Preliminary W78E51B
Typical Watch-Dog time-out period when OSC = 20 MHz
PS2 PS1 PS0 WATCHDOG TIME-OUT PERIOD
0
0
0
0
1
1
1
1
0
1
0
1
0
0
1
1
0
0
1
1
0
1
0
1
19.66 mS
39.32 mS
78.64 mS
157.28 mS
314.57 mS
629.14 mS
1.25 S
2.50 S
Clock
The W78E51B 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 W78E51B relatively insensitive to duty
cycle variations in the clock. The W78E51B 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.
Power-down Mode
When the PD bit of the PCON register is set, the processor enters the power-down mode. In this
mode all of the clocks are stopped, including the oscillator. The only way to exit power-down mode is
by a reset.
Reset
The external RESET signal is sampled at S5P2. To take effect, it must be held high for at least two
machine cycles while the oscillator is running. An internal trigger circuit in the reset line is used to
deglitch the reset line when the W78E51B is used with an external RC network. The reset logic also
has a special glitch removal circuit that ignores glitches on the reset line.
During reset, the ports are initialized to FFH, the stack pointer to 07H, PCON (with the exception of
bit 4) to 00H, and all of the other SFR registers except SBUF to 00H. SBUF is not reset.
ON-CHIP MTP ROM CHARACTERISTICS
The W78E51B has several modes to program the on-chip MTP-ROM. All these operations are
configured by the pins RST, ALE, PSEN , A9CTRL(P3.0), A13CTRL(P3.1), A14CTRL(P3.2),
OECTRL(P3.3), CE(P3.6), OE (P3.7), A0(P1.0) and VPP(EA ). Moreover, the A15- A0(P2.7- P2.0,
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