The USB physical interface

USB 1.0 and USB 1.1 support low-speed (1.5Mb/s) and full-speed (12Mb/s) USB transfer rates. USB 2.0 supports a high-speed (480 Mb/s) USB transfer rate and is backward compatible with USB 1.1. USB On-The-Go adds a connectible feather for USB peripherals on USB 2.0. Wireless USB unwires the high-speed USB and provides the functionality of wired USB without the burden of cables. In Table 1, the key features of the USB physical interface are listed. Note that unwired or high speed is not always ‘GOOD’. Losing the cable also means losing a source of power for peripherals. Wireless USB is not available for bus-powered devices. High-speed means higher current consumption and higher cost. It is not necessary to select high-speed if the bandwidth of full speed or low speed is enough for your application.

Selecting the USB cable and receptacle

Before you start your hardware design, first you will need to make a choice of the USB cable and receptacle based on the size requirement of your application. Figure 1 and Table 2 can help you select the proper USB cable and receptacle. Note that if you want to minimize the size of your product, choose the mini size receptacle for your design.

Design guidelines for USB physical interface schematics

Once you select the cable and the receptacle for your design, you then need to determine the USB controller based on the bandwidth, data bus, USB class type, current consumption and cost requirement of your application. Once this is done, you can start to design the schematics of your application based on the following guidelines.

· Follow the data sheet and the reference design of the USB controller.This is very important since some USB chips can only conditionally achieve the design requirements. Normally though, the reference design provided in the USB controller datasheet is golden and can pass all USB compliance tests

· Check the capacitance and resistance limitations between VBUS and GND For the USB upstream port, you need to ensure that the capacitive load of VBUS is more than 1.0 μF and less than 10 μF(1.0 to 6.5 μF for a dual-role device). For the USB downstream port, the VBUS power line must be bypassed with no less than 120 μF of low-ESR capacitance. Please note that the capacitance value is the total equivalent value in the circuit. The capacitor placed in the design should be less than this maximum value and more than the minimum value. The resistance between VBUS and GND should not be less than 40 kohms (40 to 100 kohms for a dual-role device) in USB suspend or idle state (see Figure 2). Also, make sure that the pull-down resistor of VBUS is not less than 40 kohms (the total equivalent value in the circuit).

· Grounding
USB specifications mention that the shield should be connected to GND at the host side, as in Figure 3, and that no USB device should connect Shield to GND. Actually though, in some designs, connecting the shield of the USB device to GND as in Figure 4 can result in better Eye Pattern. Therefore, use a 0-ohm resister to short the Shield to GND. If it is not necessary when you do USB test, you can easily remove it from the PCB board.

· Signal integrity on the D+ and DSignal
integrity is affected by the impedance on the D+ and D- line. Your design must meet the USB loading specifications. The value of the serial resistor (R1/R2) should exactly follow the data sheet of the USB controller.

· ESD (Electro-Static Discharge) and EMI (Electro-Magnetic Interference)
EMI control and ESD protection circuits are generally not required, although the datasheet mentions that it is absolutely necessary. Therefore, carefully select suitable components for EMI control and ESD protection. Note that the EMI control and ESD protection circuits will affect the impedance on D+ and D-. Redo the USB test as necessary if the EMI control and ESD protection circuits are modified. In Figure 5, the USB device is self powered from the CON3 connecter, and its power supply can be switched by VBUS. Once the VBUS is lost, the power supply will be switched off. This reference design can fully overcome the ‘backup voltage’ failing problem of the selfpowered USB device.

In Figure 6, this application can work in USB, Audio or UART mode and the USB connecter can transfer USB, audio or UART signals. The typical applications include an MP3 player or mobile phone with only one external interface (USB mini port). Users can connect it to a charger, USB host port or headphone/microphone/speaker via the same USB mini port.

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