HT66F24D View Datasheet(PDF) - Holtek Semiconductor
Part Name
Description
MFG CO.
'HT66F24D' PDF : 177 Pages View PDF
A/D Flash Type 8-bit MCU with EEPROM
HT66F24D/HT66F25D
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. 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.
Type
Name
Freq.
Pins
External High Speed Crystal
HXT
400kHz~12MHz
OSC1/OSC2
Internal High Speed RC
HIRC
4, 8 or 12MHz
—
External Low Speed Crystal
LXT
32.768 kHz
XT1/XT2
Internal Low Speed RC
LIRC
32kHz
—
Oscillator Types
System Clock Configurations
There are four system oscillators, two high speed and two low speed oscillators. The high speed
oscillators are the external crystal/ceramic oscillator – HXT and the internal RC oscillator - HIRC.
The low speed oscillators are the internal 32 kHz oscillator – LIRC and the external 32.768 kHz
crystal oscillator – LXT. 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. 1.10
36
March 25, 2013
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