1. Power supply design
High-speed DSP system PCB board design must first consider the power supply design problem. In power supply design, the following methods are usually used to solve signal integrity problems.
2. Consider the decoupling of power and ground
As the operating frequency of DSP increases, DSP and other IC components tend to be miniaturized and package-intensive. Generally, multi-layer boards are considered in circuit design. It is recommended that a dedicated layer be used for both power and ground, and for multiple power supplies, For example, the I/O power supply voltage of the DSP is different from the core power supply voltage. Two different power supply layers can be used. If the processing cost of the multi-layer board is high, a special layer can be used for the wiring or the relatively critical power supply. The power supply can be routed like a signal line, but be careful that the width of the line is sufficient.
Regardless of whether the board has a dedicated ground plane and power plane, a certain and well-distributed capacitor must be added between the power supply and ground. In order to save space and reduce the number of vias, it is recommended to use a chip capacitor. The chip capacitor can be placed on the back side of the PCB board, that is, the soldering surface, and the chip capacitor to the through hole is connected by a wide line and connected to the power supply and the ground through the through hole.
3. Consider the 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 amplifies the switching noise to bring it close to the pulse signal, so the power plane is generally required to be separated in the analog and digital sections of the board.
(2) Isolation of sensitive signals
Some sensitive signals, such as high-frequency clocks, are particularly sensitive to noise interference, and they require high-level isolation. High-frequency clocks (clocks above 20MHz, or clocks with a turn-over time of less than 5ns) must have ground escorts. The clock line width is at least 10mils, the escort ground line width is at least 20mil, and the high-frequency signal lines must be crossed at both ends. The hole is in good contact with the formation, and the hole is connected to the ground every 5cm. The clock transmission side must be connected in series with a damping resistor of 22Ω~220Ω. Interference caused by signal noise caused by these lines can be avoided.
4. Software and hardware anti-jamming design
Generally, the high-speed DSP application system PCB board is designed by the user according to the specific requirements of the system. Due to the limited design capability and laboratory conditions, if the perfect and reliable anti-interference measures are not taken, once the working environment is not ideal, there is electromagnetic Interference can lead to turbulence in the DSP program. When the normal working code of the DSP cannot be recovered, there will be a runaway program or a crash, and even some components will be damaged. Attention should be paid to the corresponding anti-interference measures.
5. Hardware anti-jamming design
The hardware anti-interference efficiency is high. In the case that the system complexity, cost and volume can be tolerated, the hardware anti-jamming design is preferred. Commonly used hardware anti-jamming techniques can be summarized as follows:
(1) Hardware filtering: The RC filter can greatly attenuate various types of high frequency interference signals. If it can suppress "glitch" 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 ground plane. The formation provides a low-impedance return path for high-frequency currents, and makes EMI and RFI smaller, while also shielding external interference. The PCB is designed to separate the analog ground from the digital ground.
(3) Shielding measures: AC power, high-frequency power, high-voltage equipment, electric spark generated by arc, will generate electromagnetic waves, become a noise source of electromagnetic interference, can be surrounded by the metal shell, and then grounded, this pair of shielding Interference caused by electromagnetic induction is very effective.
(4) Photoelectric isolation: Opto-isolators can effectively avoid mutual interference between different boards. High-speed opto-isolators are often used for interfaces between DSPs and other devices (such as sensors, switches, etc.).
6. Software anti-jamming design
Software anti-interference has the advantage that hardware anti-interference can't replace. In DSP application system, the anti-interference ability of software should also be fully exploited 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. Commonly used 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 is disturbed and jumped to this area, the boot program will boot the program that was forcibly captured to the specified address, where the program is used to program the error. Process it.
(3) Instruction redundancy: Inserting two or three bytes of the null operation instruction NOP after the two-byte instruction and the three-byte instruction can prevent the program from being automatically put on the track when the DSP system is run by the interference program.
(4) Set the watchdog timing: If the out-of-control program enters the "infinite loop", the "watchdog" technique is usually used to get the program out of the "infinite loop". The principle is to use a timer that generates a pulse according to the set period. If you do not want to generate this pulse, the DSP should clear the timer within less than the set period; but when the DSP program runs away, it will not The timer is cleared as specified, and the pulse generated by the timer acts as a DSP reset signal to reset and initialize the DSP.
7. Electromagnetic compatibility design
Electromagnetic compatibility refers to the ability of an electronic device to function properly in a complex electromagnetic environment. The purpose of electromagnetic compatibility design is to enable electronic devices to suppress various external interferences and reduce electromagnetic interference of electronic devices to other electronic devices. In the actual PCB board, there is more or less electromagnetic interference phenomenon, that is, crosstalk between adjacent signals. The size of the crosstalk is related to the distributed capacitance and distributed inductance between the loops. To solve the mutual electromagnetic interference between such signals, the following measures can be taken:
8. Choose a reasonable wire width
Since the transient interference generated by the transient current on the printed line is mainly caused by the inductance component of the printed conductor, the inductance is proportional to the length of the printed conductor and inversely proportional to the width. Therefore, it is advantageous to use short and wide wires to suppress interference. The clock leads and the bus driver's signal lines often have large transient currents, and the printed conductors should be as short as possible.毫米之间。 Selecting between 0. 2mm~1. 0mm. For the integrated circuit, the width of the printed conductor is between 0. 2mm~1. 0mm.
9. Adopt a well-shaped mesh wiring structure.
The specific method is a layer of lateral wiring on the PCB printed board, followed by a layer of vertical wiring.
10. Thermal design
In order to facilitate heat dissipation, the printed board is preferably self-supporting, the board spacing should be greater than 2cm, and at the same time pay attention to the rules of the components on the printed board. In the horizontal direction, the high-power devices are placed as close as possible to the edges of the printed board, thereby shortening the heat transfer path; the high-power devices in the vertical direction are placed as close as possible to the top of the printed board, thereby reducing the influence on the temperature of other components. Components that are sensitive to temperature should be placed in a relatively low temperature area and not placed directly above the device with large heat.
In the various designs of high-speed DSP application system, how to transform the perfect design from theory to reality depends on high-quality PCB printed board, the working frequency of DSP circuit is higher and higher, the pin is more and more dense, and the interference Increase, how to improve the quality of the signal is very important. Therefore, the performance of the system is good, and is inseparable from the quality of the designer's PCB printed board. If the design can be rationally laid out, noise reduction, noise reduction, and unnecessary mistakes are avoided, the performance of the system is not underestimated.