Hello, and welcome to this presentation of the STM32 Reset and

Hello, and welcome to this presentation of the STM32 Reset and
Hello, and welcome to this presentation of the STM32 Reset
and Clock Controller.
The RCC controller integrated inside STM32 products
manages system and peripheral clocks.
STM32F7 devices embed two internal oscillators, 2
oscillators for an external crystal or resonator, and three
phase-locked loops (PLL).
Many peripherals have their own clock, independent of
the system clock.
The RCC also manages the various resets present in the
The STM32F7 RCC provides high flexibility in the choice
of clock sources, which allows the system designer to
meet both power consumption and accuracy
The independent peripheral clocks allow a designer to
adjust the system power consumption without impacting
the communication baud rates. Finally, the RCC provides
safe and flexible reset management.
Safe and flexible reset management without any need for
external components reduces application costs.
The RCC manages three types of resets: the system
reset, the power reset and the backup domain reset.
The peripherals have individual reset control bits.
Here is the simplified block diagram of the system reset.
All internal reset sources provide a reset signal on the
NRST pin, which can be used to reset other components
of the application board. In addition, no external reset
circuitry is needed due to the internal glitch filter and the
safe power monitoring feature which guarantees the
reset of the application when VDD is below the selected
An internal pull-up on the NRST pin, allows to maintain a
high level when no reset signal is driven low.
The first type of reset is the system reset, which resets
all the registers except certain registers for the Reset
and Clock Controller. It also does not reset the backup
The system reset sources are the external reset
(generated by a low level on the NRST pin), a window
watchdog event, an independent watchdog event, a
software event through the Nested Vectored Interrupt
Controller, a low-power-mode security reset (which is
generated when Stop or Standby mode is entered but is
prohibited by the option byte configuration). The reset
source flag can be found in the RCC Control and Status
The second type of reset is the power reset. The Brownout reset (BOR) resets all registers except those in the
Backup domain powered by VBAT which contains the
RTC and the external low-speed oscillator.
When exiting Standby mode, all registers powered by the
regulator are reset. When exiting Standby mode, a reset
is generated.
The third type of reset is the backup domain reset, which
resets the RTC registers, the Backup registers, and the
RCC Backup Domain Control Register. This reset occurs
when the BDRST bit is set in the RCC Backup Domain
control register.
It also occurs when VDD and VBAT are powered on if
both supplies have previously been powered off.
The RCC offers a large choice of clock sources, which
can be selected depending on low-power, accuracy, and
performance requirements.
STM32F7 devices embed two internal clock sources: a
high-speed internal 16 MHz RC oscillator (HSI), and a
low-speed internal 32 kHz RC oscillator (LSI).
STM32F7 devices embed two oscillators for use with an
external crystal or resonator: a high-speed external 4 to
26 MHz oscillator (HSE) with a clock security system and
a low-speed external 32.768 kHz oscillator (LSE) also
with a clock security system.
STM32F7 devices embed three phase-locked loops,
each with three independent outputs for clocking
different peripherals at different frequencies.
The high-speed internal oscillator is a 16 MHz RC
oscillator which provides 1% accuracy and fast wakeup
times. The HSI is trimmed during production testing, and
can also be user-trimmed.
The HSI is selected as clock at wakeup from Stop mode,
and as the backup clock if an HSE failure is detected by
the Clock Security System.
If an HSE failure is detected, the Clock Security System
allows to put system in safe state by generating break
events to critical applications such as motor control.
The 32.768 kHz low-speed external oscillator can be
used with external quartz or resonator, or with an
external clock source in bypass mode.
The oscillator driving capability is programmable. Four
modes are available, from ultra-low power mode, to highdriving mode.
The LSE can be used to clock the RTC, the low-power
timer, the HDMI-CEC interface, and the USARTs
STM32F7 devices embed 3 phase-locked loops, each
with 3 independent outputs. The input clock of the PLL
can be selected between HSI and HSE.
The main PLL can provide the system clock.
The different PLL outputs can be used for the serial
audio interfaces, USB, Random Number Generator,
SDMMC peripherals and LTDC/ and DSI-HOST
interfaces when available.
The system clock is selected between the HSI, HSE and
PLL output.
The maximum system clock frequency is 216 MHz. The
APB1 and APB2 bus frequencies are also up to 54 MHz.
and 108 MHz respectively
The maximum clock source frequency depends on the
voltage scaling. The maximum system clock is reached
with voltage scale 1 and when enabling Over-Drive.
When Over-Drive is off, the maximum system clock
frequency is 180MHz.
Several peripherals have their own clock independent
from the system clock. This is the case for the USARTs,
I2Cs, low-power timer, HDMI-CEC interface,
independent watchdog, USB OTG HS external PHY clock
and Ethernet MAC clocks (when available in the device
package). All of these clocks can be selected from the
internal/external oscillators’ dedicated external clock pin
or bus interface clocks.
Serial audio interfaces, USB OTG FS, random number
generator, and SDMMC interfaces have independent
clock sources that are generated from the different PLL
In addition to PLL outputs the serial audio interfaces clock
can be generated from External clock mapped on the
I2S_CKIN pin or from HSI/HSE clocks if this feature is
Several peripherals available only on some F7 device
part numbers, such as the DFSDM1 interface, the
SDMMC2 interface and the DSI-HOST interface have
dedicated input clock sources. The clocks are derived
from PLL outputs or from system clocks.
In addition to PLL outputs, the DSI-HOST interface clock
can be generated from its own PLL.
The various clocks can be output on an I/O. The
Microcontroller Clock Output feature allows you to output
on a pin one of these six clocks: HSI, HSE, LSE,
The dynamic power consumption can be optimized by
using peripheral clock gating.
Each peripheral clock can be gated ON or OFF in Run
and Sleep mode, except SRAM and FLASH which are
always clocked in Run and Sleep modes. By default, the
peripheral’s clock is disabled, except the DTCMRAM
clock and RTC interface clock which are enabled by
default. When a peripheral’s clock is disabled, the
peripheral’s registers cannot be read or written.
Dedicated registers allow for configuring the peripheral’s
clock during the Sleep mode. These control bits have no
effect if the corresponding peripheral clock is disabled.
This slide lists the RCC interrupts. The PLLs ready and
all oscillators ready signals can generate an interrupt.
In addition to this training, you may find the Power
Control and Interrupt Controller trainings useful.
For more details, please refer to application note
AN2867, an oscillator design guide for STM8S, STM8A
and STM32 microcontrollers .
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