UBX-G7020 - Hardware Integration Manual
Design-in
GPS.G7-HW-10003
Objective Specification
Page 38 of 74
2.7
USB
The UBX-G7020 USB interface supports the full-speed data rate of 12 Mbit/s. It is compatible to USB 2.0 FS
standard. The interface requires some external components in order to implement the physical characteristics
required by the USB 2.0 specification. Figure 21 shows the interface pins and additional external components. In
order to comply with USB specifications, VBUS must be connected through a LDO (U2) to pin VDD_USB of the
UBX-G7020. This ensures that the internal 1.5k pull-up resistor on USB_DP gets disconnected when the USB
host shuts down VBUS.
Depending on the characteristics of the LDO (U2) it is recommended to add a pull-down resistor (R8) at its
output to ensure VDD_USB does not float if a USB cable is not connected, i.e. when VBUS is not present.
The interface can be used either in “self powered” or “bus powered” mode. The required mode can be
configured with the Low Level Configuration setting. Also, the default vendor ID, vendor string, product ID and
product string can be changed via Low Level Configuration settings and held permanently in external SQI flash
or internal eFuse, see section 2.10.2.
In order to get the 90 ohm differential impedance in between the USB_DM and USB_DP data line a 27ohm
series resistor (R1,R2) must be placed into each data line (USB_DM and USB_DP).
Figure 21: USB interface
Name
Component
Function
Comments
U2
LDO
Regulates VBUS (4.4 …5.25 V)
down to a voltage of 3.3 V).
Almost no current requirement (~1 mA) if the GPS receiver is operated as a USB
self-powered device, but if bus-powered LDO (U1) must be able to deliver the
maximum current of ~100 mA, depending if the DCDC converter option is
used.
C12,
C13
Capacitors
Required according to the specification of LDO U2
D1
Protection
diodes
Protect circuit from overvoltage
/ ESD when connecting.
Use low capacitance ESD protection such as ST Microelectronics USBLC6-2.
R1, R2 Serial
termination
resistors
Establish a full-speed driver
impedance of 28…44 Ohms
A value of 27 Ohms is recommended.
R8
Resistor
Ensures defined signal at
VDD_USB when VBUS is not
connected / powered
1k Ohms is recommended for USB self-powered setup. For bus-powered setup
R11 is not required.
Table 10: Summary of USB external components
See section A.7 and A.8 for reference schematics for self- and bus-powered operation.
If the USB interface is not used, connect VDD_USB to GND.
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