Sorry, your USB cable may be wrong!

USB It's fast 、 two-way 、 Synchronous transmission 、 Cheapness 、 Easy to use hot pluggable serial interface .

Due to fast data transmission , The interface is convenient , Support hot plug and other advantages make USB Equipment is widely used . at present , In the market USB2.0 Most of the products are interfaces , But many hardware novices are USB There are many problems in the application , Often PCB After assembly USB There are various problems with the interface .

For example, communication is unstable or unable to communicate , Check that the schematic diagram and welding are all ok , Maybe it's time to doubt PCB The design is unreasonable .

Draw satisfaction USB2.0 Data transmission requirements PCB It plays a very important role in the performance and reliability of products .

USB The protocol is defined by two differential signal lines （D+、D-） Transmit digital signals , If you want to USB If the equipment works stably, the differential signal line must be arranged and wired in strict accordance with the rules of differential signal .

According to the author for many years USB Relevant product design and commissioning experience , Summarize the following points for attention ：

1. During component layout , Try to make the differential line as short as possible , To shorten the routing distance of the differential line （√ In a reasonable way ,× In an unreasonable way ）.

2. Draw the difference line first , Try not to exceed two pairs of vias on a pair of differential lines （ The via will increase the parasitic inductance of the line , Thus affecting the signal integrity of the line ）, It shall be placed symmetrically （√ In a reasonable way ,× In an unreasonable way ）.

3. Symmetrical parallel routing , This ensures that the two wires are tightly coupled , avoid 90° Routing , Arc or 45° Are better routing methods （√ In a reasonable way ,× In an unreasonable way ）.

4. Differential series resistance capacitance , Test point , Placement of pull-down resistors （√ In a reasonable way ,× In an unreasonable way ）.

5. Because of the pin distribution 、 Through hole 、 And the existence of routing space and other factors make the difference line length easy to mismatch , And once the line length doesn't match , The timing will shift , Common mode interference will also be introduced , Reduce signal quality .

therefore , Accordingly, it is necessary to compensate for the mismatch of differential pairs , Match the line length , The length difference is usually controlled at 5mil within , The compensation principle is where the length difference occurs .

6. To reduce crosstalk , Where space permits , The spacing between other signal networks and ground separation differential lines shall be at least 20mil（20mil It's experience ）, The distance between the ground cover and the differential line is too close, which will affect the impedance of the differential line .

7. USB The output current of is 500mA, Attention should be paid to VBUS And GND The line width of , If you use 1Oz Copper foil , The line width is greater than 20mil It can meet the current carrying requirements , Of course, the wider the linewidth, the better the integrity of the power supply .

Ordinary USB Equipment differential line signal line width , And the line spacing is consistent with the signal line width and line spacing of the whole board .

However , When USB The operating speed of the equipment is 480 Mbits/s, It is not enough to do only the above , We also need to control the impedance of the differential signal , Controlling the impedance of differential signal line is very important for the integrity of high-speed digital signal .

Because the differential impedance affects the eye diagram of the differential signal 、 Signal bandwidth 、 Signal jitter and interference voltage on the signal line .

The differential line impedance is generally controlled at 90（±10%） ohm ( Refer to the chip manual for specific values ), Differential line impedance and linewidth W1、W2、T1 In inverse proportion , And dielectric constant Er1 In inverse proportion , Distance from line S1 In direct proportion to , Distance from reference layer H1 Proportional , The following figure is a cross-sectional view of the difference line .

The following figure shows the reference stack of four ply plates , The middle two layers are the reference layer , The reference layer is usually GND or Power, And the reference layer corresponding to the difference line must be complete , Can't be divided , Otherwise, the differential line impedance will be discontinuous .

If the following figure is a laminated design of four layers of plates , Usually, the design TDOA branch line adopts 4.5mil Linewidth and 5.5mil The line spacing can meet the differential impedance 90Ω.

However ,4.5mil Linewidth and 5.5mil The line spacing is only our theoretical design value , Finally, the circuit board factory will properly adjust the line width, line spacing and distance to the reference layer according to the required impedance value and in combination with the actual production situation and plate .

The routing rules described above are based on USB2.0 equipment , stay USB In the process of wiring, grasp the shortest differential line 、 Tightly coupled 、 Equal length 、 The impedance is consistent and pay attention to USB Current carrying capacity of power cord , Master the above principles USB The operation of the equipment is basically OK .

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