Integration Manual

LISA-U2 series - System Integration Manual
UBX-13001118 - R25 System description Page 35 of 182
If longer buffering time is required to allow the time reference to run during a disconnection of the
VCC supply, then an external battery can be connected to V_BCKP pin. The battery should be able to
provide a proper nominal voltage and must never exceed the maximum operating voltage for V_BCKP
(specified in the input characteristics of the supply/power pins table in the LISA-U2 series Data
Sheet [1]). The connection of the battery to V_BCKP should be done with a suitable series resistor for
a rechargeable battery, or with an appropriate series diode for a non-rechargeable battery. The
purpose of the series resistor is to limit the battery charging current due to the battery specifications,
and also to allow a fast rise time of the voltage value at the V_BCKP pin after the VCC supply has been
provided. The purpose of the series diode is to avoid a current flow from the module V_BCKP pin to
the non-rechargeable battery.
Combining a LISA-U2 series cellular module with a u-blox GNSS receiver, the VCC supply of the GNSS
receiver is controlled by the cellular module by means of the “GNSS supply enable” function provided
by the GPIO2 of the cellular module. In this case the V_BCKP supply output of the LISA-U2 series
cellular module can be connected to the V_BCKP backup supply input pin of the GNSS receiver to
provide the supply for the GNSS real time clock and backup RAM when the VCC supply of the cellular
module is within its operating range and the VCC supply of the GNSS receiver is disabled. This enables
the u-blox GNSS receiver to recover from a power breakdown with either a hot start or a warm start
(depending on the duration of the GNSS VCC outage) and to maintain the configuration settings
saved in the backup RAM. See the section 1.10 for more details regarding the application circuit with
a u-blox GNSS receiver.