HT85F2270 datasheet(57/226 Pages) HOLTEK

57 / 226 page

Rev. 1.00

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Standard 8051 8-Bit Flash MCU

HT85F2260/HT85F2270/HT85F2280

20 Oscillators

Various oscillator options offer the user a wide range of functions according to their various

application requirements. The flexible features of the oscillator functions ensure that the best

optimisation can be achieved in terms of speed and power saving. Oscillator selections and

operation are selected using internal registers.

System Oscillator Overview

In addition to being the source of the main system clock the oscillators also provide clock sources

for the Watchdog Timer and Time Base functions. External oscillators requiring some external

components as well as two fully integrated internal oscillators, requiring no external components,

are provided to form a wide range of both fast and slow system oscillators. After a reset occurs the

HIRC oscillator is selected as the initial system clock but can be later switched by the application

program using the clock control register.

Type

Name

Function

Freq.

Pins

External High Speed Cr�stal HXT Precision High Speed S�stem Clock 400kHz~�4�Hz

OSC1/OSC�

Internal High Speed RC

HIRC High Speed S�stem Clock

�.6864�Hz

—

External Low Speed Cr�stal

LXT Precision WDT and Time Base Clock

��768Hz

XT1/XT�

Internal Low Speed RC

LIRC WDT and Time Base Clock

��kHz

—

System Clock Configuration

There are four oscillators, two high speed oscillators and two low speed oscillators. The high speed

oscillators are the external crystal, HXT, and the internal RC oscillator, HIRC, which are used as

the system oscillators. The two low speed oscillators are the external 32768Hz oscillator, LXT,

and the internal 32kHz RC oscillator, LIRC, which are used as peripheral clocks for the Watchdog

Timer and Time Base functions.

External High Speed Crystal Oscillator – HXT

The simple connection of a crystal across OSC1 and OSC2 will create the necessary phase shift and

feedback for oscillation. However, for some crystals and most resonator types, to ensure oscillation

and accurate frequency generation, it is necessary to add two small value external capacitors,

C1 and C2. The exact values of C1 and C2 should be selected in consultation with the crystal or

resonator manufacturer’s specification. The external crystal frequency can be multiplied from 1

to 8 times using the internal PLL. For example, if a 4MHz crystal is used for oscillator and if the

PLL is selected as 8 times, the system clock can be increased to 32MHz. Note that if the internal

PLL is enabled, the external crystal frequency should be fixed at 4MHz; otherwise, an unexpected

frequency will be generated. When the internal PLL function is not to be used, the external crystal

frequency can be within the range, from 400kHz to 24MHz.

HT85F2270 Datasheet(PDF) 57 Page - Holtek Semiconductor Inc

HT85F2270 Datasheet(HTML) 57 Page - Holtek Semiconductor Inc

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