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Rev. 1.00
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Standard 8051 8-Bit Flash MCU
HT85F2260/HT85F2270/HT85F2280
Oscillators
Crystal/Resonator Oscillator – HXT
Crystal Recommended Capacitor Values
Crystal Oscillator C1 and C2 Values
Crystal Frequency
C1
C2
�4MHz
10pF
10pF
1�MHz
10pF
10pF
8 MHz
10pF
10pF
4 MHz
�0pF
�0pF
400kHz
300pF
300pF
Note:
C1 and C� val�es a�e fo� g�idance onl�.
Internal High Speed RC Oscillator – HIRC
The internal RC oscillator is a fully integrated system oscillator requiring no external components.
The internal RC oscillator has a single frequency of 3.6864MHz. 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. The internal RC oscillator frequency can be multiplied from
1 to 8 times using the internal PLL. If the HIRC oscillator is used as the system oscillator, then the
OSC1 and OSC2 pins should be left unconnected.
External Low Speed Crystal Oscillator – LXT
The external low speed crystal oscillator, LXT, is used as the clock source for the Watchdog Timer
and the Time Base functions. When the microcontroller enters the IDLE Mode, the CPU clock is
switched off to stop microcontroller activity and to conserve power, however the LXT oscillator
will continue to run and can maintain WDT and Time Base operation if it is selected as their clock
source. The LXT oscillator is implemented using a 32768Hz crystal connected to pins XT1/XT2.
However, for some crystals, to ensure oscillation and accurate frequency generation, it is necessary
to add two small value external capacitors, C1 and C2. The exact values of C1 and C2 should be
selected in consultation with the crystal or resonator manufacturer’s specification.