Reliability Design of PCB Board for High-speed DSP System

introduction

Due to the rapid development of microelectronics technology, digital electronic systems composed of IC chips are developing rapidly in the direction of large scale, small size and high speed, and the development speed is getting faster and faster. The application of new devices leads to high density of circuit layout in modern EDA design, and the frequency of signals is also very high. With the use of high-speed devices, there will be more and more high-speed DSP (digital signal processing) system designs, and processing signal problems in high-speed DSP application systems has become an important design issue. In this design, the characteristics are that the system data rate, clock rate and circuit density are constantly increasing, and the design of its PCB printed board shows completely different behavioral characteristics from the low-speed design, that is, signal integrity problems, interference aggravation problems, electromagnetic compatibility problems, etc.

These problems can cause or directly lead to signal distortion, timing errors, incorrect data, address and control lines, system errors and even system crashes. If not properly resolved, they will seriously affect system performance and bring immeasurable losses. The solution to these problems mainly depends on circuit design. Therefore, the design quality of PCB printed boards is very important. It is the only way to turn 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.

Power Design

The first thing to consider when designing a high-speed DSP system PCB is the power supply design. In power supply design, the following methods are usually used to solve signal integrity issues.

Consider power and ground decoupling

As the operating frequency of DSP increases, 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 supplies, such as DSP I/O power supply voltage and core power supply voltage are different, two different power supply layers can be used. If the processing cost of multi-layer boards is high, power supplies with more wiring or relatively critical can use a dedicated layer, and other power supplies can be wired like signal lines, but attention should be paid to the sufficient width of the line.

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

Consider wiring rules for power distribution

Separate analog and digital power planes

High-speed and high-precision analog components are very sensitive to digital signals. For example, amplifiers will amplify switching noise and make it close to pulse signals, so the power supply layers are generally required to be separated in the analog and digital parts of the board.

Isolating 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. High-frequency clocks (clocks above 20MHz, or clocks with a flip time of less than 5ns) must be escorted by ground wires. The clock line width must be at least 10mil, and the escort ground line width must be at least 20mil. The two ends of the protection ground wire of the high-frequency signal line must be in good contact with the ground layer through vias, and vias must be drilled every 5cm to connect to the ground layer; the clock sending side must be connected in series with a damping resistor of 22Ω to 220Ω. This can avoid interference caused by signal noise brought by these lines.

Software and hardware anti-interference design

Generally, the PCB boards of high-speed DSP application systems 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 and there is electromagnetic interference, the DSP program flow will be disordered. When the normal working code of DSP cannot be restored, the program will run away or freeze, and even some components will be damaged. It is necessary to take appropriate anti-interference measures.

Hardware anti-interference design

Hardware anti-interference is highly efficient. When system complexity, cost, and volume are tolerable, hardware anti-interference design is preferred. Common hardware anti-interference technologies can be summarized as follows:

(1) Hardware filtering: RC filters can greatly weaken various high-frequency interference signals, such as suppressing "burr" interference.

(2) Reasonable grounding: Reasonable design of the grounding system. For high-speed digital and analog circuit systems, it is important to have a low-impedance, large-area grounding layer. The ground layer can provide a low-impedance return path for high-frequency currents, reduce EMI and RFI, and shield against external interference. Separate the analog ground and digital ground during PCB design.

(3) Shielding measures: AC power supply, high-frequency power supply, high-voltage equipment, and electric arc sparks will generate electromagnetic waves, which become the noise source of electromagnetic interference. The above devices can be surrounded by metal shells and then grounded. This is very effective in shielding the interference caused by electromagnetic induction.

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

Software anti-interference design

Software anti-interference has advantages that cannot be replaced by hardware anti-interference. In DSP application systems, the anti-interference ability of software should be fully exploited to minimize the impact of interference. The following are several effective software anti-interference methods.