ST52T410 datasheet(34/84 Pages) STMICROELECTRONICS

34 / 84 page

ST52T410/ST52T420/E420

34/84

5 CLOCK, RESET & POWER SAVING MODE

5.1 System Clock

The

ST52T410/ST52x420

Clock

Generator

module generates the internal clock for the internal

Control Unit, ALU and on-chip peripherals and it is

designed to require a minimum number of external

components.

The

ST52T410/ST52x420

oscillator

circuit

generates an internal clock signal with the same

period and phase as that of the OSCin input pin.

The maximum frequency allowed is 20 Mhz.

The system clock may be generated by using

either a quartz crystal, ceramic resonator or an

external clock.

The different methods of the clock generator are

illustrated in Figure 5.1.

When an external clock is used, it must be

connected on the OSCin pin, while OSCout can be

floating.

The crystal oscillator start-up time is a function of

many variables: crystal parameters (especially

Rs),

oscillator

load

capacitance

(CL),

parameters,

environment

temperature, supply

voltage.

Figure 5.1 Oscillator Connections

Note: The crystal or ceramic leads and circuit

connections must be as short as possible. Typical

values for CL1, CL2 are 10pF for a 20 MHz crystal.

5.2 RESET

There are two Reset sources:

- RESET pin (external source.)

- WATCHDOG (internal source)

When a Reset event happens, the user program

restarts from the beginning.

The Reset pin is an input. An internal reset does

not affect this pin.

A Reset signal originated by external sources is

recognized instantaneously. The RESET pin may

be used to ensure VDD has risen to a point where

the MCU can operate correctly before the user

program runs. In working mode Reset must be set

to ‘1’ (see Table 2.1).

5.3 Power Saving Mode

There are two Power Saving modes: WAIT and

HALT mode. These conditions may be entered

using the WAIT or HALT instructions.

5.3.1 Wait Mode

Wait mode places the MCU in low power

consumption by stopping the CPU. All peripherals

OSCin

OSCout

ST52X420

OSCin

ST52X420

OSCout

CRYSTAL CLOCK

EXTERNAL CLOCK

Cl1

10pF

Cl2

10pF

CLOCK

INPUT

FLOATING

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