WM620 Hardware User Guide
V1.1
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Page 5
Figure 2-2b
2.1.2 VRTC Interface
VRTC Pin is used as an analog input from the 3 V coin cell for SMPL(Sudden Momentary Power Loss),
RTC(Real-time Clock), and crystal oscillator to keep alive power when the module is powered off. A capacitor
(rather than a coin cell) can be used if only SMPL is supported (not RTC or XTAL).
Figure 2-3
VRTC pin is also used as an analog output to charge a coin cell or a capacitor. When supply current is sourced
from the main power supply through VBAT pin. The on-chip coin cell charger is implemented using a voltage
regulator and series resistor.
If the monitored VBAT drops out-of-range (< 2.55V nominal), the SMPL feature initiates a power-on sequence
without software intervention, and then VBAT returns in-range within a programmable interval of between 0.5
and 2.0 seconds. SMPL achieves immediately and automatically recovery from momentary power loss. A valid
voltage on VRTC is required to run the SMPL timer.
If a capacitor is used instead of a coin cell, it must be connected between VRTC PIN and the ground, Figure
2-3 shows the reference RTC circuit. The capacitor must be charged to operate properly as the SMPL power
source. The capacitor value depends on the SMPL timer setting. Table 2-2 shows the capacitor value of VS
SMPL time.
