AN2476 datasheet(12/49 Pages) STMICROELECTRONICS

12 / 49 page

Power supply and clocks

AN2476

12/49

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 of approximately 5 MHz,

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

free running mode

●

OSC4M: 4 MHz Main Oscillator, based on:

–

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

–

or an 8 MHz Crystal/Ceramic oscillator to XT1/XT2 followed by an embedded

divider by 2

–

or external clock connected to XT1

which can be multiplied by the PLL to provide a wide range of frequencies (up to 64

MHz)

●

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 300 kHz 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).

The Clock Detector (CKD) protects the Microcontroller against OSC4M or external clock

failures.

The RTC provides calendar, alarm and wake-up functions and can be clocked by any of the

oscillators other than FREEOSC.

AN2476 Datasheet(PDF) 12 Page - STMicroelectronics