How to ensure the quality of PCB design for high-speed DSP

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How to ensure the quality of PCB design for high-speed DSP [Copy link]

　　As the chip integration becomes higher and higher, the number of chip pins increases, and the device packaging is also constantly changing, from DIP to OSOP, from SOP to PQFP, and from PQFP to BGA. TMS320C6000 series devices use BGA packaging. In terms of circuit application, BGA packaging has the characteristics of high success rate, low repair rate, and high reliability, and its application is becoming more and more extensive. However, since BGA packaging belongs to ball grid array patch packaging, the physical implementation of the system under development, that is, board-level design involves many high-speed digital circuit design technologies. In high-speed systems, the generation of noise interference is the first influencing factor. High-frequency circuits will also generate radiation and conflict, while faster edge rates will generate ringing, reflection and crosstalk. If the particularity of high-speed signal layout and wiring is not considered, the designed circuit board will not work properly. Therefore, the successful design of PCB boards is a very critical link in the DSPs circuit design process. Therefore, the design quality of PCB 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. 1. Power supply design The first thing to consider in the design of PCB boards for high-speed DSP systems is the power supply design problem. In power supply design, the following methods are usually used to solve signal integrity problems. 1. Consider the decoupling of power supply 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 supply and ground can use a dedicated layer. 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, the power supply with more wiring or relatively critical can be used in a dedicated layer. Other power supplies can be wired like signal lines, but attention should be paid to the width of the line. Regardless of whether the circuit board has a dedicated ground layer and power layer, a certain and reasonably distributed capacitor 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 surface mount capacitors. The surface mount capacitor can be placed on the back of the PCB board, that is, the welding surface. The surface mount capacitor is connected to the through hole with a wide line and connected to the power supply and ground layer through the through hole. 2. Consider the wiring rules of power distribution. Separate analog and digital power layers. High-speed and high-precision analog components are very sensitive to digital signals. For example, the amplifier will amplify the switching noise and make it close to the pulse signal, so the power layer is generally required to be separated in the analog and digital parts of the board. 3. 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. 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 protective ground wires of high-frequency signal lines must be well contacted with the ground layer by vias at both ends, and vias must be drilled every 5cm to connect to the ground layer; a 22Ω-220Ω damping resistor must be connected in series on the clock sending side. Interference caused by signal noise brought by these lines can be avoided. 2. 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 and there is electromagnetic interference, the DSP program flow will be disordered. When the normal working code of the 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. 1. Hardware anti-interference design Hardware anti-interference efficiency is high. When the 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. For example, it can suppress "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 not only provide a low-impedance return path for high-frequency currents, but also make EMI and RFI smaller, and also have a shielding effect on external interference. When designing PCB, separate the analog ground and digital ground. (3) Shielding measures: AC power supply, high-frequency power supply, strong electric equipment, and electric arc sparks will generate electromagnetic waves and become noise sources of electromagnetic interference. The above devices can be surrounded by metal shells and then grounded. This is very effective in shielding 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.). 2. 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 explored to minimize the impact of interference. The following are several effective software anti-interference methods. (1) Digital filtering: The noise of analog input signals can be eliminated by digital filtering. Commonly used digital filtering techniques include: median filtering, arithmetic mean filtering, etc. (2) Set traps: Set a boot program in an unused program area. When the program jumps to this area due to interference, the boot program will force the captured program to the specified address, where a special program will be used to process the faulty program. (3) Instruction redundancy: Insert two or three bytes of no-operation instructions NOP after two-byte instructions and three-byte instructions to prevent the program from automatically returning to normal when the DSP system is interfered with and the program runs away. (4) Set watchdog timing: If the out-of-control program enters a "dead loop", the "watchdog" technology is usually used to get the program out of the "dead loop". The principle is to use a timer that generates a pulse according to a set period. If you do not want to generate this pulse, the DSP should clear the timer within a time less than the set period; but when the DSP program runs away, the timer will not be cleared as required, so the pulse generated by the timer is used as a DSP reset signal to reset and initialize the DSP. III. 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 various external interferences and reduce the electromagnetic interference of electronic equipment to other electronic equipment. In actual PCB boards, there is more or less electromagnetic interference between adjacent signals, namely crosstalk. The size of the crosstalk is related to the distributed capacitance and distributed inductance between the loops. The following measures can be taken to solve this mutual electromagnetic interference between signals: 1. Choose a reasonable wire width. The impact interference generated by transient current on the printed lines is mainly caused by the inductance component of the printed wire, and its inductance is proportional to the length of the printed wire and inversely proportional to the width. Therefore, using 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, a printed wire width of about 1.5mm can meet the requirements; for integrated circuits, the printed wire width is selected between 0.2mm and 1.0mm. 2. Use a well-shaped mesh wiring structure. The specific method is to wire horizontally on one layer of the PCB board and vertically on the next layer. Fourth, heat dissipation design is conducive to heat dissipation. The printed board is preferably installed independently, and the board spacing should be greater than 2cm. At the same time, pay attention to the layout rules of components on the printed board. In the horizontal direction, high-power devices should be placed as close to the edge of the printed circuit board as possible to shorten the heat transfer path; in the vertical direction, high-power devices should be placed as close to the top of the printed circuit 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 temperatures as much as possible, and should not be placed directly above devices that generate a lot of heat. In the various designs of high-speed DSP application systems, how to transform a perfect design from theory into reality depends on high-quality PCB boards. The operating frequency of DSP circuits is getting higher and higher, the pins are getting denser, and the interference is increasing. How to improve the quality of the signal is very important. Therefore, whether the performance of the system is good or not is inseparable from the quality of the designer's PCB board. The above are the four major issues in PCB design that need to be paid attention to in ensuring high-speed DSP.

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mwkjhl

Posted by Qianghe Technology on 2017-12-4 16:55 Not bad

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