Frequently Asked Questions for USB Interface Circuit Design

USB is a fast, two-way, synchronous, low-cost, easy-to-use, hot-swappable serial interface. With advantages of fast data transfer, user-friendly interface, and hot-plug support, USB devices have become widespread. At present, most of products on market use USB2.0 as interface, but many newcomers to equipment encounter many problems in USB applications, and there are often various problems with USB interface after PCB is assembled.

For example, if connection is unstable or cannot communicate, check circuit and soldering, and there is no problem. It may be necessary to suspect at this time that circuit board design is unreasonable. Rendering a PCB that meets USB 2.0 data transfer requirements plays an extremely important role in product performance and reliability.

The USB protocol specifies that two differential signal lines (D+, D-) carry digital signals. For a USB device to work stably, differential signal line must be laid in strict accordance with differential signal rules. According to author's many years of experience in developing and debugging USB related products, following points can be summarized:

1. When placing components, try to make differential line as short as possible to reduce distance of laying differential line (√ is a reasonable way, × is an unreasonable way);

2. Prioritize drawing differential lines, try not to exceed two pairs of vias on a pair of differential lines (vias will increase parasitic inductance of line, which will affect signal integrity of line), and symmetrical placement is necessary ( √ means reasonable way, × means unreasonable way);

3. Symmetrical parallel wiring can ensure two wires are tightly connected and avoid 90° wiring. The best laying methods are arc or 45° (√ is a reasonable method, × is an unreasonable method);

4. Differential series resistance capacitance, test points, pull-up and pull-up resistor placement (√ reasonable, × impractical);

5.Due to factors such as pin distribution, vias, and routing space, differential line lengths do not easily match, and if line lengths do not match, timing will shift and introduce common mode noise and degrade signal quality. Therefore, it is necessary to compensate for mismatch of differential pair so that lengths of lines match. The difference in length is usually controlled within 5 mils. The compensation principle is where difference in length occurs;

6. To reduce crosstalk, if space permits, distance between other signal networks and ground should be at least 20 mils from differential line (20 mils is a rule of thumb) and distance between ground and differential line must be too large. This will affect differential line impedance;

7.USB output current is 500 mA. Pay attention to width of VBUS and GND lines. If 1 ounce copper foil is used, a line width over 20 mils can meet conductive requirement. Of course, line width The wider power supply, better integrity.

The signal line width and line spacing of differential line of conventional USB devices must match signal line width and line spacing of entire board. However, when operating speed of USB device is 480Mbps, it is not enough just to achieve above points. We also need to control differential signal impedance. It is very important to control differential signal line impedance. for high speed digital signal integrity.

Because differential impedance affects eye diagram, signal bandwidth, signal jitter, and interference voltage on differential signal signal line. Line differential impedance is typically adjusted to 90 (±10%) ohms (see chip manual for specific values). The differential line impedance is inversely proportional to line width W1, W2 and T1 and inversely proportional to permittivity. Er1, and proportional to line spacing S1., which is proportional to distance H1 to reference layer. The following figure shows a cross section of a differential line.

The figure below shows a reference stack of a four-layer board. The two middle layers are reference layers. The reference layer is usually GND or Power, and reference layer corresponding to differential line must be full and cannot be split, otherwise differential line will be damaged. If a four-layer board is constructed as shown in Fig. 2, width of differential line and line spacing of 5.5 mils can meet differential impedance of 90 ohms.

However, 4.5 mil line width and 5.5 mil line spacing are just our theoretical values. required impedance value, combined with actual production situation and board adjustment.

The connection rules described above are based on USB 2.0 devices. In USB connection process, use shortest differential lines, tight connection, same length and constant impedance, and pay attention to USB bandwidth. Power line Become familiar with principles of USB as described above. The device operates mostly normally.