Design Problems of High-Speed DSP System on PCB Board

Aiming at several issues that should be paid attention to in PCB board reliability design in high-speed DSP system.

The first thing that needs to be considered in the design of high-speed DSP system PCB board is the problem of power supply design. In power supply design, the following methods are usually used to solve signal integrity problems.

Consider power and ground decoupling

Regardless of whether the circuit board has a dedicated ground plane and power plane, a certain and reasonably distributed capacitance 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 more chip capacitors. The chip capacitor can be placed on the back of the PCB board, that is, the soldering surface, and the chip capacitor is connected to the through hole with a wide line and connected to the power supply and the ground layer 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 and 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 high-level isolation measures should be taken for them. High-frequency clocks (clocks above 20MHz, or clocks with an inversion time less than 5ns) must be escorted by a ground wire. The clock line width is at least 10mil, and the escort ground line width is at least 20mil. The holes are in good contact with the ground, and holes are drilled every 5cm to connect with the ground; 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.

Software and hardware anti-jamming design

General high-speed DSP application system PCB boards are designed by the user according to the specific requirements of the system. Due to the 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 Interference will cause the DSP program flow to be disordered. When the normal working code of the DSP cannot be restored, the program will run away or crash, and some components will even be damaged. Care should be taken to take appropriate anti-interference measures.

Hardware anti-jamming design

The hardware anti-jamming efficiency is high. When the system complexity, cost, and volume are tolerable, the hardware anti-jamming design is preferred. Commonly used hardware anti-jamming technologies can be summarized as follows:

1. Hardware filtering: RC filter can greatly weaken various high-frequency interference signals. For example, "burr" interference can be suppressed.

2. Reasonable grounding: Reasonably design the grounding system. For high-speed digital and analog circuit systems, it is very important to have a low-impedance, large-area grounding layer. The formation 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. Separate the analog ground and digital ground during PCB design.

3. Shielding measures: AC power supply, high-frequency power supply, strong current equipment, and electric sparks generated by arcs will generate electromagnetic waves and become noise sources of electromagnetic interference. The above-mentioned devices can be surrounded by a metal shell and then grounded. This pair of shielding passes Interference caused by electromagnetic induction is very effective.

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

Software anti-jamming has the irreplaceable advantage of hardware anti-jamming. In the DSP application system, the anti-jamming ability of software should be fully exploited, so as to suppress the influence of interference to a minimum. 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, etc.

2. Set a trap: set a boot program in the unused program area. When the program is disturbed and jumps to this area, the boot program guides the captured program to the specified address, where the error program is checked by a special program. deal with.

3. Instruction redundancy: Insert two or three bytes of no-operation instruction NOP after the double-byte instruction and three-byte instruction, which can prevent the program from being automatically put on the right track when the DSP system is disturbed by the program.

4. Set the watchdog timing: If the out-of-control program enters an "infinite loop", usually use the "watchdog" technology to make the program break out of the "infinite 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 within a time less than the set period; but when the DSP program runs away, it will not The timer will be cleared according to the regulations, so the pulse generated by the timer is used as the DSP reset signal to reset and initialize the DSP again.

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 make electronic equipment not only suppress various external interference, but also reduce the electromagnetic interference of electronic equipment to other electronic equipment. 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. The following measures can be taken to solve the mutual electromagnetic interference between such signals:

Choose a reasonable wire width

The impact interference generated by the transient current on the printed line 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 beneficial 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 should be as short as possible. For discrete component circuits, the width of the printed wires is about 1.5mm to meet the requirements; for integrated circuits, the width of the printed wires is selected between 0. 2mm - 1. 0mm.

The well-shaped mesh wiring structure is adopted, and the specific method is to lay the horizontal wiring on one layer of the PCB printed board, and the vertical wiring on the next layer.

In order to facilitate heat dissipation, it is best to install the printed board independently, and 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, the high-power devices are arranged as close as possible to the edge of the printed board, thereby shortening the heat transfer path; in the vertical direction, the high-power devices are arranged as close as possible to the top of the printed board, thereby reducing its influence on the temperature of other components. Components that are sensitive to temperature should be placed in areas with relatively low temperatures as far as possible, and should not be placed directly above devices that generate large amounts of heat.

In the various designs of high-speed DSP application systems, how to transform perfect design from theory into reality depends on high-quality PCB printed boards. How to increase 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 printed board. If the design can be rationally laid out, noise and interference can be reduced, and unnecessary mistakes can be avoided, it will not underestimate the performance of the system.