Oscillator; System Clock Configurations - Holtek HT68F Series Manual

HT68F20/HT68F30/HT68F40/HT68F50/HT68F60
HT68FU30/HT68FU40/HT68FU50/HT68FU60
Enhanced I/O Flash Type 8-Bit 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 theWatchdog Timer and Time Base Interrupts. 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 oscillators provide 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.
Externa� Cr�sta�
Externa� RC
Interna� High Speed RC
Externa� Low Speed Cr�sta�
Interna� Low Speed RC

System Clock Configurations

There are five methods of generating the system clock, three high speed oscillators and two low
speed oscillators. The high speed oscillators are the external crystal/ceramic oscillator, external
RC network oscillator and the internal 4MHz, 8MHz or 12MHz RC oscillator. The two low speed
oscillators are the internal 32kHz RC oscillator and the external 32.768kHz crystal 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 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. 2.10
Type Name
HXT �00kHz~20MHz
ERC
HIRC �� 8 or 12MHz
LXT
LIRC
Oscillator Types
�9
Freq. Pins
OSC1/OSC2
8MHz OSC1
—
32.768kHz XT1/XT2
32kHz —
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