BS85B12-3 HOLTEK [Holtek Semiconductor Inc], BS85B12-3 Datasheet - Page 38

• Manufacturer Part Number: BS85B12-3
• Description: Touch Key Flash Type 8-Bit MCU with LCD/LED Driver
• Manufacturer: HOLTEK [Holtek Semiconductor Inc]

Oscillator
Rev. 1.00
Oscillator Overview
System Clock Configurations
Internal High Speed RC Oscillator - HIRC
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.
The devices include two internal oscillators, a low speed oscillator and high speed oscillator. Both can
be chosen as the clock source for the main system clock however the slow speed oscillator is also used
as a clock source for other functions such as the Watchdog Timer, Time Base and Timer Modules. Both
oscillators require no external components for their implementation. All oscillator options are selected
using registers. The high speed oscillator provides higher performance but carries with it the
disadvantage of higher power requirements, while the opposite is of course true for the low speed
oscillator. With the capability of dynamically switching between fast and slow system clock, the
device has the flexibility to optimise the performance/power ratio, a feature especially important in
power sensitive portable applications.
There are two methods of generating the system clock, a high speed internal clock source and low
speed internal clock source. The high speed oscillator is an internal 8MHz, 12MHz or 16MHz RC
oscillator while the low speed oscillator is an internal 32kHz RC oscillator. Both oscillators are fully
integrated and do not require external components. 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 allowing the system clock to be dynamically selected.
The internal High Speed RC oscillator is a fully integrated system oscillator requiring no external
components. The internal RC oscillator has a power on default frequency of 8 MHz but can be selected
to be either 8MHz, 12MHz or 16MHz using the HIRCS1 and HIRCS0 bits in the CTRL register.
Device trimming during the manufacturing process and the inclusion of internal frequency
compensation circuits are used to ensure that the influence of the power supply voltage, temperature
and process variations on the oscillation frequency are minimised.
Internal High Speed
Internal Low Speed
Type
Touch Key Flash MCU with LCD/LED Driver
Oscillator Types
Name
HIRC
LIRC
8, 12 or 16MHz
32
32kHz
Freq.
BS85B12-3/BS85C20-3
February 1, 2011