Datasheet
Functional overview STM32F446xC/E
28/202 DocID027107 Rev 6
3.18 Real-time clock (RTC), backup SRAM and backup registers
The backup domain includes:
• The real-time clock (RTC)
• 4 Kbytes of backup SRAM
• 20 backup registers
The real-time clock (RTC) is an independent BCD timer/counter. Dedicated registers contain
the second, minute, hour (in 12/24 hour), week day, date, month, year, in BCD (binary-
coded decimal) format. Correction for 28, 29 (leap year), 30, and 31 day of the month are
performed automatically. The RTC provides a programmable alarm and programmable
periodic interrupts with wakeup from Stop and Standby modes. The sub-seconds value is
also available in binary format.
It is clocked by a 32.768 kHz external crystal, resonator or oscillator, the internal low-power
RC oscillator or the high-speed external clock divided by 128. The internal low-speed RC
has a typical frequency of 32 kHz. The RTC can be calibrated using an external 512 Hz
output to compensate for any natural quartz deviation.
Two alarm registers are used to generate an alarm at a specific time and calendar fields can
be independently masked for alarm comparison. To generate a periodic interrupt, a 16-bit
programmable binary auto-reload downcounter with programmable resolution is available
and allows automatic wakeup and periodic alarms from every 120 µs to every 36 hours.
A 20-bit prescaler is used for the time base clock. It is by default configured to generate a
time base of 1 second from a clock at 32.768 kHz.
The 4-Kbyte backup SRAM is an EEPROM-like memory area. It can be used to store data
which need to be retained in VBAT and standby mode. This memory area is disabled by
default to minimize power consumption (see
Section 3.19: Low-power modes). It can be
enabled by software.
The backup registers are 32-bit registers used to store 80 bytes of user application data
when V
DD
power is not present. Backup registers are not reset by a system, a power reset,
or when the device wakes up from the Standby mode (see
Section 3.19: Low-power
modes).
Additional 32-bit registers contain the programmable alarm subseconds, seconds, minutes,
hours, day, and date.
Like backup SRAM, the RTC and backup registers are supplied through a switch that is
powered either from the V
DD
supply when present or from the V
BAT
pin.
LQFP144 Yes No
Yes
PDR_ON
set to V
DD
Yes
PDR_ON
set to VSS
UFBGA144
Yes
BYPASS_REG
set to Vss
Yes
BYPASS_REG
set to VDD
WLCSP81
Table 4. Regulator ON/OFF and internal reset ON/OFF availability
Package Regulator ON Regulator OFF Internal reset ON Internal reset OFF
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