LISA-U2 series - System integration manual
UBX-13001118 - R27
System description
Page 33 of 183
C1-Public
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 levels could be required if the line is externally accessible on the application
board. Higher protection levels 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 the supply/power
pins table in the LISA-U2 series data sheet
). 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 the
V_BCKP
voltage value drops below the specified operating range minimum limit (1 V):
the RTC value read after a system restart might be not reliable, as explained in
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
