Rev. 1.10
36
November 26, 2019
Rev. 1.10
37
November 26, 2019
HT68FB240
USB Low Speed Flash MCU
Internal RC Oscillator – HIRC
The internal RC oscillator is a fully integrated system oscillator requiring no external components.
The internal RC oscillator has
a
fixed frequency
of 12MHz.
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. Note that if this internal system clock option is selected, as it
requires no external pins for its operation, I/O pins are free for use as normal I/O pins.
Internal 32kHz Oscillator – LIRC
The Internal 32kHz System Oscillator is the low frequency oscillator. It is a fully integrated
RC oscillator with a typical frequency of 32kHz at 5V, requiring no external components for its
implementation. 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.
Operating Modes and System Clocks
Present day applications require that their microcontrollers have high performance but often still
demand that they consume as little power as possible, conflicting requirements that are especially
true in battery powered portable applications. The fast clocks required for high performance will
by their nature increase current consumption and of course vice-versa, lower speed clocks reduce
current consumption. As Holtek has provided the device with both high and low speed clock sources
and the means to switch between them dynamically, the user can optimise the operation of their
microcontroller to achieve the best performance/power ratio.
System Clocks
The device has many different clock sources for both the CPU and peripheral function operation. By
providing the user with a wide range of clock options using register programming, a clock system
can be configured to obtain maximum application performance.
The main system clock, can come from either a high frequency f
H
or low frequency f
SUB
source,
and is selected using the HLCLK bit and CKS2~CKS0 bits in the SMOD register. The high speed
system clock can be sourced from the HIRC oscillator. The low speed system clock source can be
sourced from the LIRC oscillator. The other choice, which is a divided version of the high speed
system oscillator has a range of f
H
/2~f
H
/64.