CD00001237 datasheet(19/105 Pages) STMICROELECTRONICS

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ST6215C ST6225C

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5 CLOCKS, SUPPLY AND RESET

5.1 CLOCK SYSTEM

The main oscillator of the MCU can be driven by

any of these clock sources:

– external clock signal

– external AT-cut parallel-resonant crystal

– external ceramic resonator

– external RC network (RNET).

In addition, an on-chip Low Frequency Auxiliary

Oscillator (LFAO) is available as a back-up clock

system or to reduce power consumption.

An optional Oscillator Safeguard (OSG) filters

spikes from the oscillator lines, and switches to the

LFAO backup oscillator in the event of main oscil-

lator failure. It also automatically limits the internal

clock frequency (fINT) as a function of VDD, in order

to guarantee correct operation. These functions

are illustrated in Figure 10, and Figure 11.

Table 5 illustrates various possible oscillator con-

figurations using an external crystal or ceramic

resonator, an external clock input, an external re-

sistor (RNET), or the lowest cost solution using only

the LFAO.

For more details on configuring the clock options,

refer to the Option Bytes section of this document.

The internal MCU clock frequency (fINT) is divided

by 12 to drive the Timer, the Watchdog timer and

the A/D converter, by 13 to drive the CPU core and

the SPI and by 1 or 3 to drive the ARTIMER, as

shown in Figure 9.

With an 8 MHz oscillator, the fastest CPU cycle is

therefore 1.625µs.

A CPU 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 CPU cycles for execution.

Figure 9. Clock Circuit Block Diagram

MAIN

OSCILLATOR

OSG

LFAO

CORE

: 13

: 12

8-BIT TIMER

WATCHDOG

fINT

OSCOFF BIT

ADC

filtering

OSCILLATOR SAFEGUARD (OSG)

OSG ENABLE OPTION BIT (See OPTION BYTE SECTION)

(ADCR REGISTER)

fOSC

Oscillator

Divider

SPI

: 1

: 3

8-BIT ARTIMER

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