Design of the differential signal line of the USB2.0 Interface

Address: http://www.cnblogs.com/jiansiming/archive/2011/10/29/2228459.html

 

[Switch] Design of differential signal line of USB2.0 Interface

Introduction
The general serial bus (Universal Serial Bus) has evolved from its birth to today, and the USB protocol has transitioned from 1.1 to 2.0. As an important indicator of its device transmission speed, it has gone from 1.5 Mbps; the low speed and the full speed of 12 Mbps increase to the current high speed of 480 Mbps. The USB interface is widely used for its advantages such as fast speed, low power consumption, plug-and-play support, and convenient installation. At present, there are more and more products on the market that use USB as the interface. Drawing PCB boards that meet the high-speed data transmission requirements of USB plays an extremely important role in product performance and reliability, and bring obvious economic benefits.

The USB interface is the preferred interface for many high-speed data transmission devices. practice shows that correct PCB design can give full play to the high-speed performance of USB master and slave devices. However, if the PCB is improperly designed, the transmission rate may not reach the expected goal, and even the high-speed USB device can only work at full speed.

The design of high-speed data transmission PCB is introduced below.

1. Design of differential signal line of USB2.0 Interface
The USB 480 Protocol defines that two differential signal lines (D and D-) are used to transmit high-speed digital signals. The highest transmission rate is Mbps.The differential voltage on the differential signal line is 400 mV.,The differential impedance (zdiff) is 90 (1 ± O. 1) Ω. When designing PCB, it is very important to control the differential impedance of the differential signal line for the integrity of high-speed digital signals, the difference impedance affects the eye view, signal bandwidth, signal jitter, and interference voltage on the signal line. The 2D model 1 of the differential line is shown in.

 

The difference line is drawn by two parallel cables on the PCB Surface (top layer or bottom layer) with edge coupling effect.Microstrip)The impedance is determined by the impedance and sum of the two micro-strip lines. The impedance (Zo) of the micro-strip line is composed of the width (w) of the micro-strip line and the copper thickness (t) of the micro-strip line) the distance from the microline to the nearest reference plane (H) and the dielectric constant (ER) of PCB materials are determined by the formula Zo = {87/SQRT (ER 1.41)]}. ln [5.98 H/(0.8 w t)]. The main parameters affecting the differential line impedance are the line spacing (s) between the strip line impedance and the two strip lines ).When the line spacing of the two micro-strip lines increases, the coupling effect of the difference line decreases, and the difference impedance increases. When the line spacing decreases, the coupling effect of the difference line increases and the difference impedance decreases.The formula for calculating the differential line impedance is zdiff = 2zo (1-0.48exp (-0.96 s/h )).The formula for calculating the microline and the differential line is true in the case of 0.1 <W/H <2.0 and 0.2 <S/H <3.0.To achieve ideal signal quality and transmission characteristics,High-speed USB equipment requires that the number of PCB layers be at least 4 layers. You can choose the top layer (signal layer), ground layer, power layer and bottom layer (signal Layer ). It is not recommended to use signal lines in the middle layer to avoid separating the integrity of the ground layer and power layer.The thickness of General PCB is 1.6
Mm,The distance from the differential line on the signal layer to the nearest reference plane is approximately 11mil.,The copper strip thickness t is approximately o.65mil.Filled material is generally FR-4, dielectric constant Er is 4.2. Under the conditions determined by h, T, and er, a proper line width W and line spacing s can be obtained from the 2D impedance model of the differential line and the formulas for calculating the line Impedance Between the microline and the differential line. Zdiff = 87 Ω when w = 16mil and S = 7mil. However, it is complicated to use the formula above to derive the proper dimension.PCB impedance control design software polarYou can easily get the appropriate results. Polar can get zdiff = 92.2 Ω when w = 11mil, S = 5mil.

Pay attention to the following requirements when drawing the Interface Differential line of the USB device:
① In component layout,The USB chip should be placed on the signal layer closest to the formation, and should be close to the USB socket as much as possible to shorten the distance between differential cables..
②Filter measures such as magnetic beads or capacitors should not be added to the differential line; otherwise, the impedance of the differential line will be seriously affected..

We can see that there is a USB differential online string resistance. Is this reasonable!

