Clock frequencies generate the main source of energy in a USB design. The USB differential DP/DM pairs operate in high-speed mode at 480 Mbps. 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 ~100 KHz. The USB socket pin 4 (analog 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 750 kHz (low speed).
- External crystal circuit (device terminals XI and X0): 12 MHz, 19.2 MHz, 24 MHz, and 48 MHz fundamental. When using an external crystal as a reference clock, a 24 MHz and higher crystal is highly recommended.