Reset and clock control (RCC)
RM0365
115/1080
DocID025202 Rev 7
The Backup registers are also reset when one of the following events occurs:
1.
RTC tamper detection event.
2. Change of the read out protection from level 1 to level 0.
9.2 Clocks
Three different clock sources can be used to drive the system clock (SYSCLK):
•
HSI 8 MHZ RC oscillator clock
•
HSE oscillator clock
•
PLL clock
The devices have the following additional clock sources:
•
40 kHz low speed internal RC (LSI RC) which drives the independent watchdog and
optionally the RTC used for Auto-wakeup from Stop/Standby mode.
•
32.768 kHz low speed external crystal (LSE crystal) which optionally drives the real-
time clock (RTCCLK)
Each clock source can be switched on or off independently when it is not used, to optimize
power consumption.
Several prescalers can be used to configure the AHB frequency, the high speed APB
(APB2) and the low speed APB (APB1) domains. The maximum frequency of the AHB and
APB2 domains is 72 MHz. The maximum allowed frequency of the APB1 domain is 36 MHz.
All the peripheral clocks are derived from their bus clock (HCLK, PCLK1 or PCLK2) except:
•
The Flash memory programming interface clock (FLITFCLK) which is always the HSI
clock.
•
The 48-MHz USB clock which is derived from the PLL VCO
•
The option byte loader clock which is always the HSI clock
•
The ADCs clock which is derived from the PLL output. It can reach 72 MHz and can
then be divided by 1,2,4,6,8,10,12,16,32,64,128 or 256.
•
The U(S)ARTs clock which is derived (selected by software) from one of the four
following sources:
–
system clock
–
HSI clock
–
LSE clock
–
APB1 or APB2 clock (PCLK1 or PCLK2 depending on which APB is mapped the
USART)
•
The I2C1/2 (I2C1/2/3 on STM32F302xD/E) clock which is derived (selected by
software) from one of the two following sources:
–
system clock
–
HSI clock
•
The I2S2 and I2S3 clocks which can be derived from an external dedicated clock
source.
•
The RTC clock which is derived from the LSE, LSI or from the HSE clock divided by 32.
•
The IWDG clock which is always the LSI clock.