Rev. 1.10
34
November 26, 2019
Rev. 1.10
35
November 26, 2019
HT68FB240
USB Low Speed Flash MCU
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 Function. Fully integrated internal oscillators, requiring no external
components, are provided to form a wide range of both fast and slow system oscillators. 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.
Internal High Speed RC
HIRC
12MHz
Internal Low Speed RC
LIRC
32kHz
Oscillator Types
System Clock Configurations
There are several oscillator sources, a high speed oscillator and a low speed oscillator. The high
speed system clock is sourced from
the internal
12
MHz RC oscillator
and
the automatic adjust
clock. The low speed oscillator is the 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 the high speed and the low speed oscillators is chosen via registers.
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.
In addition, the internal automatic adjust
frequency clock, whose clock source is supplied by the internal
12
MHz RC oscillator, can be enabled
by a software control bit to generate various frequencies for the USB interface and system clock.
High Speed
Oscillator
HIRC
LIRC
f
H
6-stage Prescaler
HLCLK,
CKS2~CKS0 bits
f
H
/2
f
H
/4
f
H
/8
f
H
/16
f
H
/32
f
H
/64
÷2
SYSCLK
Bit
6MHz
USBCKEN bit
SYSCLK
To USBCK
Circuits
f
SYS
Low Speed
Oscillator
f
L
System Clock Configurations
