Datasheet

Description STM32F405xx, STM32F407xx
30/202 DocID022152 Rev 8
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 V
BAT
and standby mode. This memory area is disabled by
default to minimize power consumption (see
Section 2.2.19: 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 2.2.19: 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.
2.2.19 Low-power modes
The STM32F405xx and STM32F407xx support three low-power modes to achieve the best
compromise between low-power consumption, short startup time and available wakeup
sources:
• Sleep mode
In Sleep mode, only the CPU is stopped. All peripherals continue to operate and can
wake up the CPU when an interrupt/event occurs.
• Stop mode
The Stop mode achieves the lowest power consumption while retaining the contents of
SRAM and registers. All clocks in the V
12
domain are stopped, the PLL, the HSI RC
and the HSE crystal oscillators are disabled. The voltage regulator can also be put
either in normal or in low-power mode.
The device can be woken up from the Stop mode by any of the EXTI line (the EXTI line
source can be one of the 16 external lines, the PVD output, the RTC alarm / wakeup /
tamper / time stamp events, the USB OTG FS/HS wakeup or the Ethernet wakeup).
• Standby mode
The Standby mode is used to achieve the lowest power consumption. The internal
voltage regulator is switched off so that the entire V
12
domain is powered off. The PLL,
the HSI RC and the HSE crystal oscillators are also switched off. After entering