ST62T85B datasheet(19/78 Pages) STMICROELECTRONICS

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ST62T85B/E85B

3 CLOCKS, RESET, INTERRUPTS AND POWER SAVING MODES

3.1 CLOCK SYSTEM

3.1.1 Main Oscillator

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.

Figure 8 illustrates various possible oscillator con-

figurations using an external crystal or ceramic res-

onator, an external clock input. CL1 an CL2 should

have a capacitance in the range 12 to 22 pF for an

oscillator frequency in the 4-8 MHz range.

The internal MCU clock Frequency (FINT) is divid-

ed by 13 to drive the CPU core and by 12 to drive

the A/D converter and the watchdog timer, while

clock used to drive on-chip peripherals depends

on the peripheral as shown in the clock circuit

block diagram.

With an 8 MHz oscillator frequency, the fastest ma-

chine 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.

Figure 8. Oscillator Configurations

Figure 9. Clock Circuit Block Diagram

OSCin

OSCout

CL1n

CL2

ST6xxx

CRYSTAL/RESONATOR CLOCK

OSCin

OSCout

ST6xxx

EXTERNAL CLOCK

VA0016

VA0015A

MAIN

OSCILLATOR

Core

: 13

: 12

Timer 1

Watchdog

POR

fINT

ADC

OSCin

OSCout

fOSC

fINT

LCD

CONTROLLER

DRIVER

UART

ARTIMER

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