P89C668 Philips Semiconductors, P89C668 Datasheet - Page 31

• Manufacturer Part Number: P89C668
• Description: 80C51 8-bit Flash microcontroller family 64KB ISP FLASH with 8KB RAM
• Manufacturer: Philips Semiconductors

Philips Semiconductors
HARDWARE WATCHDOG TIMER (ONE-TIME
ENABLED WITH RESET-OUT FOR P89C668)
The WDT is intended as a recovery method in situations where the
CPU may be subjected to software upset. The WDT consists of a
14-bit counter and the WatchDog Timer reset (WDTRST) SFR. The
WDT is disabled at reset. To enable the WDT, user must write 01EH
and 0E1H in sequence to the WDTRST, SFR location 0A6H. When
WDT is enabled, it will increment every machine cycle while the
oscillator is running and there is no way to disable the WDT except
through reset (either hardware reset or WDT overflow reset). When
WDT overflows, it will drive an output reset HIGH pulse at the
RST-pin (see the note below).
Using the WDT
To enable the WDT, user must write 01EH and 0E1H in sequence to
the WDTRST, SFR location 0A6H. When WDT is enabled, the user
needs to service it by writing to 01EH and 0E1H to WDTRST to
avoid WDT overflow. The 14-bit counter overflows when it reaches
16383 (3FFFH) and this will reset the device. When WDT is
enabled, it will increment every machine cycle while the oscillator is
running. This means the user must reset the WDT at least every
16383 machine cycles. To reset the WDT, the user must write 01EH
and 0E1H to WDTRST. WDTRST is a write only register. The WDT
counter cannot be read or written. When WDT overflows, it will
generate an output RESET pulse at the reset pin (see note below).
The RESET pulse duration is 98
To make the best use of the WDT, it should be serviced in those
sections of code that will periodically be executed within the time
required to prevent a WDT reset.
2001 Jul 27
80C51 8-bit Flash microcontroller family
64KB ISP Flash with 8KB RAM
1FFF
100
Figure 29. Internal and External Data Memory Address Space with EXTRAM = 0
7936 BYTES
ERAM
T
OSC
, where T
FF
80
00
OSC
INTERNAL RAM
INTERNAL RAM
= 1/f
128 BYTES
128 BYTES
LOWER
UPPER
OSC
.
31
FF
80
00
FUNCTION
REGISTER
SPECIAL
FFFF
0000
EXTERNAL
MEMORY
DATA
1EFF
1F00
P89C668
SU01107
Preliminary data