P80C592 Philips Semiconductors, P80C592 Datasheet - Page 26

• Manufacturer Part Number: P80C592
• Description: 8-bit microcontroller with on-chip CAN
• Manufacturer: Philips Semiconductors

Philips Semiconductors
11.3
In addition to Timer T2 and the standard timers (Timer 0
and Timer 1), a Watchdog Timer (WDT) comprising an
11-bit prescaler and an 8-bit timer (T3) is also provided
(see Fig.12).
The timer T3 is incremented every 1.5 ms, derived from
the oscillator frequency of 16 MHz by the following
formula:
When a timer T3 overflow occurs, the microcontroller is
reset and a reset-output-pulse is generated at pin RST.
This short output pulse (3 machine cycles) may be
suppressed if the RST pin is connected to a capacitor.
To prevent a system reset (by an overflow of the WDT), the
user program has to reload T3 within periods that are
shorter than the programmed Watchdog time interval.
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.
1996 Jun 27
8-bit microcontroller with on-chip CAN
Watchdog Timer (T3)
1/12 f CLK
f
timer
EW
=
------------------------- -
12 2048
f
CLK
write
T3
PRESCALER
Fig.12 Functional diagram of T3 Watchdog Timer.
11-BIT
CLEAR
TIMER T3 (8-BIT)
PCON.4
INTERNAL BUS
INTERNAL BUS
LOAD
26
CLEAR
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 timer 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.
In the Idle state the Watchdog Timer and reset circuitry
remain active.
The Watchdog Timer (WDT) is controlled by the Enable
Watchdog pin (EW) (see Table 28).
Table 28 EW controlling WDT and Power-down mode
WLE
PIN EW
LOADEN
HIGH
LOW
internal
reset
overflow
disabled
enabled
LOADEN
WDT
PD
PCON.1
POWER-DOWN MODE
Product specification
V DD
P
disabled
enabled
R RST
P8xC592
MGA157
RST