Rev. 1.00
42
September 11, 2018
Rev. 1.00
43
September 11, 2018
HT45F4050
A/D NFC Flash MCU
HT45F4050
A/D NFC Flash MCU
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 through a combination of configuration options and relevant control 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 register programming. 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~16MHz
OSC1/OSC2
Internal High Speed RC
HIRC
4, 8, 12MHz
—
External Low Speed Crystal
LXT
32.768kHz
XT1/XT2
Internal Low Speed RC
LIRC
32kHz
—
Oscillator Types
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/ceramic oscillator – HXT and
the internal
4
MHz, 8MHz, 12MHz RC oscillator – HIRC. The two low speed oscillators are the
internal 32kHz RC oscillator – LIRC and the external 32.768kHz crystal oscillator – LXT. Selecting
whether the low or high speed oscillator is used as the system oscillator is implemented using the
CKS2~CKS0 bits in the SCC register and as the system clock can be dynamically selected.
The actual source clock used for the low speed oscillators is chosen by the FSS bit in the SCC
register while for the high speed oscillator the source clock is selected by the FHS bit in the SCC
register. The frequency of the slow speed or high speed system clock is determined using the
CKS2~CKS0 bits in the SCC 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.