P89LPC925FDH Datasheet, PDF (35/49 Pages) NXP Semiconductors – 8-bit microcontrollers with accelerated two-clock 80C51 core 4 kB/8 kB 3 V low-power Flash with 8-bit A/D converter

Philips Semiconductors
P89LPC924/925
8-bit microcontrollers with accelerated two-clock 80C51 core
8.23 Watchdog timer
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. Figure 11 shows the
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 µs to a few seconds. Please refer to the P89LPC924/925 User’s Manual for
more details.
MOV WFEED1, #0A5H
MOV WFEED2, #05AH
Watchdog
oscillator
÷32
PCLK
WDL (C1H)
PRESCALER
8-BIT DOWN
COUNTER
CONTROL REGISTER
RESET
see note (1)
SHADOW
REGISTER
FOR WDCON
WDCON (A7H) PRE2 PRE1 PRE0
–
–
WDRUN WDTOF WDCLK
002aaa423
(1) Watchdog reset can also be caused by an invalid feed sequence, or by writing to WDCON not immediately followed by a
feed sequence.
Fig 11. Watchdog timer in Watchdog mode (WDTE = ‘1’).
8.24 Additional features
8.24.1 Software reset
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.
8.24.2 Dual data pointers
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.
9397 750 14471
Product data
Rev. 03 — 15 December 2004
© Koninklijke Philips Electronics N.V. 2004. All rights reserved.
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