ST62T52B datasheet(16/68 Pages) STMICROELECTRONICS

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ST62T52B ST62T62B/E62B

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).

Figure 8. illustrates various possible oscillator con-

figurations using an external crystal or ceramic res-

onator, 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.

Aprogrammable 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 Figure 9..

With an 8 MHz oscillator frequency, the fastest

machine cycle is therefore 1.625µs.

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 CRYS-

TAL/RESONATOR option is selected, it must be

used with a quartz crystal, a ceramic resonator or

an external signal provided on the OSCin pin. When

the RC NETWORK option is selected, the system

clock is generated by an external resistor.

Figure 8. Oscillator Configurations

OSCin

OSCout

CL1n

CL2

ST6xxx

CRYSTAL/RESONATOR CLOCK

CRYSTAL/RESONATOR option

OSCin

OSCout

ST6xxx

EXTERNAL CLOCK

CRYSTAL/RESONATOR option

OSCin

OSCout

RNET

ST6xxx

RC NETWORK

RC NETWORK option

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