SZF2002 datasheet(22/76 Pages) PHILIPS

22 / 76 page

1998 Aug 26

Philips Semiconductors

Product speciﬁcation

Low voltage 8-bit microcontroller with

6-kbyte embedded RAM

SZF2002

12.5

Watchdog Timer (T3)

In addition to Timer 2 and the standard timers, a Watchdog

Timer (consisting of an 11-bit prescaler and an 8-bit timer)

is also available.

The Watchdog Timer is controlled by the Watchdog

Enable Register (WDTKEY). When WDTKEY = 55H, the

timer is disabled and the Power-down mode is enabled.

Otherwise, the timer is enabled and the Power-down mode

is disabled. In the Idle mode the Watchdog Timer and reset

circuitry remain active.

The Watchdog Timer is shown in Fig.11.

The timer frequency is derived from the clock frequency

using the formula shown below:

When a timer overflow occurs, the microcontroller is reset.

To prevent a system reset the timer must be reloaded in

time by the application software.

timer

clk

62048

() T3

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

If the processor suffers a hardware/software malfunction,

the software will fail to reload the timer.This failure will

produce a reset upon overflow thus preventing the

processor running out of control.

The Watchdog Timer can only be reloaded if the condition

flag WLE (PCON.4) has been previously set by software.

At the moment the counter is loaded the condition flag is

automatically cleared. After reset the Watchdog Timer is

off. The Watchdog Timer is started by loading a value into

T3.

The time interval between the timer reloading and the

occurrence of a reset is dependent upon the reloaded

value. The time interval is derived from the clock and the

value programmed into T3 and may be calculated as

shown below:

For example, this time period may range from 2 to 500 ms

when using a clock frequency fclk = 6 MHz.

reload

256

–

()

timer

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

Fig.11 Functional diagram of the Watchdog Timer (T3).

handbook, full pagewidth

MGM141

INTERNAL BUS

write

PRESCALER

11-BIT

TIMER T3 (8-BIT)

LOAD

CLEAR

overflow

internal

reset

LOADEN

SFR WDTKEY

LOADEN

PCON.4

PCON.1

CLEAR

WLE

RST

INTERNAL BUS

fclk/6

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