P80C557E4 Philips Semiconductors, P80C557E4 Datasheet - Page 28

• Manufacturer Part Number: P80C557E4
• Description: Single-chip 8-bit microcontroller
• Manufacturer: Philips Semiconductors

Philips Semiconductors
6.8 Watchdog Timer T3
In addition to Timer T2 and the standard timers, a watchdog timer
(T3) consisting of an 11-bit prescaler and an 8-bit timer is also
incorporated (see Figure 27).
The timer is incremented every 1.5 ms, derived from the system
clock frequency of 16 MHz by the following:
When a timer overflow occurs, the microcontroller is reset and a
reset output pulse is generated at pin RSTOUT. Also the PLL control
register is reset.
To prevent a system reset the timer must be reloaded in time by the
application software. If the processor suffers a hardware/software
malfunction, the software will fail to reload the timer. This failure will
1999 Mar 02
Single-chip 8-bit microcontroller
f
timer
f
12
CLK
f
CLK
/12
EW
2048
Write T3
Prescaler
Clear
(11-bit)
Figure 27. Watchdog timer.
PCON.4
Internal Bus
Internal Bus
28
LOAD LOADEN
Timer T3
Clear
(8-bit)
WLE
P83C557E4/P80C557E4/P89C557E4
In the Idle state the watchdog timer and reset circuitry remain active.
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.
The time interval between the timer’s reloading and the occurrence
of a reset depends on the reloaded value. For example, this may
range from 1.5 ms to 0.375 s when using an oscillator frequency of
16 MHz.
The watchdog timer is controlled by the watchdog enable pin (EW).
A LOW level enables the watchdog timer and disables the
Power-down Mode. A HIGH level disables the watchdog timer and
enables the Power-down Mode.
LOADEN
to reset circuitry (see Figure 46)
PD
PCON.1
Product specification