ZOE-M8 series - Hardware integration manual
UBX-16030136 - R09
Design-in
Page 11 of 34
Production information
In case of improper shielding, it is recommended to use resistors or ferrite beads (see Appendix A.7)
on the I/O lines in series. Choose these components with care because they also affect the signal rise
times. Alternatively, feed-through capacitors with good GND connection close to the GNSS receiver
can be used (see Appendix A.8).
EMI protection measures are particularly useful when RF emitting devices are placed next to the
GNSS receiver and/or to minimize the risk of EMI degradation due to self-jamming. An adequate
layout with a robust grounding concept is essential in order to protect against EMI. For more
information, see section 2.14.6.3.
2.4
Real-time clock (RTC)
The use of the RTC is optional to maintain time in the event of power failure at VCC. It requires
V_BCKP to be supplied in case of power failure at VCC. The RTC is required for hot start, warm start,
AssistNow Autonomous, AssistNow Offline and in some power save mode operations.
The time information can either be generated by connecting an external RTC crystal to the SiP, by
connecting an external 32.768 kHz signal to the RTC input, or by time aiding of the GNSS receiver at
every start-up.
If a power save mode is used, an external RTC crystal must be connected. Optionally an external
32.768 kHz signal can be provided.
2.4.1
RTC using a crystal
The easiest way to provide time information to the receiver is to connect an RTC crystal to the
corresponding pins of the RTC oscillator, RTC_I and RTC_O. There is no need to add load capacitors
to the crystal for frequency tuning because they are already integrated in the chip. Using an RTC
crystal provides the lowest current consumption to V_BCKP in case of a power failure. On the other
hand, it increases the BOM costs and requires space for the RTC crystal.
Figure 3: RTC crystal
2.4.2
RTC using an external clock
Some applications can provide a suitable 32.768 kHz external reference to drive the SiP RTC. The
external reference can simply be connected to the RTC_I pin. Make sure that the 32.768 kHz reference
signal is always turned on and the voltage at the RTC_I pin does not exceed 350 mVpp. Adjustment
of the voltage level (typically 200 mVpp) can be achieved with a resistive voltage divider followed by a
DC blocking capacitor in the range of 1 nF to 10 nF. Also make sure that the frequency versus
temperature behavior of the external clock is within the recommended crystal specifications shown
in section A.1.
2.4.3
Time aiding
Time can also be sent by UBX message at every start-up of the ZOE-M8 GNSS SiPs. This can be done
to enable warm starts, AssistNow Autonomous and AssistNow Offline. This can be done when no RTC
is maintained.
To enable hot starts correctly, the time information must be known accurately and thus the
TimeMark feature must be used.
