ST62T55B datasheet(18/74 Pages) STMICROELECTRONICS

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ST62T55B ST62T65B/E65B

3 CLOCKS, RESET, INTERRUPTS AND POWER SAVING MODES

3.1 CLOCK SYSTEM

The MCU features a Main Oscillator which can be

driven by an external clock, or used in conjunction

with an AT-cut parallel resonant crystal or a suita-

ble ceramic resonator, or with an external resistor

(RNET).

8 illustrates various possible oscillator configura-

tions using an external crystal or ceramic resonator,

an external clock input, an external resistor (RNET).

CL1 an CL2 should have a capacitance in the range

12 to 22 pF for an oscillator frequency in the 4-8 MHz

range.

A programmable divider is provided in order to adjust

the internal clock of the MCU to the best power con-

sumption and performance trade-off.

The internal MCU clock frequency (fINT) drives di-

rectly the AR TIMER while it is divided by 12 to

drive the TIMER, the A/D converter and the

Watchdog timer, and by 13 to drive the CPU core,

as may be seen in 9.

With an 8MHz oscillator frequency, the fastest ma-

chine cycle is therefore 1.625

µs.

A machine cycle is the smallest unit of time needed

to execute any operation (for instance, to increment

the Program Counter). An instruction may require

two, four, or five machine cycles for execution.

3.1.1 Main Oscillator

The oscillator configuration may be specified by se-

lecting the appropriate option. When the CRYSTAL/

RESONATOR option isselected,it mustbeusedwith

a quartz crystal, a ceramic resonator or an external

signal provided on the OSCinpin. When theRCNET-

WORK option is selected, the system clock is gen-

erated by an external resistor.

Figure 8. Oscillator Configurations

OSCin

OSCout

CL1n

CL2

ST6xxx

CRYSTAL/RES ONATOR CLOCK

CRYSTA L/RESONATOR option

OSCin

OSCout

ST6xxx

EXTE RNAL CLOCK

CRYSTAL /RESONATOR option

OSCin

OSCout

RNET

ST6xxx

RC NETWORK

RC NETWORK option

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