Maxim DS87C530 Clock User Manual


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DS87C530/DS83C530 EPROM/ROM Microcontrollers with Real-Time Clock
27 of 47
There are five 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 discussed below. 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).
INTERRUPTS
The DS87C530/DS83C530 provide 14 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
RTCI RTC Interrupt 6Bh 14 DALLAS
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