W79E825A Datasheet, PDF (95/127 Pages) Winbond – 8-BIT MICROCONTROLLER

W79E825A/824A/823B/822B Data Sheet
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 W79E825 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 W79E825
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.
21.3 CPU Clock Rate select
The CPU clock of W79E825 series may be selected by the DIVM register. If DIVM = 00H, the CPU
clock is running at 4 CPU clock per machine cycle, and without any division from source clock
(Fosc). When the DIVM register is set to N value, the CPU clock is divided by 2(DVIM+1), so
CPU clock frequency division is from 4 to 512. The user may use this feature to set CPU at a
lower speed rate for reducing power consumption. This is very similar to the situation when
CPU has entered Idle mode. In addition this frequency division function affect all peripheral
timings as they are all sourcing from the CPU clock(Fcpu).
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Publication Release Date: February 21, 2008
Revision A9