EVA-8M and EVA-M8 series - Hardware Integration Manual
UBX-16010593 - R06
Design-in
Page 15 of 47
Early Production Information
2.4
Real-Time Clock (RTC)
The use of the RTC is optional to maintain time in the event of power failure at
VCC_IO
. 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 module, by deriving the RTC from the internal crystal oscillator, by connecting an
external 32.768 kHz signal to the RTC input, or by time aiding of the GNSS receiver at every startup.
If a power save mode is used then an external RTC crystal must be connected. Optionally the RTC
frequency can be derived from the system clock or 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 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 4: RTC crystal
2.4.2
RTC derived from the system clock: “single crystal” feature
The crystal based EVA-8M / EVA-M8M GNSS modules can be configured in such way that the
reference frequency for the RTC is internally derived from the 26 MHz crystal oscillator. For this
feature
RTC_I
must be connected to ground and
RTC_O
left open. The capacity of the backup battery
at
V_BCKP
must be dimensioned accordingly, taking into account the higher than normal current
consumption at
V_BCKP
in the event of power failure at
VCC_IO
.
☞
Deriving RTC clock from internal oscillator is not available on TCXO based EVA-M8Q module.
☞
With EVA-8M / EVA-M8M modules the single crystal feature can be configured by sending the
following sequence to the receiver:
B5 62 06 41 0C 00 00 00 03 1F 06 C3 CC B4 FF FF FD FF B8 CF
☞
Applying this sequence results in a permanent change and cannot be reversed. An unstable supply
voltage at the
VCC_IO
pin while applying this sequence can also damage the receiver.
2.4.3
RTC using an external clock
Some applications can provide a suitable 32.768 kHz external reference to drive the module 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. Adjusting 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 the frequency versus temperature
behavior of the external clock is within the recommended crystal specification shown in section B.1.
