NEO-M8U - Hardware Integration Manual
UBX-15016700 - R07
Hardware description
Page 8 of 28
Production Information
1.4.5
TX Ready signal
The TX Ready signal indicates that the receiver has data to transmit. A listener can wait on the TX
Ready signal instead of polling the DDC or SPI interfaces. The UBX-CFG-PRT message lets you
configure the polarity and the number of bytes in the buffer before the TX Ready signal goes active.
The TX Ready signal can be mapped to UART
TXD
(PIO 06). The TX Ready function is disabled by
default.
☞
The TX Ready functionality can be enabled and configured by AT commands sent to the u-blox
cellular module supporting the feature. For more information, see the
GPS Implementation and
Aiding Features in u-blox wireless modules [6].
1.5
I/O pins
RESET_N: Reset input
Driving
RESET_N
low activates a hardware reset of the system. Use this pin only to reset the module.
Do not use
RESET_N
to turn the module on and off, since the reset state increases power
consumption, or as a POR. With the NEO-M8U module the
RESET_N
pin
is an input only.
D_SEL: Interface select
The
D_SEL
pin selects the available interfaces. SPI cannot be used simultaneously with the
UART/DDC. If open, UART and DDC are available. If pulled low, the SPI interface is available. More
information see the
NEO-M8U Data Sheet [1].
LNA_EN: LNA enable
In Power Save Mode, the system can turn on/off an optional external LNA using the LNA_EN signal in
order to optimize power consumption.
TIMEPULSE
A configurable time pulse signal is available with the NEO-M8U module. It generates pulse trains
synchronized with GPS or UTC time grid with intervals configurable over a wide frequency range. The
time pulse signal is disabled by default. For more information, see the
u-blox 8 / u-blox M8 Receiver
Description Including Protocol Specification [2].
☞
The NEO-M8U time-pulse output is configured using messages for “TIMEPULSE2”.
☞
The time-pulse output must
not
be held LOW during start-up.
1.5.1
Electromagnetic interference on I/O lines
Any I/O signal line with a length greater than approximately 3 mm can act as an antenna and may pick
up arbitrary RF signals transferring them as noise into the GNSS receiver. This specifically applies to
unshielded lines, in which the corresponding GND layer is remote or missing entirely, and lines close
to the edges of the printed circuit board.
If, for example, a cellular signal radiates into an unshielded high-impedance line, it is possible to
generate noise in the order of volts and not only distort receiver operation but also damage it
permanently.
On the other hand, noise generated at the I/O pins will emit from unshielded I/O lines. Receiver
performance may be degraded when this noise is coupled into the GNSS antenna (see Figure 15).
To avoid interference by improperly shielded lines, it is recommended to use resistors (e.g. R>20
Ω
),
ferrite beads (e.g. BLM15HD102SN1) or inductors (e.g. LQG15HS47NJ02) on the I/O lines in series.
These components should be chosen with care because they will affect also the signal rise times.
