W78LE51/W78L051A

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OSC

1/12

PRESCALER

14-BIT TIMER

CLEAR

CLRW

EXTERNAL

RESET

INTERNAL

RESET

WIDL

IDLE

ENW

Watchdog Timer Block Diagram

Typical Watch-Dog time-out period when OSC = 20 MHz

PS2 PS1 PS0

WATCHDOG TIME-OUT PERIOD

0

0

0

19.66 mS

0

1

0

39.32 mS

0

0

1

78.64 mS

0

1

1

157.28 mS

1

0

0

314.57mS

1

0

1

629.14 mS

1

1

0

1.25 S

1

1

1

2.50 S

Clock

The W78L051 is designed to be used with either a crystal oscillator or an external clock. Internally, the

clock is divided by two before it is used. This makes the W78L051 relatively insensitive to duty cycle

variations in the clock. The W78L051 incorporates a built-in crystal oscillator. To make the oscillator

work, a crystal must be connected across pins XTAL1 and XTAL2. In addition, a load capacitor must

be connected from each pin to ground. An external clock source should be connected to pin XTAL1.

Pin XTAL2 should be left unconnected. The XTAL1 input is a CMOS-type input, as required by the

crystal oscillator.

Power Management

Idle Mode

The idle mode is entered by setting the IDL bit in the PCON register. In the idle mode, the internal

clock to the processor is stopped. The peripherals and the interrupt logic continue to be clocked. The

processor will exit idle mode when either an interrupt or a reset occurs.