AN2339

Clock management

7/20

3

Clock management

The STR91x offers a flexible way for selecting the core and peripheral clocks, the devices

have up to four external clock source inputs: Main Oscillator, RTC, USB Clock and TIM

clocks. It also provides one output clock.

Figure 3.

Clock Management

3.1

Main oscillator

The source for the main oscillator input is a 4 to 25 MHz external crystals connected to

STR91xF pins X1_CPU and X2_CPU or an external oscillator device connected to pin

X1_CPU, in this case the X2_CPU pin can be left open and not used.

The recommended circuitry for a crystal is shown below. C1, C2 and R1 values depend

greatly on the crystal type and manufacturer. You should ask your crystal supplier for the

best values for these components.

MII_PHYCLK

25MHz

PHYSEL

X1_CPU

X2_CPU

X1_RTC

X2_RTC

EXTCLK_T0T1

USB_CLK48M

4-25MHz

48MHz

Main

OSC

RTC

OSC

16-bit prescaler

PLL

32.768kHz

TIM01CLK

Master CLK

1/2

1/2

BRCLK

To UART

USBCLK

To USB

To TIM0 & TIM1

EXTCLK_T2T3

16-bit prescaler

TIM23CLK

To TIM2 & TIM3