p89lpc904 NXP Semiconductors, p89lpc904 Datasheet - Page 30

• Manufacturer Part Number: p89lpc904
• Description: 8-bit Microcontrollers With Two-clock Accelerated 80c51 Core 1 Kb 3 V Byte-erasable Flash With 8-bit A/d Converter
• Manufacturer: NXP Semiconductors

Philips Semiconductors
9397 750 13521
Preliminary data
Fig 9. Watchdog timer in Watchdog mode (WDTE = 1).
(1) Watchdog reset can also be caused by an invalid feed sequence, or by writing to WDCON not immediately followed by a
feed sequence.
watchdog
oscillator
MOV WFEED1, #0A5H
MOV WFEED2, #05AH
PCLK
8.26.1 Software reset
8.26.2 Dual data pointers
8.25 Watchdog timer
8.26 Additional features
WDCON (A7H)
The Watchdog timer causes a system reset when it underflows as a result of a failure
to feed the timer prior to the timer reaching its terminal count. It consists of a
programmable 12-bit prescaler, and an 8-bit down counter. The down counter is
decremented by a tap taken from the prescaler. The clock source for the prescaler is
either the PCLK or the nominal 400 kHz Watchdog oscillator. The Watchdog timer
can only be reset by a power-on reset. When the Watchdog feature is disabled, it can
be used as an interval timer and may generate an interrupt.
Watchdog timer in Watchdog mode. Feeding the Watchdog requires a two-byte
sequence. If PCLK is selected as the Watchdog clock and the CPU is powered-down,
the Watchdog is disabled. The Watchdog timer has a time-out period that ranges
from a few s to a few seconds. Please refer to the P89LPC904 User’s Manual for
more details.
The SRST bit in AUXR1 gives software the opportunity to reset the processor
completely, as if an external reset or Watchdog reset had occurred. Care should be
taken when writing to AUXR1 to avoid accidental software resets.
The dual Data Pointers (DPTR) provides two different Data Pointers to specify the
address used with certain instructions. The DPS bit in the AUXR1 register selects
one of the two Data Pointers. Bit 2 of AUXR1 is permanently wired as a logic 0 so that
the DPS bit may be toggled (thereby switching Data Pointers) simply by incrementing
the AUXR1 register, without the possibility of inadvertently altering other bits in the
register.
32
PRE2
PRESCALER
PRE1
Rev. 02 — 25 June 2004
8-bit microcontrollers with two-clock accelerated 80C51 core
PRE0
SHADOW REGISTER
-
-
8-BIT DOWN
WDL (C1H)
COUNTER
WDRUN
© Koninklijke Philips Electronics N.V. 2004. All rights reserved.
WDTOF
Figure 9
P89LPC904
WDCLK
002aaa905
shows the
reset (1)
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