ZOE-M8 series - Hardware Integration Manual
UBX-16030136 - R07 Production Information Design-in
Page 11 of 32
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.
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 will provide the lowest current
consumption to V_BCKP in case of a power failure. On the other hand, it will increase 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 (typ. 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.
For more information about time aiding or timemark, see the u-blox 8 / u-blox M8 Receiver Description Including
Protocol Specification [2].
For information of this use case, it is mandatory to contact u-blox support team.
For Power Save Mode operations where the RTC is needed, the time aiding cannot be used. This is because
the host does not have any information about when the ZOE-M8 GNSS SiPs turns from OFF status to ON
status during an ON/OFF operation of Power Save Mode.
2.5 RF input
The ZOE-M8 GNSS SiPs RF-input is already matched to 50 Ohms and has an internal DC block. The ZOE-M8 SiPs
are optimized to work with a passive antenna.
The ZOE-M8 GNSS SiPs can receive and track multiple GNSS systems (e.g. GPS, Galileo, GLONASS, BeiDou and
QZSS signals). Because of the dual-frequency RF front-end architecture, two GNSS signals (from: GPS L1C/A,
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