Integration Manual

LISA-U series - System Integration Manual
3G.G2-HW-10002-A3 Preliminary System description
Page 33 of 160
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-U1 series
Data Sheet [1] and LISA-U2 series Data Sheet [2]). 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 doesn't necessarily stop operation (i.e. the RTC counting doesn't 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 the following Table 12.
V_BCKP voltage value
RTC value reliability
Notes
1.00 V < V_BCKP < 1.90 V (LISA-U2 series)
1.00 V < V_BCKP < 2.50 V (LISA-U1 series)
RTC oscillator doesn't 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 doesn't 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 12: 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 (refer to the
Input characteristics of Supply/Power pins table in the LISA-U1 series Data Sheet [1] and in the LISA-U2 series
Data Sheet [2] for the detailed specification), whereas at 70°C and an equal voltage the power consumption
increases to 5-10 µA.
The internal regulator for V_BCKP is optimized for low leakage current and very light loads. It is not
recommended to use V_BCKP to supply external loads.
If V_BCKP is left unconnected and the module main voltage supply is removed from VCC, the RTC is supplied
from the bypass capacitor mounted inside the module. However, this capacitor is not able to provide a long
buffering time: within few milliseconds the voltage on V_BCKP will go below the valid range (1 V min). This has
no impact on wireless connectivity, as all the functionalities of the module do not rely on date and time setting.
Leave V_BCKP unconnected if the RTC is not required when the VCC supply is removed. The date and
time will not be updated when VCC is disconnected. If VCC is always supplied, then the internal
regulator is supplied from the main supply and there is no need for an external component on V_BCKP.
If RTC is required to run for a time interval of T [s] at 25°C when VCC supply is removed, place a capacitor with a
nominal capacitance of C [µF] at the V_BCKP pin. Choose the capacitor using the following formula:
C [µF] = (Current_Consumption [µA] x T [s]) / Voltage_Drop [V]
= 1.92 x T [s] for LISA-U1 series
= 2.50 x T [s] for LISA-U2 series
For example, a 100 µF capacitor (such as the Murata GRM43SR60J107M) can be placed at V_BCKP to provide a
long buffering time. This capacitor will hold V_BCKP voltage within its valid range for around 50 s at 25°C, after
the VCC supply is removed. If a very long buffering time is required, a 70 mF super-capacitor (e.g. Seiko