Rev. 1.10
60
October 23, 2020
Rev. 1.10
61
October 23, 2020
BC66F5652
2.4GHz RF Transceiver A/D Flash MCU
BC66F5652
2.4GHz RF Transceiver A/D 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 option 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 range of both fast and slow system oscillators. 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 oscillator. 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
Frequency
Pins
External High Speed Crystal Oscillator
HXT
400kHz~16MHz
OSC1/OSC2
Internal High Speed RC Oscillator
HIRC
8/12/16MHz
—
External Low Speed Crystal Oscillator
LXT
32.768kHz
XT1/XT2
Internal Low Speed RC Oscillator
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 8/12/16MHz 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 the system clock can be dynamically selected.
The actual source clock used for the low speed oscillators is chosen via 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 also 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.