EVA-M8M - Hardware Integration Manual
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
The EVA-M8M 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
.
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 47 F2 D7 AD FF FF FC FF 2B 3D
Applying this sequence results in a permanent change and cannot be reversed.
2.4.3
RTC using an external clock
Some applications can provide a suitable 32.768 kHz external reference to drive the EVA-M8M 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.
2.4.4
Time aiding
Time can also be sent by UBX message at every startup of the EVA-M8M modules. 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
has to be used.
For more information about time aiding or timemark see the
u-blox M8 Receiver Description Including Protocol
Specification
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 EVA-M8M series turns from OFF status
to ON status during ON/OFF operation of Power Save Mode.
2.5
RF input
The EVA-M8M modules RF-input is already matched to 50 Ohms and has an internal DC block. To achieve the
performance values as written in the
EVA-M8M Data Sheet
[1], an active antenna with a good LNA inside or the
mandatory LNA with passive antenna in front of EVA-M8M (must have a noise figure below 1dB).
UBX-14006179 - R01
Advance Information
Design-in
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