Special attention should be paid to the processing of the DSP board layout of the PCB.

With the wide application of DSP (Digital Signal Processor), the design of DSP-based high-speed signal processing PCB is particularly important. In PCB design, layout is an important link. The quality of the layout results will directly affect the effect of the wiring. Therefore, it can be considered that a reasonable layout is the first step in the success of PCB design.

For PCB boards of high-speed DSP image processing boards used for real-time image signal detection, it is very important to optimize the layout of components in order to achieve the best performance of the DSP system. In general, first place DSP, Flash, SRAM, and CPLD devices. This requires careful consideration of the trace space, then place other ICs on a functionally independent basis, and finally consider the placement of I/O ports. Consider the above layout and then consider the size of the PCB: if the size is too large, the printed lines will be too long, the impedance will increase, the noise resistance will decrease, and the board cost will increase. If the PCB is too small, the heat dissipation is not good and the space is limited. The adjacent lines are susceptible to interference. Therefore, it is necessary to select the device according to actual needs, and combine the wiring space to 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 line between DSP and Flash, SRAM, so the distance between the devices should be as close as possible, the connection is as short as possible, and directly connected. Therefore, in order to reduce the impact of the transmission line on the signal quality, the high-speed signal trace should be as short as possible. Also consider a lot of DSP chips that reach speeds of a few hundred MHz, and you need to do a delay tune.

(2) Digital-to-analog device layout

Most of the DSP systems are not a single functional circuit. A large number of CMOS digital devices and digital analog hybrid devices are used, so the digital/analog should be laid out separately. The analog signal devices are concentrated as much as possible so that the analog ground can draw an independent area of the analog signal in the middle of the digital ground to avoid interference of the digital signal with the analog signal. For some digital-analog hybrid devices, such as D/A converters, they are traditionally viewed as analog devices, placed on analog ground, and provided with a digital loop that allows digital noise to be fed back to the source, reducing digital noise. The impact on the simulated ground.

(3) Clock layout

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 is critical to its processing. Always keep the clock generator as close as possible to the DSP chip, making the clock line as short as possible. The housing of the clock crystal oscillator is preferably grounded.

(4) Decoupling layout

In order to reduce the instantaneous overshoot of the voltage on the integrated circuit chip power supply, a decoupling capacitor is added to the integrated circuit chip, which can effectively remove the influence of the glitch on the power supply and reduce the power supply loop reflection on the PCB. The decoupling capacitor can bypass the high frequency noise of the integrated circuit device, and can also be used as a storage capacitor to provide and absorb the charge and discharge energy of the integrated circuit switch gate.

In the DSP system, decoupling capacitors are placed on each integrated circuit, such as DSP, SRAM, Flash, etc., and are added between each power supply and ground of the chip, and special attention should be paid to the decoupling capacitor as close as possible to the power supply end (source) ) and the part pin of the IC. Ensure the purity of the current from the power supply (sotlrce) and into the IC, and try to shorten the path of the noise. As shown in Figure 2, when processing capacitors, use large vias or multiple vias, and the wiring between vias and capacitors should be as short and thick as possible. When the distance between the two vias is too long, because the path is too large, it is not good; the best is that the closer the two vias of the decoupling capacitor are, the better the noise can be to the ground with the shortest path.

It is also advantageous to add high frequency capacitors at the power input or from the battery. In general, the value of the decoupling capacitor is not very strict. Generally, it is calculated as C=1/f, that is, a capacitor of 0.1 μF is taken at a frequency of 10 MHz.

(5) Power supply layout

Power supplies need to be carefully considered when developing DSP systems. Because some power chips generate a lot of heat, they should be placed in a position that is good for heat dissipation, and should be separated from other components by a certain distance. Heat dissipation can be done by adding a heat sink or by laying copper under the device. Be careful not to place hot components on the bottom of the development board.

(6) Other attention

For the layout of other components of the DSP system, the requirements of convenient welding, convenient debugging and aesthetics should be considered as much as possible. For example, adjustable devices such as potentiometers, adjustable inductors, variable capacitors, and DIP switches should be placed in combination with the overall structure. For devices over 15 g, it is necessary to add a fixing bracket and then solder, paying special attention to the position of the positioning hole of the PCB and the position of the fixing bracket. The distance between the components on the edge of the PCB is generally less than 2 mm from the edge of the PCB. The PCB is preferably rectangular with an aspect ratio of 3:2 or 4;