4 Circuit Design
Copyright © Fibocom Wireless Inc.
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4.4
Baseband Interface
4.4.1
USB
The module supports USB 2.0 and is compatible with USB High-Speed (480 Mbits/s) and
USB Full-Speed (12 Mbits/s). For details about the timing and electrical characteristics for
the USB interface, see
Universal Serial Bus Specification 2.0.
Schematic Diagram Design
Connector
VBUS
DM
DP
GND
USB_DM
USB_DP
USB_VBUS
1uF
Module
Figure 13. USB interface circuit design
Since the module supports USB 2.0 High-Speed, it is recommended to use TVS with a
capacitance of 0.5 pF on the USB_DM/DP differential signal line. It is recommended to
connect a 0 ohm resistor on each USB_DM/DP differential line to facilitate debugging.
USB enumeration requires an additional voltage to USB_VBUS. Otherwise, USB cannot be
enumerated normally. The voltage range is 3.3 V to 5.5 V.
PCB Design
USB_DP and USB_DM are high-speed differential signal lines that should be equal in
length and parallel to avoid right-angle route. The difference of cabling length is
controlled within
≤
2 mm, and the differential impedance is controlled at 90Ω±15%.
The USB data cable cannot be routed under the crystal, oscillator, magnetic device, or RF
signal. It is recommended to take an inner differential cable that is wrapped with copper
connected to the ground at all directions.
