p87c770aar NXP Semiconductors, p87c770aar Datasheet - Page 11

• Manufacturer Part Number: p87c770aar
• Description: Microcontrollers Ntsc With On-screen Display Closed Caption
• Manufacturer: NXP Semiconductors

Philips Semiconductors
8
In addition to the standard timers, an 8-bit Watchdog Timer
is also incorporated. 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.
If the processor suffers a hardware/software malfunction,
the software will fail to reload the timer. This failure will
result in a reset upon overflow thus preventing the
processor running out of control.
The timer is incremented every 2 ms. The timer interval
between the timer reloading and the occurrence of a reset
depends on the reloaded value. This may range from
2 to 512 ms according to the following formula:
8.1
Table 4 Watchdog Timer Register (SFR address FFH)
Table 5 Description of the T3 bits
1999 Jun 11
T
handbook, full pagewidth
timer
Microcontrollers for NTSC TVs with On-Screen
Display (OSD) and Closed Caption (CC)
WATCHDOG TIMER (T3)
7 to 0
BIT
D7
7
Watchdog Timer Register (WDT)
=
256 T3 value
–
1/12 f osc
write T3
SYMBOL
D7 to D0
D6
6
2 ms
Watchdog Timer reload value. These 8 bits determine the timer interval. If WDT holds
FFH the timer interval is 2 ms. If WDT holds 00H the timer interval is 512 ms.
D5
5
PRESCALER
Fig.4 Watchdog Timer block diagram.
11-BIT
CLEAR
D4
4
11
WDT REGISTER
The Watchdog Timer can only be reloaded if the condition
flag WLE in SFR PCON has been previously set HIGH by
software. At the moment the counter is loaded WLE is
automatically cleared.
The Watchdog Timer is controlled by the EW bit in SFR
BWC (see Section 11.5). If EW = 1, the Watchdog Timer is
enabled and the Power-down mode disabled. If EW = 0,
the Watchdog Timer is disabled and the Power-down
mode enabled.
In the Idle mode the Watchdog Timer and reset circuitry
remain active.
INTERNAL BUS
INTERNAL BUS
PCON.4
LOAD
(8-BIT)
D3
CLEAR
3
DESCRIPTION
WLE
LOADEN
D2
2
R RESET
LOADEN
IDL
PCON.0
RESET
internal reset
P8xCx70 family
D1
MGL298
1
Product specification
D0
0