P83C524 datasheet(29/76 Pages) PHILIPS

29 / 76 page

1997 Dec 15

Philips Semiconductors

Product speciﬁcation

8-bit microcontrollers

P83C524; P80C528; P83C528

11.3

Watchdog Timer T3

The Watchdog Timer (WDT) see Fig.17, consists of an

11-bit prescaler and an 8-bit timer formed by SFR T3. The

prescaler is incremented by an on-chip oscillator with a

fixed frequency of 1 MHz. The maximum tolerance on this

frequency is

−50% and +100%. The 8-bit timer increments

every 2048 cycles of the on-chip oscillator. When a timer

overflow occurs, the microcontroller is reset and a

reset-output-pulse of 16 x 2048 cycles of the on-chip

oscillator is generated at pin RST. The internal RESET

signal is not inhibited when the external RST pin is kept

LOW by e.g. an external reset circuit. The RESET signal

drives Ports 1, 2 and 3 outputs into the High state and Port

0 into high impedance, no matter if the XTAL-clock is

running or not.

The WDT is controlled by WDCON SFR with the direct

address location A5H. WDCON can be read and written by

software. A value of A5H in WDCON halts the on-chip

oscillator and clears both the prescaler and Timer T3. After

RESET, WDCON contains A5H. Every value other than

A5H in WDCON enables the WDT. When the WDT is

enabled it runs independent of the XTAL-clock.

Timer T3 can be read on the fly. Timer T3 can be written

only if WDCON has previously been loaded with 5AH,

otherwise T3 and the prescaler are not affected. A

successful write operation to T3 also clears the prescaler

and clears WDCON. During a read or write operation

addressing T3, the output of the on-chip oscillator is

inhibited to prevent timing problems due to asynchronous

increments of T3. To prevent an overflow of the WDT, the

user program has to reload T3 within periods that are

shorter than the programmed Watchdog time interval. This

time interval is determined by the 8-bit reload value that is

written into register T3.

The advantages of this implementation are:

• Only an internal reset connection to the microcontroller

core

• The Power-down mode and the Watchdog (WDT)

function can be used concurrently

• The WDT also monitors the XTAL oscillator. In case of a

failure the port outputs are forced to a defined High state

• Interference will not disable the WDT because it is

unlikely that it will force WDCON to A5H

• Tolerances of the on-chip oscillator can be adjusted by

testing the T3 value and adapting the reload value

• The WDT can be enabled and disabled under control of

the user software. This gives the possibility to use both

the Watchdog function and the Power-down function

• The direct address A5H of WDCON and its disable value

A5H will not unintentionally be present at a random

location in the field of program code, except for

immediate data, because the opcode A5H is not used in

the instruction set.

Watchdog time interval

256

)

(

–

[] 2048

on-chip oscillator frequency

-------------------------------------------------------------------------

Fig.17 Watchdog Timer T3.

handbook, full pagewidth

MBC471 - 1

8 - BIT TIMER

11 - BIT

PRESCALER

WDCON

clear

input

A5H

5AH

clear

ON - CHIP -

OSCILLATOR

halt

write

read

clear

WR - T3

RD - T3

over-flow

VDD

VSS

internal

RESET

RST

this signal is active if WDCON

contains this hex value

IBS

R RST

(1)

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