HT85F2240 datasheet(55/147 Pages) HOLTEK

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Rev. 1.10

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A/D Flash 8051 Core Low Pin Count MCU

HT85F2230/HT85F2240

The main system clock source, known as fSYS, and which is used by the CPU and the peripheral

functions, can come from one of two sources. These are the internal HIRC oscillator or the external

crystal HXT oscillator. The selection is implemented using the SCKS0 and SCKS1 bits in the

SCCR register. The HXT and HIRC oscillators also have independent enable control bits, which

are the HXTEN and HIRCEN bits in the HSOCR register. There are also two oscillator status bits,

HIRCRDY and HXTRDY, in the HSOCR register to indicate whether the oscillators are ready for

operation. After power on, these status bits should be monitored by the program to indicate the

“ready or not” status of the respective oscillator before they are used with instruction execution.

After power on, the device will automatically select the HIRC oscillator as its default system clock,

which can be changed later by the application program.

There is an additional internal 32KHz low frequency clock for the peripheral circuits. It is the

internal LIRC oscillator, which is enabled using the LIRCEN bit in the LSOCR register. There is

a low frequency oscillator status bit, LIRCRDY, to indicate the “ready or not” status of the LIRC

oscillator. This bit should be monitored by the program to indicate the “ready or not” status of the

oscillator before it is used for instruction execution. This bit will be automatically cleared to zero

by hareware when the LIRC is disabled and set high once the LIRC is stable.

System Clock Control Register – SCCR

SFR Address: ECh

Bit

Name

—

SCKS1

SCKS0

R/W

—

R/W

POR

—

Bit 7~2

Unimplemented, read as “0”

Bit 1~0

SCKS1, SCKS0: High Frequency System clock select

00: HIRC oscillator clock source

01: HIRC oscillator clock source

10: HXT oscillator clock source

11: PLL clock source

The HIRC will be the default system clock source after a power on reset.

When switching between different clock sources an oscillator stabilisation time delay

must be provided before continuing with program execution.

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