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USB 2.0 board design and layout guidelines

Posted: 01 Dec 2007     Print Version  Bookmark and Share

Keywords:Clock frequencies  USB design  USB board  differential DP/DM pairs 

Clock frequencies generate the main source of energy in a USB design. The USB differential DP/DM pairs operate in high-speed mode at 480Mbps. System clocks can operate at 12 MHz, 48 MHz, and 60 MHz.

The USB cable can behave as a monopole antenna; take care to prevent RF currents from coupling onto the cable.

When designing a USB board, the signals of most interest are:

� Device interface signals: Clocks and other signal/data lines that run between devices on the PCB.

� Power going into and out of the cable: The USB connector socket pin 1 (VBUS ) may be heavily filtered and need only pass low frequency signals of less than ~100kHz. The USB socket pin 4 (analogue ground) must be able to return the current during data transmission, and must be filtered sparingly.

� Differential twisted pair signals going out on cable, DP and DM: Depending upon the data transfer rate, these device terminals can have signals with fundamental frequencies of 240 MHz (high speed), 6 MHz (full speed), and 750kHz (low speed).

� External crystal circuit (device terminals XI and X0): 12 MHz, 19.2 MHz, 24 MHz, and 48 MHz fundamental. When using

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