HT85F2270 datasheet(62/226 Pages) HOLTEK

62 / 226 page

Rev. 1.00

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Standard 8051 8-Bit Flash MCU

HT85F2260/HT85F2270/HT85F2280

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

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

crystal HXT oscillator or a frequency multiplied version of these oscillators using the internal

PLL. 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 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 are two additional internal 32kHz low frequency clocks for the peripheral circuits. These are

the external crystal LXT oscillator and the internal LIRC oscillators. The selection is implemented

using the LSOSEL bit in the LSOCR register. There is a low frequency oscillator status bit,

LSORDY, to indicate the “ready or not” status of the low frequency oscillator. This bit is common

to both low frequency oscillators, and 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 during low speed oscillator switching and set high once the chosen oscillator 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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