Datasheet

DocID17050 Rev 9 27/175
STM32F21xxx Functional overview
174
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 1.2 V domain is powered off. The
PLL, the HSI RC and the HSE crystal oscillators are also switched off. After entering
Standby mode, the SRAM and register contents are lost except for registers in the
backup domain and the backup SRAM when selected.
The device exits the Standby mode when an external reset (NRST pin), an IWDG reset,
a rising edge on the WKUP pin, or an RTC alarm / wakeup / tamper /time stamp event
occurs.
Note: The RTC, the IWDG, and the corresponding clock sources are not stopped when the device
enters the Stop or Standby mode.
3.19 V
BAT
operation
The V
BAT
pin allows to power the device V
BAT
domain from an external battery or an
external supercapacitor.
V
BAT
operation is activated when V
DD
is not present.
The V
BAT pin supplies the RTC, the backup registers and the backup SRAM.
Note: When the microcontroller is supplied from V
BAT
, external interrupts and RTC alarm/events
do not exit it from V
BAT
operation.
3.20 Timers and watchdogs
The STM32F21x devices include two advanced-control timers, eight general-purpose
timers, two basic timers and two watchdog timers.
All timer counters can be frozen in debug mode.
Table 4 compares the features of the advanced-control, general-purpose and basic timers.
Table 4. Timer feature comparison
Timer type Timer
Counter
resolution
Counter
type
Prescaler
factor
DMA
request
generation
Capture/
compare
channels
Complementary
output
Max
interface
clock
Max
timer
clock
Advanced-
control
TIM1,
TIM8
16-bit
Up,
Down,
Up/down
Any integer
between 1
and 65536
Yes 4 Yes 60 MHz
120
MHz