at89ls51-16si ATMEL Corporation, at89ls51-16si Datasheet - Page 10
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at89ls51-16si
Manufacturer Part Number
at89ls51-16si
Description
At89ls51, Low-voltage Microcontroller With 4k Bytes In-system Programmable Flash
Manufacturer
ATMEL Corporation
Datasheet
1.AT89LS51-16SI.pdf
(32 pages)
7.1
7.2
8. UART
9. Timer 0 and 1
10
Using the WDT
WDT During Power-down and Idle
AT89LS51
To enable the WDT, a user must write 01EH and 0E1H in sequence to the WDTRST register
(SFR location 0A6H). When the WDT is enabled, the user needs to service it by writing 01EH
and 0E1H to WDTRST to avoid a WDT overflow. The 14-bit counter overflows when it reaches
16383 (3FFFH), and this will reset the device. When the WDT is enabled, it will increment every
machine cycle while the oscillator is running. This means the user must reset the WDT at least
every 16383 machine cycles. To reset the WDT the user must write 01EH and 0E1H to
WDTRST. WDTRST is a write-only register. The WDT counter cannot be read or written. When
WDT overflows, it will generate an output RESET pulse at the RST pin. The RESET pulse dura-
tion is 98xTOSC, where TOSC=1/FOSC. To make the best use of the WDT, it should be
serviced in those sections of code that will periodically be executed within the time required to
prevent a WDT reset.
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 interrupt, 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 does whenever the AT89LS51 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 for
the interrupt used to exit Power-down mode.
To ensure that the WDT does not overflow within a few states of exiting Power-down, it is best to
reset the WDT just before entering Power-down mode.
Before going into the IDLE mode, the WDIDLE bit in SFR AUXR is used to determine whether
the WDT continues to count if enabled. The WDT keeps counting during IDLE (WDIDLE bit = 0)
as the default state. To prevent the WDT from resetting the AT89LS51 while in IDLE mode, the
user should always set up a timer that will periodically exit IDLE, service the WDT, and reenter
IDLE mode.
With WDIDLE bit enabled, the WDT will stop to count in IDLE mode and resumes the count
upon exit from IDLE.
The UART in the AT89LS51 operates the same way as the UART in the AT89C51. For further
information on the UART operation, please click on the document link below:
Timer 0 and Timer 1 in the AT89LS51 operate the same way as Timer 0 and Timer 1 in the
AT89C51. For further information on the timers’ operation, please click on the document link
below:
http://www.atmel.com/dyn/resources/prod_documents/DOC4316.PDF
http://www.atmel.com/dyn/resources/prod_documents/DOC4316.PDF
3053B–MICRO–3/05