③If the USB interface chip needs to be connected to the serial port resistor or the D-wire is connected to the pulling resistor, make sure that these resistors are placed as close as possible to the chip..
④ ChangeThe USB differential signal line is distributed on the signal layer closest to the ground layer..
⑤Wiring of the USB differential line and other differential lines should be completed before other signal lines are drawn on the PCB..
⑥Maintains the bottom layer integrity of the USB differential line.If the formation at the lower end of the differential line is separated, the impedance of the differential line is discontinuous, and the influence of external noise on the differential line is increased.
7. During the cabling of the USB differential cableAvoid placing a pass (VIA) on the differential line), Through the hole will cause differential line impedance imbalance. If the wiring of the differential line must be completed by placing a hole, try to use a small hole and keep the USB differential line on a signal layer.
Distance ensures the consistency of the line spacing of the differential line during the Cabling Process. Shove can be used for drawing with Cadence, but special attention should be paid when drawing with Protel. IfThe gap of the differential line changes during the line process, which may cause the inconsistency of the differential line impedance..
During the process of drawing a differential line, use a 45 ° angle or an arc angle instead of a 90 ° angle, and try not to use other signal lines within the 150 mil range around the differential line, in particular, the steep edge of the digital signal line, pay attention to its line does not affect the USB differential line.
BytesThe differential line should be as long as possible. If the two lines have a large difference in length, you can draw a snake line to increase the short line length..

How can we achieve the same duration without changing the spacing? I can only walk one line when I walk the snake-like line, which means the distance is not enough. I am depressed.

 

2 Design of the power cord and ground wire at the Interface End of USB2.0 bus
The USB interface has five endpoints: USB power supply (vbus), D-, D, Gnd, and shield ). The above has already introduced how to design D and D-differential signals. Correct Design of USB bus power supply, signal location, and protection are equally important to the normal operation of USB system.

When the USB power cord voltage is 5 V and the maximum current is 500mA, the power cord should be arranged on the signal layer near the power supply layer, rather than on the same layer as the USB differential line, the line width should be above 30 mil to reduce its interference to the differential signal line.At present, many manufacturers use a USB slave control chip with a working voltage of 3.3 V. When it works in the bus power supply mode, 3.3 ~ For a 5 V power conversion chip, the output end of the Power Conversion Chip should be close to the voltage input end of the USB chip as much as possible, and the input and output end of the Power Conversion Chip should increase the capacity capacitor and small capacity capacitor for filtering. When the USB slave control chip works in self-powered mode, the USB power cord can be connected to a large resistor to the ground.

The signal location of the USB interface should be in good contact with the signal location on the PCB Board. The protected area can be placed on any layer of the PCB Board, which is separated from the signal location, A large resistor can be used between two places to parallel a capacitor with a high voltage value, as shown in figure 2.

 

The distance between the protected site and the signal ground should not be less than 25mil to reduce the edge coupling between the two places. Do not overwrite copper in a large area in the protected area. a mli copper foil wire can meet the functional requirements of the protected area..

Pay attention to the following points when drawing the USB power cord, signal location, and protected area:
① USB socket 1, 2, 3, 4 feet should be in the signal area of the surrounding area, rather than in the protected area of the surrounding area.
② The USB differential signal line and other signal lines should not overlap with the protection layer during cabling.
③ The power supply layer and signal layer should not overlap with the protection layer when copper is covered.
④The power supply layer is 20 d more than the signal layer, and D is the distance between the power supply layer and the signal layer..

⑤ If the signal of the layer where the differential line is located must be copper-coated in a large area, ensure that the spacing between the signal ground and the differential line is over 35 mil to avoid reducing the impedance of the differential line after copper-coated.
⑥ Some passing holes with the signal property can be placed in other signal layers to increase signal connectivity and shorten the signal current reflux path.
7. the power cord of the USB bus and PCB can be added with magnetic beads to increase the anti-interference capability of the power supply.

3. Topology Design of other signals of USB2.0
USB 480 provides a transmission rate of up to Mbps. Therefore, an external high-frequency crystal oscillator is required for the chip. For example, cypress CY7C68013 needs an external 24 MHz crystal oscillator. The crystal oscillator should be as close as possible to the clock input foot of the USB chip. The clock line cannot cross the USB differential line,Do not set any signal lines under the Crystal Oscillator,And a complete signal must be covered around the clock line to reduce the interference of the clock line to other signal lines.In particular, interference with the differential line. Ensure that the line spacing is not less than 8 mil when drawing the data line connecting the USB chip to other chips.

Conclusion
Based on EMC, EMI principles and signal integrity requirements, the PCB of the USB2.0 device can transmit at a rate of over 300 Mbps. The design of high-speed digital signal transmission PCB is a complex field. It has high requirements on designers and a long design period.