Datasheet

Functional overview STM32F21xxx
22/175 DocID17050 Rev 9
3.14 Power supply schemes
• V
DD
= 1.8 to 3.6 V: external power supply for I/Os and the internal regulator (when
enabled), provided externally through V
DD
pins.
• V
SSA
, V
DDA
= 1.8 to 3.6 V: external analog power supplies for ADC, DAC, Reset
blocks, RCs and PLL. V
DDA
and V
SSA
must be connected to V
DD
and V
SS
, respectively.
• V
BAT
= 1.65 to 3.6 V: power supply for RTC, external clock, 32 kHz oscillator and
backup registers (through power switch) when V
DD
is not present.
Refer to Figure 17: Power supply scheme for more details.
3.15 Power supply supervisor
The devices have an integrated power-on reset (POR) / power-down reset (PDR) circuitry
coupled with a Brownout reset (BOR) circuitry.
At power-on, POR/PDR is always active and ensures proper operation starting from 1.8 V.
After the 1.8 V POR threshold level is reached, the option byte loading process starts, either
to confirm or modify default BOR threshold levels, or to disable BOR permanently. Three
BOR thresholds are available through option bytes.
The device remains in reset mode when V
DD
is below a specified threshold, V
POR/PDR
or
V
BOR
, without the need for an external reset circuit. .
The devices also feature an embedded programmable voltage detector (PVD) that monitors
the V
DD
/V
DDA
power supply and compares it to the V
PVD
threshold. An interrupt can be
generated when V
DD
/V
DDA
drops below the V
PVD
threshold and/or when V
DD
/V
DDA
is
higher than the V
PVD
threshold. The interrupt service routine can then generate a warning
message and/or put the MCU into a safe state. The PVD is enabled by software.
3.16 Voltage regulator
The regulator has four operating modes:
• Regulator ON
– Main regulator mode (MR)
– Low power regulator (LPR)
– Power-down
• Regulator OFF
– Regulator OFF/internal reset ON
3.16.1 Regulator ON
The regulator ON modes are activated by default on LQFP packages. On UFBGA176
package, they are activated by connecting REGOFF to V
SS
.
V
DD
minimum value is 1.8 V.