Installation Instructions

LISA-U2 series - System Integration Manual
UBX-13001118 - R19 Early Production Information System description
Page 32 of 175
1.5.4 RTC Supply (V_BCKP)
The V_BCKP pin connects the supply for the Real Time Clock (RTC) and Power-On / Reset internal logic. This
supply domain is internally generated by a linear regulator integrated in the Power Management Unit. The
output of this linear regulator is always enabled when the main voltage supply provided to the module through
VCC is within the valid operating range, with the module switched-off or powered-on.
V_BCKP supply output pin provides internal short circuit protection to limit start-up current and protect the
device in short circuit situations. No additional external short circuit protection is required.
Name
Description
Remarks
V_BCKP
Real Time Clock supply
V_BCKP output voltage = 1.8 V (typical)
Generated by the module to supply Real Time Clock when
VCC supply voltage is within valid operating range.
Table 12: Real Time Clock supply pin
The V_BCKP pin ESD sensitivity rating is 1 kV (Human Body Model according to JESD22-A114F). Higher
protection level could be required if the line is externally accessible on the application board. Higher
protection level can be achieved by mounting an ESD protection (e.g. EPCOS CA05P4S14THSG varistor
array) on the line connected to this pin, close to accessible point.
The RTC provides the time reference (date and time) of the module, also in power-off mode, when the V_BCKP
voltage is within its valid range (specified in the Input characteristics of Supply/Power pins table in LISA-U2 series
Data Sheet [1]). The RTC timing is normally used to set the wake-up interval during idle-mode periods between
network paging, but is able to provide programmable alarm functions by means of the internal 32.768 kHz
clock.
The RTC can be supplied from an external back-up battery through the V_BCKP, when the main voltage supply
is not provided to the module through VCC. This lets the time reference (date and time) run until the V_BCKP
voltage is within its valid range, even when the main supply is not provided to the module.
The RTC oscillator does not necessarily stop operation (i.e. the RTC counting does not necessarily stop) when
V_BCKP voltage value drops below the specified operating range minimum limit (1.00 V): the RTC value read
after a system restart could be not reliable, as explained in Table 13.
V_BCKP voltage value
RTC value reliability
Notes
1.00 V < V_BCKP < 1.90 V
RTC oscillator does not stop operation
RTC value read after a restart of the system is reliable
V_BCKP within operating range
0.05 V < V_BCKP < 1.00 V
RTC oscillator does not necessarily stop operation
RTC value read after a restart of the system is not reliable
V_BCKP below operating range
0.00 V < V_BCKP < 0.05 V
RTC oscillator stops operation
RTC value read after a restart of the system is reliable
V_BCKP below operating range
Table 13: RTC value reliability as function of V_BCKP voltage value
Consider that the module cannot switch on if a valid voltage is not present on VCC even when the RTC is
supplied through V_BCKP (meaning that VCC is mandatory to switch-on the module).
The RTC has very low power consumption, but is highly temperature dependent. For example at 25°C, with the
V_BCKP voltage equal to the typical output value, the power consumption is approximately 2 µA (see the Input
characteristics of Supply/Power pins table in the LISA-U2 series Data Sheet [1] for the detailed specification),
whereas at 70°C and an equal voltage the power consumption increases to 5-10 µA.