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| 型号： | P87C770AAR |
| 厂商： | NXP SEMICONDUCTORS |
| 元件分类： | 微控制器/微处理器 |
| 英文描述： | Microcontrollers for NTSC TVs with On-Screen Display OSD and Closed Caption CC |
| 中文描述： | 8-BIT, OTPROM, 12 MHz, MICROCONTROLLER, PDIP52 |
| 文件页数： | 11/80页 |
| 文件大小： | 266K |
| 代理商： | P87C770AAR |
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1999 Jun 11
11
Philips Semiconductors
Product specification
Microcontrollers for NTSC TVs with On-Screen
Display (OSD) and Closed Caption (CC)
P8xCx70 family
8
WATCHDOG TIMER (T3)
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:
T
timer
256
T3 value
–
(
)
2 ms
×
=
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.
8.1
Watchdog Timer Register (WDT)
Table 4
Watchdog Timer Register (SFR address FFH)
Table 5
Description of the T3 bits
7
6
5
4
3
2
1
0
D7
D6
D5
D4
D3
D2
D1
D0
BIT
SYMBOL
DESCRIPTION
7 to 0
D7 to D0
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.
handbook, full pagewidth
MGL298
INTERNAL BUS
1/12 fosc
PRESCALER
11-BIT
WDT REGISTER
(8-BIT)
LOAD
CLEAR
LOADEN
write T3
LOADEN
PCON.4
PCON.0
CLEAR
WLE
IDL
internal reset
INTERNAL BUS
RESET
RRESET
Fig.4 Watchdog Timer block diagram.
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