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USB PCB design is a fast, bidirectional, synchronous transmission, cheap, easy to use, hot-swappable serial interface. USB PCB design devices are widely used due to their fast data transmission, convenient interface, and hot-swappable support. At present, most products on the market use USB2.0 as the interface, but many hardware novices encounter many troubles in USB applications, and often various problems occur in the USB interface after the PCB is assembled.
For example, if the communication is unstable or unable to communicate, and there are no problems with the schematic diagram and welding, perhaps at this time you need to suspect that the PCB design is unreasonable. Drawing a PCB that meets the USB2.0 data transmission requirements plays an extremely important role in the performance and reliability of the product.
The USB protocol defines that digital signals are transmitted by two differential signal lines (D+, D-). If the USB device is to work stably, the differential signal lines must be laid out and wired strictly according to the rules of differential signals. Based on the author's many years of experience in designing and debugging USB-related products, the following points are summarized:
1. When laying out components, try to make the differential lines as short as possible to shorten the differential line routing distance (√ is a reasonable method, × is an unreasonable method);
Shortest differential line
2. Draw differential lines first. Try not to have more than two pairs of vias on a pair of differential lines (vias will increase the parasitic inductance of the line, thereby affecting the signal integrity of the line), and they need to be placed symmetrically (√ is a reasonable way, × is an unreasonable way);
Draw differential lines first
3. Symmetrical parallel routing can ensure that the two lines are tightly coupled, avoid 90° routing, arc or 45° are both better routing methods (√ is a reasonable way, × is an unreasonable way);
Differential series capacitance
4. Placement of differential series resistors and capacitors, test points, pull-up and pull-down resistors (√ is a reasonable way, × is an unreasonable way);
Line length matching
5. Due to factors such as pin distribution, vias, and routing space, the differential line length is easy to mismatch. Once the line length does not match, the timing will be offset, and common mode interference will be introduced, reducing signal quality. Therefore, the mismatch of differential pairs should be compensated accordingly to match the line lengths. The length difference is usually controlled within 5mil. The compensation principle is to compensate for the length difference wherever it occurs;
Crosstalk
6. In order to reduce crosstalk, if space permits, the distance between other signal networks and ground and differential lines should be at least 20mil (20mil is an empirical value). If the distance between the ground and the differential line is too close, it will affect the impedance of the differential line;
VBUS
7. The output current of USB is 500mA. Pay attention to the line width of VBUS and GND. If 1Oz copper foil is used, the line width greater than 20mil can meet the current carrying requirements. Of course, the wider the line width, the better the integrity of the power supply.
The signal line width and line spacing of the differential line of ordinary USB devices are consistent with the signal line width and line spacing of the whole board. However, when the working speed of the USB device is 480 Mbits/s, it is not enough to just do the above points. We also need to control the impedance of the differential signal. Controlling the impedance of the differential signal line is very important for the integrity of high-speed digital signals.
Because the differential impedance affects the eye diagram, signal bandwidth, signal jitter and interference voltage on the signal line of the differential signal. The differential line impedance is generally controlled at 90 (±10%) ohms (refer to the chip manual for specific values). The differential line impedance is inversely proportional to the line width W1, W2, T1, inversely proportional to the dielectric constant Er1, proportional to the line spacing S1, and proportional to the distance H1 of the reference layer. The following figure is a cross-sectional view of the differential line.
The following figure is a reference stack of a four-layer board, in which the middle two layers are reference layers. The reference layer is usually GND or Power, and the reference layer corresponding to the differential line must be complete and cannot be divided, otherwise it will cause discontinuous differential line impedance. If a four-layer board is designed according to the stackup in Figure 2, the differential line is usually designed with a line width of 4.5 mil and a line spacing of 5.5 mil to meet the differential impedance of 90Ω.
However, the line width of 4.5 mil and the line spacing of 5.5 mil are only our theoretical design values. In the end, the circuit board factory will make appropriate adjustments to the line width, line spacing and distance to the reference layer based on the required impedance value and the actual production situation and the board material.
USB2.0
The wiring rules described above are based on USB2.0 devices. During the USB wiring process, keep the differential lines short, tightly coupled, of equal length, with consistent impedance, and pay attention to the current-carrying capacity of the USB power line. If you master the above principles, there will be basically no problem in the operation of USB devices.