P89LPC9331 datasheet(54/94 Pages) NXP

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P89LPC9331_9341_9351_9361

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Product data sheet

Rev. 5 — 10 January 2011

54 of 94

NXP Semiconductors

P89LPC9331/9341/9351/9361

8-bit microcontroller with accelerated two-clock 80C51 core

7.28 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 can be the PCLK, the nominal

400 kHz watchdog oscillator or low speed crystal 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 22 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 P89LPC9331/9341/9351/9361 User manual for more details.

7.29 Additional features

7.29.1 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.

7.29.2 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

(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 22. Watchdog timer in Watchdog mode (WDTE = 1)

PRE2

PRE1

PRE0

WDRUN

WDTOF

WDCLK

WDCON (A7H)

SHADOW REGISTER

PRESCALER

002aae015

8-BIT DOWN

COUNTER

WDL (C1H)

watchdog

oscillator

crystal

oscillator

PCLK

XTALWD

÷32

MOV WFEED1, #0A5H

MOV WFEED2, #05AH

reset(1)

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