AT89C51RB2-RLTCM ATMEL [ATMEL Corporation], AT89C51RB2-RLTCM Datasheet - Page 76

• Manufacturer Part Number: AT89C51RB2-RLTCM
• Description: 8-bit Microcontroller with 16K/ 32K Bytes Flash
• Manufacturer: ATMEL [ATMEL Corporation]

WDT During Power-down
and Idle
76
AT89C51RB2/RC2
Table 60. 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 two 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
AT89C51RB2/RC2 is reset. Exiting Power-down with an interrupt is significantly differ-
ent. 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 power-down,
it is better to reset the WDT just before entering power-down.
In the Idle mode, the oscillator continues to run. To prevent the WDT from resetting the
AT89C51RB2/RC2 while in Idle mode, the user should always set up a timer that will
periodically 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
S2
0
0
0
0
1
1
1
1
S1
5
0
0
1
1
0
0
1
1
-
S0
0
1
0
1
0
1
0
1
Selected Time-out
(2
(2
(2
(2
(2
(2
(2
(2
4
-
14
15
16
17
18
19
20
21
- 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
S1
1
= 12 MHz
= 12 MHz
= 12 MHz
= 12 MHz
= 12 MHz
= 12 MHz
4180B–8051–04/03
S0
0