Integration Guide

LEON-G100/G200 - System Integration Manual
GSM.G1-HW-09002-F3 Preliminary System description
Page 30 of 101
1.5.5 RTC Supply (V_BCKP)
V_BCKP connects the Real Time Clock (RTC) supply, generated internally by a linear regulator integrated in the
module chipset. The output of this linear regulator is enabled when the main voltage supply providing the
module through VCC is within the valid operating range, or if the module is switched-off.
Name
Description
Remarks
V_BCKP
Real Time Clock supply
V_BCKP = 2.0 V (typical) generated by the module to supply
Real Time Clock when VCC supply voltage is within valid
operating range.
Table 9: Real Time Clock supply pin
V_BCKP pin ESD rating is 1 kV (contact discharge). A higher protection level could be required if the line
is externally accessible on the application board. A higher protection level can be achieved by mounting
an ESD protection (e.g. EPCOS CA05P4S14THSG varistor array) on the line connected to this pin if it is
externally accessible on the application board.
The RTC provides the time reference (date and time) of the module, also in power-off mode, since the RTC runs
when the V_BCKP voltage is within its valid range (specified in LEON-G100/G200 Data Sheet [1]). The RTC block
is able to provide programmable alarm functions by means of the internal 32.768 kHz clock.
The RTC block has very low, but highly temperature dependent power consumption. For example at 25°C and a
V_BCKP voltage of 2.0 V the power consumption is approximately 2 µA, whereas at 85°C and an equal voltage
it increases to 5 µA.
The RTC can be supplied from an external back-up battery through V_BCKP, when the main voltage supply is
not provided to the module through VCC. This enables the time reference (date and time) to run even when the
main supply is not provided to the module. The module cannot switch on if a valid voltage is not present on
VCC, even when RTC is supplied through V_BCKP (meaning that VCC is mandatory to switch-on the module).
If V_BCKP is left unconnected and the main voltage supply of the module is removed from VCC, the RTC is
supplied from the 1 µF buffer capacitor mounted inside the module. However, this capacitor is not able to
provide a long buffering time: within 0.5 seconds the voltage on V_BCKP will fall below the valid range (1 V
min).
If RTC is not required when VCC supply is removed, V_BCKP can be left floating on the application
board.
If RTC has to run for a time interval of T [seconds] at 25°C and VCC supply is removed, place a capacitor of
nominal capacitance of C [µF] at the V_BCKP pin. Choose the capacitor using the following formula:
C [µF] = (Current_Consumption [µA] x T [seconds]) / Voltage_Drop [V] = 2 x T [seconds]
The current consumption of the RTC is around 2 µA at 25°C, and the voltage drop is equal to 1 V (from the
V_BCKP typical value of 2.0 V to the valid range minimum limit of 1.0 V).
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 seconds 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 Instruments XH414H-IV01E) can be placed at V_BCKP, with a 4.7 k series resistor to hold the V_BCKP
voltage within its valid range for around 10 hours at 25°C, after the VCC supply is removed. The purpose of the
series resistor is to limit the capacitor charging current due to the big capacitor specifications, and also to let a
fast rise time of the voltage value at the V_BCKP pin after VCC supply has been provided. These capacitors will
allow the time reference to run during a disconnection of the VCC supply.