Hardware design of DSP system

- Dec 18, 2017-

DSP hardware design includes hardware design, DSP and peripheral device selection, schematic design, PCB design and simulation, hardware debugging and so on. The first part is about two parts of hardware design, DSP and peripheral device selection. We will talk about schematic design, PCB design and hardware debugging in detail.

1. System resource planning

One of the preconditions of hardware design is to plan the resources of the whole system, and finally get the resource allocation table of the system, that is, Memory Map. Table 1 provides an address mapping table for an image processing system designed with TMS320DM642.

Two. Design of hardware schematic diagram

DSP chip manufacturers usually provide corresponding EVM (evaluation board) reference schematic design when designing every DSP chip, you can download it through the Internet or buy the original EVM board. In detail, we will not elaborate on the principle of a board. According to the author's experience in DSP design for many years, we can summarize the skills in design.

When the hardware is designed, the following points should be paid attention to.

(1) clock circuit. The DSP clock can be provided by the outside and can be provided by the crystal oscillator on the board. However, the external clock input is often used in the DSP system, because the clock has high precision, good stability and convenient use when using the external clock. Because the DSP work is based on the clock, if the clock quality is not high, the reliability and stability of the system will be difficult to guarantee. Therefore, if the external clock is used, the stability and burr should be tested in a comprehensive way when the crystal oscillator is selected so that the DSP system can work reliably.

(2) reset circuit. It is necessary to design both the power reset circuit and the artificial reset circuit at the same time. It is convenient to reset the system when the system runs out of fault. For reset circuit, on the one hand, we should ensure that the reset low level time is long enough (generally more than 20ms), and ensure reliable reset of DSP. On the other hand, we should ensure good stability and prevent DSP from being reset.

Three, high frequency PCB design

Digital devices are developing towards high speed, low consumption, small volume and high anti-interference. This trend brings many new requirements for printed circuit board design. Based on years of experience in hardware design, the author summarizes some techniques for high frequency wiring for reference.

(1) high frequency circuits often have high integration and large wiring density. Using multilayer boards is not only necessary for wiring, but also an effective means to reduce interference.

(2) the less the lead bending between the pins of the high speed circuit devices, the better. The wiring of high frequency circuit is preferably straight line. It needs turning. It can be turned at 45 degree broken line or circular arc to meet this requirement, which can reduce the external emission and mutual coupling between high-frequency signals.

(3) the shorter the lead between the pins of the high frequency circuit, the better.

(4) the lower the interlayer of the lead between the pins of the high frequency circuit, the better the better. The so-called "less interlayer alternation" is the less the better the Via used in the connection process. According to the measurement, a through hole can bring about 0.5 pF distributed capacitance and reduce the number of through-hole, which can significantly improve the speed.

(5) high frequency circuit wiring should pay attention to the "cross interference" introduced by the signal line near distance parallel line. If we can not avoid parallel distribution, we can reduce the interference greatly by arranging a large area of ground on the opposite side of the parallel signal line. Parallel lines in the same layer can hardly be avoided, but in the two adjacent layers, the direction of the line must be taken perpendicular to each other.

(6) the measures to enclose the ground line of a particularly important signal line or local unit, that is, to draw the outer profile of the selected object. By using this function, we can automatically handle the so-called "packet handling" of the selected important signal lines. Of course, it is also very useful for the high-speed system to use this function locally for the clock and other units.

(7) all kinds of signals can not form a loop, and the ground line can not form a current loop.

(8) a high frequency decoupling capacitor should be set near each integrated circuit block.

(9) the high frequency choke link should be used when the analog ground line and the digital ground line are connected to the common ground line. In the actual assembly of high frequency choke link is often used in high frequency ferrite bead center hole wear of the wire, in the circuit principle diagram of it are generally not expressed, thus formed the network table (netlist) does not contain this element, wiring it and ignore it. For this reality, it can be used as an inductor in the schematic diagram, and a component package is defined for it in the PCB component library. It is moved to the appropriate location near the common ground junction before wiring.

(10) the analog circuit and the digital circuit should be arranged separately. The power supply and the ground should be connected by a single point after the independent wiring, and the mutual interference should be avoided.

(11) before DSP, extra program memory and data memory are connected to the power supply, we should add the filter capacitor and make it close to the chip power pin as far as possible, so as to filter the power noise. In addition, shielding is suggested around key parts such as DSP and out of chip memory and data storage, which can reduce external interference.

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