The third generation universal serial port (USB 3.0) inherits the convenience and diversity of USB, improves the transmission speed ten times in one fell swoop, and maintains the downward compatibility with existing USB devices. It is expected to be rapidly popularized in various applications such as personal computers, consumer electronics, communications and so on. The existing USB 2.0 cable and connector provide four signal lines, including VBUS 5V 500mA DC power supply, a pair of DP / DM half duplex bidirectional differential signal lines and GND grounding. These four lines provide 480mbps data transmission and DC power supply of USB 2.0. In order to provide data transmission rate up to 5Gbps, USB 3.0 adds five additional signal lines, including two pairs of unidirectional superspeed differential signals: sstx +, sstx -, ssrx +, ssrx -, and a group of grounding contacts, and improves the DC power supply capacity to 5V 900mA. Two pairs of differential signals are responsible for transmission and reception respectively, providing bidirectional full duplex 5Gbps transmission capacity.
Figure 1: USB 3.0 connector, USB 3.0 superspeed signal contact at the rear.
Figure 2: schematic diagram of USB 3.0 cable. The blue part is superspeed 5Gbps signal.
System design and measurement challenge of superspeed 5Gbps signal quality
The USB 3.0 superspeed bit transmission rate is up to 5Gbps, and the open interface is adopted. The signal quality will directly affect the device compatibility and transmission efficiency, which is closely related to the user experience. Main control chip, printed circuit board, connector, cable and even device chip set are the key factors affecting signal quality. The analog transmission design capability of chip manufacturers, the variation of semiconductor process and the printed circuit design layout of system manufacturers are all new tests. Different from the experience of USB 2.0, the reference index of signal quality will no longer be just the eye diagram of the transmission end. USB 3.0 has independent transmission and receiving channels, and the system receiving capacity has also become an important measurement index of signal quality and system design. The receiving end measurement mainly tests the time base error tolerance of the object to be measured to different frequencies. The chip with stronger tolerance means that its system design can be more relaxed, longer signal routing can be used, and better device compatibility.
Figure 3: eye diagram of signal measurement at USB 3.0 superspeed transmission end
Figure 4: measurement results received by USB 3.0. The horizontal axis is the frequency of the time base error, and the vertical axis is the amplitude of the time base error. Green dots indicate test benchmarks with bit error rates below 10-12. The black line is the test benchmark specification.
Extension design of motherboard and laptop
USB 2.0 is a very common front-end connection port, which connects the motherboard and the outer box of the motherboard in the way of internal connection, providing users with convenient hot plug applications. The USB 3.0 connection port is bound to be introduced into the design of the front board connection port. Additional connectors and internal connecting wires are an additional test for signal quality.
Figure 5: USB 3.0 internal connector pin definition.
In addition, in the design of laptop, flexible cables are widely used to connect different sub boards to match the lightweight appearance and mechanism design. The impedance control of flexible cable and the signal continuity of flexible cable connector are not as easy to control as printed circuit board and standard connector. These will be the design challenges of USB 3.0 interface in notebook computers in the future. How to import the system design such as front board and flexible cable without the additional cost of signal regulator (re driver) and maintain good 5Gbps signal quality has become an important component selection key.
Figure 6: USB 3.0 internal connector and cable
USB3.0 master control chip fl1009 of American business Ruisi technology integrates high-performance analog entity design to fully verify different system design requirements in response to the above different system design challenges. It includes a line layout length of up to 23 cm (9 inches), releases the limitations of the system design on the line layout, and allows additional wiring of USB3.0 on the front board as long as 45 cm without the cost of additional re driver, so that the system manufacturers and card manufacturers can create products with the highest performance and effectively reduce the design time and cost. In addition, fl1009 is also the world’s first superspeed USB master chip supporting xhci 1.0, providing super efficiency and excellent compatibility.