
DocID024597 Rev 5
153/1830
RM0351
Power control (PWR)
198
5.1.4 Independent
LCD
supply
The VLCD
pin is provided to control the contrast of the glass LCD. This pin can be used in
two ways:
•
It can receive from an external circuitry the desired maximum voltage that is provided
on segment and common lines to the glass LCD by the microcontroller.
•
It can also be used to connect an external capacitor that is used by the microcontroller
for its voltage step-up converter. This step-up converter is controlled by software to
provide the desired voltage to segment and common lines of the glass LCD.
The voltage provided to segment and common lines defines the contrast of the glass LCD
pixels. This contrast can be reduced when you configure the dead time between frames.
•
When an external power supply is provided to the VLCD
pin, it should range from 2.5 V
to 3.6 V. It does not depend on V
DD
.
•
When the LCD is based on the internal step-up converter, the VLCD
pin should be
connected to a capacitor (see the product datasheet for further information).
5.1.5 Battery
backup
domain
To retain the content of the Backup registers and supply the RTC function when V
DD
is
turned off, the VBAT
pin can be connected to an optional backup voltage supplied by a
battery or by another source.
The VBAT pin powers the RTC unit, the LSE oscillator and the PC13 to PC15 I/Os, allowing
the RTC to operate even when the main power supply is turned off. The switch to the V
BAT
supply is controlled by the power-down reset embedded in the Reset block.
Warning:
During
t
RSTTEMPO
(temporization at V
DD
startup) or after a PDR
has been detected, the power switch between V
BAT
and V
DD
remains connected to V
BAT
.
During the startup phase, if V
DD
is established in less than
t
RSTTEMPO
(refer to the datasheet for the value of t
RSTTEMPO
)
and V
DD
> V
BAT
+ 0.6 V, a current may be injected into V
BAT
through an internal diode connected between V
DD
and the
power switch (V
BAT
).
If the power supply/battery connected to the VBAT pin cannot
support this current injection, it is strongly recommended to
connect an external low-drop diode between this power
supply and the VBAT pin.
If no external battery is used in the application, it is recommended to connect V
BAT
externally to V
DD
with a 100 nF external ceramic decoupling capacitor.
When the backup domain is supplied by V
DD
(analog switch connected to V
DD
), the
following pins are available:
•
PC13, PC14 and PC15, which can be used as GPIO pins
•
PC13, PC14 and PC15, which can be configured by RTC or LSE (refer to
RTC functional description on page 1188
•
PA0/RTC_TAMP2 and PE6/RTC_TAMP3 when they are configured by the RTC as
tamper pins