them. The high-frequency clock (a clock above 20MHz, or a clock with a flip time of less than 5ns) must have a ground wire escort, the clock line width should be at least 10 mils, and the escort ground wire width should be at least 20 mils. The hole is in good contact with the ground, and every 5cm is punched to connect with the ground; a 22Ω～220Ω damping resistor must be connected in series on the clock sending side. The interference caused by the signal noise brought by these lines can be avoided.

Software and hardware anti-interference design
Generally, high-speed DSP application system PCB boards are designed by users according to the specific requirements of the system. Due to limited design capabilities and laboratory conditions, if perfect and reliable anti-interference measures are not taken, once the working environment is not ideal, there is electromagnetic Interference will cause the DSP program flow to be disordered. When the DSP's normal working code cannot be restored, the program will run away or crash, and some components may even be damaged. Attention should be paid to taking corresponding anti-interference measures.

Hardware anti-jamming design
The hardware anti-interference efficiency is high. When the system complexity, cost, and volume are tolerable, the hardware anti-interference design is preferred. Commonly used hardware anti-jamming technologies can be summarized as the following:

(1) Hardware filtering: RC filter can greatly attenuate all kinds of high-frequency interference signals. For example, the interference of "burr" can be suppressed.

(2) Reasonable grounding: Reasonable design of grounding system. For high-speed digital and analog circuit systems, it is very important to have a low-impedance, large-area grounding layer. The ground layer can not only provide a low-impedance return path for high-frequency currents, but also make EMI and RFI smaller, and it also has a shielding effect on external interference. Separate the analog ground from the digital ground during PCB design.

(3) Shielding measures: AC power, high-frequency power, high-voltage equipment, and electric sparks generated by electric arcs will generate electromagnetic waves and become noise sources of electromagnetic interference. Metal shells can be used to surround the above devices and ground them. This pair of shields The interference caused by electromagnetic induction is very effective.

(4) Photoelectric isolation: Photoelectric isolators can effectively avoid mutual interference between different circuit boards. High-speed photoelectric isolators are often used in the interface of DSP and other devices (such as sensors, switches, etc.).

Heat dissipation design

In order to facilitate heat dissipation, the printed board is best to be installed on its own, and the board spacing should be greater than 2cm. At the same time, pay attention to the layout rules of the components on the printed board. In the horizontal direction, high-power devices are placed as close to the edge of the printed board as possible to shorten the heat transfer path; in the vertical direction, high-power devices are placed as close to the top of the printed board as possible to reduce their impact on the temperature of other components. Components that are more sensitive to temperature should be placed in areas with relatively low temperature as much as possible, and should not be placed directly above devices that generate large amounts of heat.

In the various designs of high-speed DSP application systems, how to transform a perfect design from theory to reality depends on high-quality PCB printed boards. Increase, how to improve the quality of the signal is very important. Therefore, whether the performance of the system is good is inseparable from the quality of the designer's PCB printed board.

Electromagnetic compatibility design

Electromagnetic compatibility refers to the ability of electronic equipment to work normally in a complex electromagnetic environment. The purpose of electromagnetic compatibility design is to enable electronic equipment to suppress all kinds of external interference, but also to reduce the electromagnetic interference of electronic equipment to other electronic equipment. In the actual PCB board, there is more or less electromagnetic interference phenomenon, that is, crosstalk between adjacent signals. The magnitude of crosstalk is related to the distributed capacitance and distributed inductance between the loops. To solve this kind of mutual electromagnetic interference between signals, the following measures can be taken:

Choose a reasonable wire width

The impact interference produced by the transient current on the printed lines is mainly caused by the inductance of the printed wires, and its inductance is proportional to the length of the printed wires and inversely proportional to the width. Therefore, the use of short and wide wires is beneficial to suppress interference. Clock leads and bus driver signal lines often have large transient currents, and their printed wires should be as short as possible. For discrete component circuits, the printed wire width is about 1.5mm to meet the requirements; for integrated circuits, the printed wire width is selected between 0.2mm～1.0mm.

It adopts a tic-tac-toe network wiring structure.

The specific method is to wire horizontally on the first layer of the PCB printed board, and wire vertically on the next layer.