Power control (PWR) RM0367
144/1043 RM0367 Rev 7
6.1.2 Independent LCD supply
The V
LCD
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 V
LCD
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 V
LCD
pin should be
connected to a capacitor (see the product datasheets for further information).
6.1.3 RTC and RTC backup registers
The real-time clock (RTC) is an independent BCD timer/counter. The RTC provides a time-
of-day clock/calendar, two programmable alarm interrupts, and a periodic programmable
wakeup flag with interrupt capability. The RTC contains 5 backup data registers (20 bytes).
These backup registers are reset when a tamper detection event occurs. For more details
refer to Real-time clock (RTC) section.
RTC registers access
After reset, the RTC Registers (RTC registers and RTC backup registers) are protected
against possible stray write accesses. To enable access to the RTC Registers, proceed as
follows:
1. Enable the power interface clock by setting the PWREN bits in the RCC_APB1ENR
register.
2. Set the DBP bit in the PWR_CR register (see Section 6.4.1).
3. Select the RTC clock source through RTCSEL[1:0] bits in RCC_CSR register.
4. Enable the RTC clock by programming the RTCEN bit in the RCC_CSR register.
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