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ST9 USER GUIDE

3.6 RESET AND CLOCK CONTROL UNIT (RCCU)

The RCCU is composed of the Clock Control Unit (CCU) and the Reset and Stop Manager.

3.6.1 CLOCK CONTROL UNIT

The CCU generates the peripheral clock INTCLK and the CPU clock CPUCLK. It is a useful

clock generator with low power function and low external frequency oscillator to reduce elec-

tromagnetic emissions.

The diagram can be reduced as in Figure 24.

Figure 24

. CCU Simplified Block Diagram

The CCU is composed of 5 main blocks:

BLOCK

COMMENTS

Quartz oscillator gives the main frequency generator. The frequency is in the range of 3 to 5

MHz.

External clock for very low power consumption with a very low frequency.

The Clock Multiplier can reduce or increase the input frequency by using prescaler and

PLL.The increase is for the normal use and the decrease for low power consumption.

The selector aims the clock coming from the Clock Multiplier for normal or low power use or

aims the clock CK_AF for a very low power consumption.

The prescaler is used also for low power consumption. The INTCLK is not change giving a high

frequency to the internal peripheral. This allows the user to slow down program execution

during non processor intensive routines.

Quartz

Oscillator

CK_AF

Source

Clock Multiplier

[1/32 to 14]

Prescaler

CPUCLK

INTCLK

ST9 Datasheet(PDF) 60 Page - STMicroelectronics