UBX-G7020 - Hardware Integration Manual
Design-in
GPS.G7-HW-10003
Objective Specification
Page 30 of 74
2.5
Real-Time Clock (RTC)
The RTC section is located in the backup domain of the UBX-G7020. It is used to maintain time in the event of
power failure at main supply, VDD_IO. The RTC is required for Hotstart, Warmstart, AssistNow Autonomous,
AssistNow Offline and in some Power Save Mode operations.
The time information can either be generated by connecting an external crystal to the RTC oscillator, by deriving
the RTC from the system clock, by connecting an external 32.768 kHz signal to the RTC input or by time aiding
of the GPS receiver at every startup.
2.5.1
RTC using a crystal
The easiest way to provide time information to the receiver is to connect a RTC crystal to the corresponding pins
of the RTC oscillator, RTC_I and RTC_O. There is no need for adding load capacitors to the crystal for frequency
tuning, because they are already integrated into the UBX-G7020. Using a RTC crystal will provide lowest current
consumption into V_BCKP in case a power failure. On the other hand it will increase the BOM costs and requires
space for the RTC crystal.
Figure 13: RTC crystal
2.5.2
RTC derived from the system clock; “Single Crystal” Feature
The UBX-G7020 can be configured by Low Level Configuration, section 2.10.2, in such way that the reference
frequency for the RTC is internally derived from the TCXO or XTO system clock frequency (26MHz). This feature
is called “single crystal” operation. In addition, the TCXO or XTO (see section 2.4) will be supplied by the backup
battery at V_BCKP in the event of a power failure at VDD_IO. Consequently, the power consumption into
V_BCKP is higher compared to the usage of an ordinary RTC crystal. On the other hand it requires less space and
saves cost, owing to a smaller BOM.
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 account of the higher than normal current consumption at
V_BCKP in the event of power failure at VDD_IO.
2.5.3
RTC using an external clock
Some applications can provide a suitable 32.768 kHz external reference to drive the UBX-G7020 RTC. The
external reference can simply be connected to the RTC_I pin. Take care that the 32.768 kHz reference signal is
always on and the input voltage is within the RTC_I specification. Adjusting the level can be achieved with a
voltage divider. Also make sure the frequency versus temperature behavior of the external clock is within the
recommended crystal specification in section 3.3.
Make sure the voltage levels are within datasheet values; see
UBX-G7020-Kx Data Sheet
[1] or
UBX-
2.5.4
Time aiding
Time can also be sent by UBX message at every startup of the UBX-G7020. This can be done to enable
warmstart, AssistNow Autonomous and AssistNow Offline. This can be done when no RTC is maintained.
To enable Hotstarts correctly, the time information must be known accurately and thus the TimeMark feature
has to be used.
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