Datasheet

DS87C520/DS83C520 EPROM/ROM High-Speed Microcontrollers
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will set an interrupt flag. Regardless of whether the user enables this interrupt, there are then 512 clocks
left until the reset flag is set. Software can enable the interrupt and reset individually. Note that the
Watchdog is a free running timer and does not require an enable.
There are 5 control bits in special function registers that affect the Watchdog Timer and two status flags
that report to the user. WDIF (WDCON.3) is the interrupt flag that is set at timer termination when there
are 512 clocks remaining until the reset flag is set. WTRF (WDCON.2) is the flag that is set when the
timer has completely timed out. This flag is normally associated with a CPU reset and allows software to
determine the reset source.
EWT (WDCON.1) is the enable for the Watchdog timer reset function. RWT (WDCON.0) is the bit that
software uses to restart the Watchdog Timer. Setting this bit restarts the timer for another full interval.
Application software must set this bit before the timeout. Both of these bits are protected by Timed
Access. As mentioned previously, WD1 and 0 (CKCON .7 and 6) select the timeout. The Reset
Watchdog Timer bit (WDCON.0) should be asserted prior to modifying the Watchdog Timer Mode
Select bits (WD1, WD0) to avoid corruption of the watchdog count. Finally, the user can enable the
Watchdog Interrupt using EWDI (EIE.4). The Special Function Register map is shown above.
INTERRUPTS
The DS87C520/DS83C520 provide 13 interrupt sources with three priority levels. The Power-Fail
Interrupt (PFI) has the highest priority. Software can assign high or low priority to other sources. All
interrupts that are new to the 8051 family, except for the PFI, have a lower natural priority than the
originals.
Table 8. Interrupt Sources and Priorities
NAME FUNCTION VECTOR
NATURAL
PRIORITY
8051/DALLAS
PFI Power-Fail Interrupt 33h 1 DALLAS
INT0
External Interrupt 0 03h 2 8051
TF0 Timer 0 0Bh 3 8051
INT1
External Interrupt 1 13h 4 8051
TF1 Timer 1 1Bh 5 8051
SCON0 TI0 or RI0 from serial port 0 23h 6 8051
TF2 Timer 2 2Bh 7 8051
SCON1 TI1 or RI1 from serial port 1 3Bh 8 DALLAS
INT2 External Interrupt 2 43h 9 DALLAS
INT3
External Interrupt 3 4Bh 10 DALLAS
INT4 External Interrupt 4 53h 11 DALLAS
INT5
External Interrupt 5 5Bh 12 DALLAS
WDTI Watchdog Timeout Interrupt 63h 13 DALLAS