Electronic Design - High-speed PCB anti-jamming design based on DSP

Introduction

With the wide application of DSP (digital signal processor), the design of a high-speed signal processing PCB board based on DSP is particularly important. In a DSP system, the operating frequency of the DSP microprocessor can be as high as hundreds of MHz. Its reset line, interrupt line and control line, integrated circuit switches, high-precision A/D conversion circuits, and circuits containing weak analog signals are all It is very susceptible to interference; therefore, to design and develop a stable and reliable DSP system, anti-interference design is very important.

1 DSP system interference generation analysis

For DSP systems, the main interference comes from the following aspects:

1. I/O channel interference. Refers to interference entering the system through the forward channel and the backward channel, such as the data acquisition link of a DSP system. The interference is superimposed on the signal through the sensor, which increases the error of data acquisition. In the output link, interference can increase the output data error, or even make a complete error, causing the system to crash. Optocoupler devices can be reasonably used to reduce the interference of input and output channels, and the interference of the sensor and the DSP main system can be used to electrically isolate the interference.

2. The interference of the power system. The main source of interference for the entire DSP system. The power supply adds its noise to the power supply while providing power to the system. The power supply line must be decoupled during the circuit design of the power supply chip.

3.  Space radiation coupling interference. The coupling through radiation is usually called crosstalk. Crosstalk occurs in the electromagnetic field generated when current flows through the wires, and the electromagnetic field induces transient currents in adjacent wires, causing nearby signals to be distorted or even errors. The strength of crosstalk depends on the geometrical size and separation distance of devices and wires. In DSP wiring, the larger the signal line spacing and the closer to the ground line, the more effective it is to reduce crosstalk.

2 Design PCB for the cause of interference

The following shows how to reduce various interferences in the PCB manufacturing process of the DSP system.

2.1 Stacked design of multilayer board

In DSP high-speed digital circuits, in order to improve signal quality, reduce wiring difficulty, and increase the EMC of the system, a multilayer design is generally adopted. The stacked design can provide the shortest return path, reduce the coupling area, and suppress differential mode interference. In the stacked design, the distribution of the dedicated power layer and ground layer, and the tight coupling of the ground layer and the power layer is good for suppressing common-mode interference (using adjacent planes to reduce the AC impedance of the power plane). The 4-layer board is taken as an example to illustrate the stacked design scheme.

There are many advantages to adopting this 4-layer PCB design structure. There is a power layer under the top layer, and the power pins of the components can be directly connected to the power supply without passing through the ground plane. The key signal is selected on the bottom layer (bottom layer) so that the important signal wiring space is larger, and the devices are placed on the same layer as much as possible.

2.2 Layout design

In order to get the best performance of the DSP system, the layout of the components is very important. Place DSP, Flash, SRAM, and CPLD devices first, carefully consider the wiring space, then place other ICs according to the principle of functional independence, and finally consider the placement of I/O ports. Combine the above layout and consider the PCB size: if the size is too large, the printed lines will be too long, the impedance will increase, the noise resistance will be reduced, and the board cost will increase; if the PCB is too small, the heat dissipation will not be good, and the space will be limited, Adjacent lines are easily disturbed. Therefore, the device should be selected according to actual needs, combined with the wiring space, and roughly calculate the size of the PCB. When laying out the DSP system, pay special attention to the placement of the following devices.

(1) High-speed signal layout

In the entire DSP system, the main high-speed digital signal lines are between DSP and Flash, and SRAM, so the distance between the devices should be as close as possible, and their connections should be as short as possible and directly connected. Therefore, in order to reduce the influence of transmission lines on signal quality, high-speed signal traces should be as short as possible. Also, consider that many DSP chips with speeds up to several hundred MHz require a snake-shaped winding (delay tune). This will be emphasized in the wiring below.

(2) Digital-analog device layout

Most of the DSP systems are not have a single functional circuit, a large number of CM0S digital devices and digital-analog hybrid devices are used, so the digital/analog layout should be separated. The analog signal devices are concentrated as much as possible so that the analog ground can draw an independent area belonging to the analog signal in the middle of the entire digital ground, so as to avoid the interference of the digital signal to the analog signal. Some digital-analog hybrid devices, such as D/A converters, are traditionally regarded as analog devices, placed on the analog ground, and provided with a digital loop to allow digital noise to be fed back to the signal source to reduce digital noise The impact on the analog ground.

(3) The layout of the clock

For clock, chip select, and bus signals, keep away from I/O lines and connectors as much as possible. The clock input of the DSP system is very susceptible to interference, and its processing is very critical. Always ensure that the clock generator is as close to the DSP chip as possible, and make the clock line as short as possible. The outer shell of the clock crystal oscillator is preferably grounded.

(4) Decoupling layout

In order to reduce the instantaneous overshoot of the voltage on the power supply of the integrated circuit chip, a decoupling capacitor is added to the integrated circuit chip, which can effectively remove the influence of the burr on the power supply and reduce the power loop reflection on the PCB. Adding a decoupling capacitor can bypass the high-frequency noise of the integrated circuit device, and can also be used as an energy storage capacitor to provide and absorb the instantaneous charging and discharging of the energy of the integrated circuit door opening and closing.

In the DSP system, place decoupling capacitors for each integrated circuit, such as DSP, SRAM, Flash, etc., and add them between each power supply and ground of the chip, and pay special attention to the decoupling capacitors as close as possible to the power supply terminal (source ) And IC component pins. Ensure the purity of the current from the power supply terminal (source terminal) and the IC, and shorten the noise path as much as possible.

(5) Layout of power supply

When developing a DSP system, the power supply needs to be carefully considered. Because some power chips generate a lot of heat, they should be placed in a position that is conducive to heat dissipation and should be separated from other PCB components by a certain distance. You can use heat sinks or lay copper under the device for heat dissipation. Be careful not to place heat-generating components on the bottom layer of the development board.