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Standard 8051 8-Bit Flash MCU
HT85F2260/HT85F2270/HT85F2280
Operating Modes and System Clocks
21
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
important in battery powered portable applications. This usually requires the microcontroller can
provide a range of clock sources which can be dynamically selected.
System Clocks Description
The fast clocks required for high performance will inherently have a higher power consumption
and of course vice-versa, lower speed clocks will have a lower power consumption. As Holtek has
provided these devices with a range of oscillators and a PLL function the user can optimise the
system clock frequency to achieve the best performance/power ratio. In addition to the two high
frequency system oscillators, two low frequency 32kHz oscillators are also provided as clock
sources for the WDT and Time Base.
The MCU system clock is sourced from the high speed external crystal, HXT oscillator, or internal,
HIRC oscillator. These oscillators can be used directly as the system clock and can be routed via an
internal PLL to give a wide range of operating frequencies. The PLL frequency can be dynamically
changed to suit varying operating conditions and to achieve maximum performance.
The system clock, namely f
SYS
, can also be used as a clock source for the peripheral functions, such
as WDT, Time Base, Timers, UART, I
2
C, SPI, ADC and DAC. Refer to the related sections for the
clock source selections.