
W79E804A/803A/802A
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21. OSCILLATOR
The W79E804 series provides three oscillator input option. These are configured at CONFIG register
(CONFIG1) that include On-Chip RC Oscillator Option, External Clock Input Option and Crystal
Oscillator Input Option. The Crystal Oscillator Input frequency may be supported from 4MHz to
20MHz, and without capacitor or resister.
Figure
21-1: Oscillator
21.1 On-Chip RC Oscillator Option
The On-Chip RC Oscillator is fixed at 6MHz +/- 25% frequency to support clock source. When FOSC1,
FOSC0 = 01H, the On-Chip RC Oscillator is enabled. A clock output on P2.0 (XTAL2) may be enabled
when On-Chip RC oscillator is used.
21.2 External Clock Input Option
The clock source pin (XTAL1) is from External Clock Input by FOSC1, FOSC0 = 11H, and frequency
range is form 0Hz up to 20MHz. A clock output on P2.0 (XTAL2) may be enabled when External Clock
Input is used.
The W79E804 series supports a clock output function when either the on-chip RC oscillator or the
external clock input options is selected. This allows external devices to synchronize to the W79E804
serial. When enabled, via the ENCLK bit in the P2M1 register, the clock output appears on the
XTAL2/CLKOUT pin whenever the on-chip oscillator is running, including in Idle Mode. The frequency
of the clock output is 1/4 of the CPU clock rate. If the clock output is not needed in Idle Mode, it may
be turned off prior to entering Idle mode, saving additional power. The clock output may also be
enabled when the external clock input option is selected.