ST52T400 datasheet(42/94 Pages) STMICROELECTRONICS

42 / 94 page

ST52T400/T440/E440/T441

42/94

5 CLOCK, RESET & POWER SAVING MODE

5.1 Clock System

The ST52x400/440/441 Clock Generator module

generates the internal clock for the internal Con-

trol Unit, ALU and on-chip peripherals and is

designed to require a minimum of external compo-

nents.

The ST52x400/440/441 oscillator circuit gener-

ates an internal clock signal with the same period

and phase as at the OSCin input pin. The maxi-

mum frequency allowed is 20 MHz.

The system clock may be generated by using

either a quartz crystal, ceramic resonator (CER-

ALOC), or an external clock.

The different clock generator options connection

methods are illustrated in Oscillator Connec-

tions.When an external clock is used, it must be

connected to the pin OSCin while OSCout should

be left floating.

The crystal oscillator start-up time is a function of

many variables: crystal parameters (especially

Rs), oscillator load capacitance (CL), IC parame-

ters, environment temperature, supply voltage.

It must be observed that the crystal or ceramic

lead and circuit connections must be as short as

possible. Typical values for CL1, CL2 are 10pF for

a20MHz crystal.

Figure 5.1 Oscillator Connections

OSCin

OSCout

ST52X440

OSCin

ST52X440

OSCout

CRYSTAL CLOCK

EXTERNAL CLOCK

Cl1

10pF

Cl2

10pF

CLOCK

INPUT

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