at89c51re2-slsem ATMEL Corporation, at89c51re2-slsem Datasheet - Page 126

• Manufacturer Part Number: at89c51re2-slsem
• Description: At89c51re2 8-bit Flash Microcontroller
• Manufacturer: ATMEL Corporation

WDT During Power Down
and Idle
7663B–8051–03/07
Table 87. WDTPRG Register
WDTPRG - Watchdog Timer Out Register (0A7h)
Reset value = XXXX X000
In Power Down mode the oscillator stops, which means the WDT also stops. While in
Power Down mode the user does not need to service the WDT. There are 2 methods of
exiting Power Down mode: by a hardware reset or via a level activated external inter-
rupt which is enabled prior to entering Power Down mode. When Power Down is exited
with hardware reset, servicing the WDT should occur as it normally should whenever the
AT89C51RE2 is reset. Exiting Power Down with an interrupt is significantly different.
The interrupt is held low long enough for the oscillator to stabilize. When the interrupt is
brought high, the interrupt is serviced. To prevent the WDT from resetting the device
while the interrupt pin is held low, the WDT is not started until the interrupt is pulled high.
It is suggested that the WDT be reset during the interrupt service routine.
To ensure that the WDT does not overflow within a few states of exiting of powerdown, it
is better to reset the WDT just before entering powerdown.
In the Idle mode, the oscillator continues to run. To prevent the WDT from resetting the
AT89C51RE2 while in Idle mode, the user should always set up a timer that will periodi-
cally exit Idle, service the WDT, and re-enter Idle mode.
Number
Bit
7
-
7
6
5
4
3
2
1
0
Mnemonic Description
Bit
S2
S1
S0
6
-
-
-
-
-
-
Reserved
The value read from this bit is undetermined. Do not try to set this bit.
WDT Time-out select bit 2
WDT Time-out select bit 1
WDT Time-out select bit 0
S2S1 S0
0
0
0
0
1
1
1
1
0
0
1
1
0
0
1
1
5
-
0 (2
1 (2
0 (2
1 (2
0 (2
1 (2
0 (2
1 (2
Selected Time-out
14
15
16
17
18
19
20
21
4
-
- 1) machine cycles, 16. 3 ms @ F
- 1) machine cycles, 32.7 ms @ F
- 1) machine cycles, 65. 5 ms @ F
- 1) machine cycles, 131 ms @ F
- 1) machine cycles, 262 ms @ F
- 1) machine cycles, 542 ms @ F
- 1) machine cycles, 1.05 s @ F
- 1) machine cycles, 2.09 s @ F
3
-
S2
2
OSCA
OSCA
OSCA
OSCA
OSCA
OSCA
OSCA
OSCA
=12 MHz
=12 MHz
=12 MHz
=12 MHz
=12 MHz
=12 MHz
=12 MHz
=12 MHz
S1
1
S0
0
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