What should be paid attention to when designing the PCB board of a high-speed DSP system
The high-speed DSP system PCB board design first needs to consider the power supply design issue. In power supply design, the following methods are usually used to solve the problem of signal integrity.
Consider power and ground decoupling
With the increase of the DSP operating frequency, DSP and other IC components tend to be miniaturized and densely packed. Usually, multi-layer boards are considered in circuit design. It is recommended that both the power supply and the ground can use a special layer, and for multiple power supplies, For example, the I / O power supply voltage of the DSP and the core power supply voltage are different. Two different power supply layers can be used. If the processing cost of the multilayer board is high, a special layer for more wiring or a relatively critical power supply can be used. Others The power supply can be routed the same as the signal line, but pay attention to the width of the line.
Regardless of whether the circuit board has a dedicated ground plane and power plane, a certain and reasonably distributed capacitor must be added between the power source and 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 of the PCB board, that is, the soldering surface. The chip capacitor is connected to the through hole with a wide line and connected to the power source and the ground through the through hole.
Consider wiring rules for power distribution
(1) Separate the 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 bring it closer to the pulse signal, so the power supply layer is generally required to be separated on the analog and digital parts of the board.
(2) Isolate sensitive signals
Some sensitive signals (such as high-frequency clocks) are particularly sensitive to noise interference, and high-level isolation measures must be adopted for them. High-frequency clocks (clocks above 20MHz, or clocks with a flip time less than 5ns) must be escorted by a ground wire with a clock line width of at least 10mil and an escorted ground line width of at least 20mil. The hole is in good contact with the ground, and the hole is connected to the ground every 5cm; the clock sending side must be connected in series with a 22Ω-220Ω damping resistor. Interference caused by signal noise from 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 and there is electromagnetic Interference will cause the DSP program flow to be disordered. When the normal working code of the DSP cannot be recovered, a runaway program or a crash will occur, and some components will even be damaged. Attention should be paid to appropriate anti-interference measures.
Hardware anti-interference design
Hardware anti-interference efficiency is high. Under the circumstances of system complexity, cost and volume tolerance, hardware anti-interference design is preferred. The commonly used hardware anti-interference technologies can be summarized as follows:
(1) Hardware filtering: RC filters can greatly attenuate various high-frequency interference signals. If you can suppress "glitch" interference.
(2) Reasonable grounding: Reasonably design the grounding system. For high-speed digital and analog circuit systems, it is important to have a low-impedance, large-area ground plane. The ground layer can not only provide a low-impedance return path for high-frequency currents, but also make EMI and RFI smaller, and at the same time have a shielding effect on external interference. Separate the analog ground from the digital ground when designing the PCB.
(3) Shielding measures: Electric sparks generated by AC power, high-frequency power, high-power equipment, and arcs will generate electromagnetic waves and become noise sources of electromagnetic interference. The above-mentioned devices can be surrounded by a metal case and grounded. 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 optical isolators are often used for the interface of DSP and other equipment (such as sensors, switches, etc.).
Software anti-interference design
Software anti-interference has advantages that cannot be replaced by hardware anti-interference. In the DSP application system, the anti-interference ability of the software should be fully exploited, so as to minimize the influence of interference. Several effective software anti-jamming methods are given below.
(1) Digital filtering: The noise of the analog input signal can be eliminated by digital filtering. Common digital filtering techniques are: median filtering, arithmetic mean filtering, and so on.
(2) Set trap: Set a boot program in the 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, and use a special program to correct the error program there. For processing.
(3) Instruction redundancy: Insert two or three bytes of no-operation instruction NOP after the two-byte instruction and the three-byte instruction, which can prevent the DSP system from automatically putting the program on the right track when it is disturbed by the program.
(4) Set the watchdog timing: If the program that is out of control enters the "endless loop", usually the "watchdog" technology is used to make the program out of the "endless loop". The principle is to use a timer that generates a pulse at a set period. If you do not want to generate this pulse, the DSP should clear the timer to a time less than the set period. However, when the DSP program runs, it does not The timer will be cleared to zero as required, so the pulse generated by the timer is used as the DSP reset signal to reset and initialize the DSP.
Electromagnetic compatibility design
Electromagnetic compatibility refers to the ability of electronic equipment to work normally in complex electromagnetic environments. The purpose of electromagnetic compatibility design is to make electronic equipment not only suppress various external interference, but also 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, that is, crosstalk. The amount of crosstalk is related to the distributed capacitance and distributed inductance between the loops. The following measures can be taken to resolve mutual electromagnetic interference between such signals:
Choose a reasonable wire width
The impact interference caused by the 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, it is advantageous to use short and wide wires to suppress interference. Clock leads and signal lines of bus drivers often have large transient currents, and their printed wires must be as short as possible. For discrete component circuits, the width of the printed wiring can meet the requirements of about 1.5mm; for integrated circuits, the width of the printed wiring is selected between 0. 2mm-1. 0mm.
Adopting a chevron-shaped network wiring structure
The specific method is to horizontally route one layer of the PCB printed board and the next layer to vertically.
In order to facilitate heat dissipation, it is best to install printed boards on their own. The board spacing should be greater than 2cm. At the same time, pay attention to the rules for the layout of components on the printed boards. In the horizontal direction, the high-power devices are arranged as close to the edge of the printed board as possible to shorten the heat transfer path; in the vertical direction, the high-power devices are arranged as close to the printed board as possible to reduce its influence on the temperature of other components. Components that are more sensitive to temperature should be placed in areas where the temperature is relatively low, and should not be placed directly above the components that generate large amounts of heat.
In 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. The working frequency of DSP circuits is getting higher and higher, and the pins are getting denser and more disturbing. It is important to increase the signal quality. Therefore, whether the performance of the system is good or not is inseparable from the quality of the designer's PCB. If the layout can be designed reasonably, noise and interference can be reduced, and unnecessary mistakes can be avoided, which will not underestimate the performance of the system.