Oscillator; Oscillato� Ove�View; System Clock Configurations - Holtek HT66F488 Manual

HT66F488/HT66F489
A/D Flash MCU with EEPROM

Oscillator

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 through a combination of configuration options and registers.
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 Interrupts. The external oscillators requiring some external
components as well as fully integrated internal oscillators requiring no external components are
provided to form a wide range of both fast and slow system oscillators. All oscillator options
are selected through the configuration options. The higher frequency oscillator provides higher
performance but carry with it the disadvantage of higher power requirements, while the opposite
is of course true for the lower frequency oscillators. With the capability of dynamically switching
between fast and slow system clock, the device has the flexibility to optimize the performance/
power ratio, a feature especially important in power sensitive portable applications.
Type
Exte�nal C�ystal
Inte�nal High Speed RC
Exte�nal low speed C�ystal
Inte�nal Low Speed RC

System Clock Configurations

There are four methods of generating the system clock, two high speed oscillators and two low speed
oscillators. The high speed oscillators are the external crystal oscillator and the internal 8MHz RC
oscillator. The low speed oscillators are the external 32.768kHz crystal oscillator and internal 32kHz
RC oscillator. Selecting whether the low or high speed oscillator is used as the system oscillator is
implemented using the HLCLK bit and CKS2 ~ CKS0 bits in the SMOD register and as the system
clock can be dynamically selected.
The actual source clock used for each of the high speed and the low speed oscillators is chosen
via configuration options. The frequency of the slow speed or high speed system clock is also
determined using the HLCLK bit and CKS2 ~ CKS0 bits in the SMOD register. Note that two
oscillator selections must be made namely one high speed and one low speed system oscillators. It is
not possible to choose a no-oscillator selection for either the high or low speed oscillator.
Rev. 1.21
Name Freq.
HXT 4��kHz~2�MHz
HIRC 8MHz
LXT 32.�68kHz
LIRC 32kHz
Oscillator Types
35
Pins
OSC1/OSC2
—
XT1/XT2
—
�ove��e� ��� 2�1