Several problems that should be paid attention to in the design of PCB board reliability in high-speed DSP system.
Power design
The first thing to consider when designing a high-speed DSP system PCB board is power supply design issues. In power supply design, the following methods are usually used to solve the problem of signal integrity. Consider the decoupling of power and ground
With the increase of DSP operating frequency, DSP and other IC components tend to be miniaturized and package-intensive. Usually circuit design considers multilayer boards. It is recommended that both power supply and ground can use a special layer, and for multiple power supplies, For example, the I/O supply voltage and the core supply voltage of the DSP may be different, and two different power layers may be used. If the cost of processing the multilayer board is high, a more dedicated or a more critical power supply may be used for the other layers. The power supply can be wired like a signal line, but be aware that the width of the line is sufficient.
Regardless of whether the board has a dedicated ground plane or power plane, a certain, well-distributed capacitor must be placed between the power supply and the ground. In order to save space and reduce the number of vias, it is recommended to use more chip capacitors. The chip capacitor can be placed on the back side of the PCB board, that is, the solder surface. The chip capacitor is connected to the through hole with a wide wire and is connected with the power source and the ground through the through hole. Consider wiring rules for power distribution (1), separate analog and digital power layers
High-speed, 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 in the analog and digital parts of the board, the power plane is generally required to be separated. (2) Isolate sensitive signals
Some sensitive signals (such as high-frequency clocks) are particularly sensitive to noise interference and require high-level isolation measures. High-frequency clock (clock above 20MHz, or clock with less than 5ns flipping time) must be escorted by a ground wire. The clock line width must be at least 10 mils. The line width of the escaping ground line should be at least 20 mils. Both sides of the protection ground of the high-frequency signal line must pass. The hole is in good contact with the ground and the hole is drilled every 5cm to connect with the ground; a damping resistance of 22Ω to 220Ω must be connected in series on the clock sending side. The interference caused by the signal noise caused by these lines can be avoided. Software and hardware anti-jamming design
General high-speed DSP application system PCB board is designed by the user according to the specific requirements of the system, due to the design capabilities, laboratory conditions are limited, if not take perfect, reliable anti-jamming measures, once encountered working environment is not ideal, there is electromagnetic Disturbance will cause the DSP program flow to be disordered. When the DSP normal work code cannot be recovered, runaway programs or crashes will occur, and even certain components will be damaged. Should pay attention to take appropriate anti-jamming measures. Hardware anti-jamming design
The hardware anti-jamming efficiency is high. In the case of system complexity, cost, and volume can be tolerated, hardware anti-jamming design is preferred. The commonly used hardware anti-jamming technology can be summarized as follows:
(1) Hardware filtering: RC filter can greatly weaken various types of high-frequency interference signals. If you can suppress the "glitch" interference.
(2) Reasonable grounding: Reasonably designing the grounding system is very important for high-speed digital and analog circuit systems with a low-impedance, large-area ground plane. The formation can provide a low-impedance return path for high-frequency currents, and it can make EMI, RFI smaller, and also shield external disturbances. The PCB design separates the analog ground from the digital ground.
(3) Shielding measures: AC power supply, high-frequency power supply, strong electrical equipment, and electric spark generated by an electric arc will generate electromagnetic waves and become electromagnetic noise sources. The metal housing can be used to surround the above components and be grounded. The interference caused by electromagnetic induction is very effective.
(4) Opto-isolation: Opto-isolators can effectively avoid mutual interference between different circuit boards. High-speed opto-isolators are commonly used in interfaces between DSPs and other devices (such as sensors, switches, etc.). Software anti-jamming design
Software anti-jamming has the advantage that hardware anti-jamming can't replace it. In the DSP application system, the anti-interference ability of the software should also be fully exploited so as to minimize the influence of interference. Here are some effective software anti-jamming methods.
(1) Digital filtering: The noise of the analog input signal can be eliminated by digital filtering. The commonly used digital filtering techniques are: median filtering, arithmetic average filtering, and the like.
(2) Set traps: Set a boot program in the unused program area. When the program is disturbed to jump to this area, the boot program will force the captured program to the specified address, where special program to error program Processing.
(3) Instruction Redundancy: The insertion of two or three bytes of the no-operation instruction NOP after a double-byte instruction and a three-byte instruction can prevent the program from being automatically placed on the track when the DSP system is run away by an interference program.
(4) Set Watchdog Timer: If the out-of-control program enters an "endless loop", the "watchdog" technique is usually used to make the program "out of loop". The principle is to use a timer, which generates a pulse according to the set period. If you do not want to generate this pulse, the DSP should clear the timer in less than the set period; but when the DSP program runs away, it does not The timer will be cleared as required. The pulse generated by the timer is then used as a DSP reset signal to reset and initialize the DSP. Electromagnetic Compatibility Design
Electromagnetic compatibility refers to the ability of an electronic device to work properly in a complex electromagnetic environment. The purpose of electromagnetic compatibility design is to make electronic devices not only suppress all kinds of external interference, but also reduce the electromagnetic interference from electronic devices to other electronic devices. There are more or less electromagnetic interference phenomena, that is, crosstalk, between adjacent signals in an actual PCB board. The size of crosstalk is related to the distributed capacitance and distributed inductance between loops. Solve the mutual electromagnetic interference between these signals can take the following measures: Select a reasonable wire width
The impact of transient currents on printed lines is mainly caused by the inductance components of printed conductors, and the inductance is proportional to the length of printed conductors and inversely proportional to the width. Therefore, it is advantageous to use short and wide wires to suppress interference. Clock lines, bus driver signal lines often have large transient currents, and their printed conductors must be as short as possible. For discrete component circuits, a printed conductor width of about 1.5 mm may satisfy the requirement; for an integrated circuit, the width of the printed conductor is selected between 0. 2 mm and 1.0 mm. Use a grid pattern of grid structure.
The specific approach is to route a layer of PCB PCB layout, followed by a vertical layer of wiring. Thermal design
In order to facilitate heat dissipation, the printed board is preferably installed by itself. 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 arranged as close to the edge of the printed board as possible, thereby shortening the heat transfer path; in the vertical direction, high-power devices are arranged as close to the printed board as possible, thereby reducing the influence on the temperature of other components. Components that are sensitive to temperature are placed as far as possible in the lower temperature area, and cannot be placed directly above the heat generating device. Conclusion
In the design of high-speed DSP application system, how to transform the perfect design from theory to reality depends on high-quality PCB PCB, the working frequency of DSP circuit is higher and higher, the pin is more and more dense, and interference To increase, how to improve the signal quality is very important. Therefore, whether the performance of the system is good is inseparable from the designer's PCB printed board quality. If we can rationally design the design, reduce noise, reduce disturbances, and avoid unnecessary mistakes, we can't underestimate the performance of the system.
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