ST6208C datasheet(22/104 Pages) STMICROELECTRONICS

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ST6208C/ST6209C/ST6210C/ST6220C

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CLOCK SYSTEM (Cont’d)

5.1.3

Low

Frequency

Auxiliary

Oscillator

(LFAO)

The Low Frequency Auxiliary Oscillator has three

main purposes. Firstly, it can be used to reduce

power consumption in non timing critical routines.

Secondly, it offers a fully integrated system clock,

without any external components. Lastly, it acts as

a backup oscillator in case of main oscillator fail-

ure.

This oscillator is available when the OSG ENA-

BLED option is selected in the option byte (refer to

the Option Bytes section of this document). In this

case, it automatically starts one of its periods after

the first missing edge of the main oscillator, what-

ever the reason for the failure (main oscillator de-

fective, no clock circuitry provided, main oscillator

switched off...). See Figure 11.

User code, normal interrupts, WAIT and STOP in-

structions, are processed as normal, at the re-

duced fLFAO frequency. The A/D converter accura-

cy is decreased, since the internal frequency is be-

low 1.2 MHz.

At power on, until the main oscillator starts, the re-

set delay counter is driven by the LFAO. If the

main oscillator starts before the 2048 cycle delay

has elapsed, it takes over.

The Low Frequency Auxiliary Oscillator is auto-

matically switched off as soon as the main oscilla-

tor starts.

5.1.4 Register Description

ADC CONTROL REGISTER (ADCR)

Address: 0D1h

—

Read/Write

Reset value: 0100 0000 (40h)

Bit 7:3, 1:0 = ADCR[7:3], ADCR[1:0]

ADC Control

These bits are used to control the A/D converter (if

available on the device) otherwise they are not

used.

Bit 2 = OSCOFF

Main Oscillator Off.

0: Main oscillator enabled

1: Main oscillator disabled

Note: The OSG must be enabled using the OS-

GEN option in the Option Byte, otherwise the OS-

COFF setting has no effect.

ADCR

OSC

OFF

ADCR

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