STR91XF datasheet(14/72 Pages) STMICROELECTRONICS

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Functional overview

STR91xF

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2.10

Clock control unit (CCU)

The CCU generates a master clock of frequency fMSTR. From this master clock the CCU also

generates individually scaled and gated clock sources to each of the following functional blocks

within the STR91xF.

●

CPU, fCPUCLK

●

Advanced High-performance Bus (AHB), fHCLK

●

Advanced Peripheral Bus (APB), fPCLK

●

Flash Memory Interface (FMI), fFMICLK

●

External Memory Interface (EMI), fBCLK

●

UART Baud Rate Generators, fBAUD

●

USB, fUSB

2.10.1 Master clock sources

The master clock in the CCU (fMSTR) is derived from one of three clock input sources. Under

firmware control, the CPU can switch between the three CCU inputs without introducing any

glitches on the master clock output. Inputs to the CCU are:

●

Main Oscillator (fOSC). The source for the main oscillator input is a 4 to 25 MHz external

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

connected to pin X1_CPU.

●

PLL (fPLL). The PLL takes the 4 to 25 MHz oscillator clock as input and generates a master

clock output up to 96 MHz (programmable). By default, at power-up the master clock is

sourced from the main oscillator until the PLL is ready (locked) and then the CPU may

switch to the PLL source under firmware control. The CPU can switch back to the main

oscillator source at any time and turn off the PLL for low-power operation. The PLL is

always turned off in Sleep mode.

●

RTC (fRTC). A 32.768 kHz external crystal can be connected to pins X1_RTC and

X2_RTC, or an external oscillator connected to pin X1_RTC to constantly run the real-time

clock unit. This 32.768 kHz clock source can also be used as an input to the CCU to run

the CPU in slow clock mode for reduced power.

As an option, there are a number of peripherals that do not have to receive a clock sourced

from the CCU. The USB interface can receive an external clock on pin P2.7, TIM timers TIM0/

TIM1 can receive an external clock on pin P2.4, and timers TIM2/TIM3 on pin P2.5.

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