EVA-M8M - Hardware Integration Manual
UBX-14006179 - R02
Early Production Information
Design-in
Page 11 of 45
2.2.3
SPI Interface
The SPI interface can be used to provide a serial communication with a host CPU. If the SPI interface is used,
UART and DDC are deactivated, because they share the same pins.
To make use of the SPI interface, the
D_SEL
pin has to be connected to GND.
2.2.4
USB interface
The USB interface of the EVA-M8M modules support the full-speed data rate of 12
Mbit/s. It is compatible to the
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 2 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
V_USB
of the EVA-M8M series. This ensures that the internal 1.5
k
Ω
pull-up resistor on
USB_DP
gets
disconnected when the USB host shuts down VBUS.
Depending on the characteristics of the LDO (U2), for a self-powered design it is recommended to add a pull-
down resistor (R8) at its output to ensure
V_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 using the UBX-CFG-USB message. Also, the vendor ID, vendor string, product ID and product string
can be changed.
In order to get the 90
Ω
differential impedance in between the
USB_DM
and
USB_DP
data line, a 27
Ω
series
resistor (R1, R2) must be placed into each data line (
USB_DM
and
USB_DP
).
Figure 2: 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 GNSS receiver is operated as a
USB self-powered device, but if bus-powered LDO (U2) must be able to deliver
the maximum current of ~100 mA.
C2,C3 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
Ω
A value of 27
Ω
is recommended.
R8
Resistor
Ensures defined signal at
V_USB
when VBUS is not
connected / powered
100 k
Ω
is recommended for USB self-powered setup. For bus-powered setup
R8 is not required.
Table 2: Summary of USB external components
See Appendix A.5 and Appendix A.6 for reference schematics for self- and bus-powered operation.
If the USB interface is not used, connect
V_USB
to GND.
