AN2419

Clocks

9/22

2

Clocks

2.1

Clock overview

Figure 6.

Clock overview

Several on-chip oscillators can feed the MCU system clock (CK_SYS) from which the HCLK

and PCLK derive:

●

FREEOSC: Internal Free Running Oscillator providing a clock between 1 and 10MHz,

also used as emergency clock. It consists of the internal VCO of the PLL configured in

free running mode

●

OSC4M: 4MHz or 8MHz Main Oscillator (Crystal or Ceramic oscillator or external

clock)

–

a 4 MHz Crystal/Ceramic oscillator connected to XT1/XT2

–

or an 8 MHz Crystal/Ceramic oscillator to XT1/XT2 followed by a divider by 2

–

or external clock connected to XT1

●

OSC32K: 32.768kHz Oscillator (Crystal or Ceramic oscillator) which can drive either

the system clock and/or the RTC.

●

LPOSC: Internal Low Power RC Oscillator providing a clock around 300kHz which can

drive either the system clock and/or the RTC.

Several configurable dividers provide a high degree of flexibility to the application in the

choice of the APB or AHB frequency, while keeping a fixed frequency value for the USB

clock (48 MHz).

XT1

XT2

XRTC1

XRTC2

USB_CK

OSC4M

PLL

AHB & APB

1-10 MHz

UP TO 64 MHz

48 MHz

4MHz

CKD FLAG

HCLK

PCLK

CK_TIM

UP TO 64MHz

UP TO 32MHz

UP TO 64MHz

DIVIDERS

CK_USB

48 MHz

RTC

OSC32K

LPOSC

/128

32kHz

ALARM

WAKEUP

CK_SYS

300kHz

CLOCK

DETECTOR

PLL

FREEOSC

CKD FLAG

For further details see the Reference manual UM0191.