DS87C520 datasheet(21/42 Pages) DALLAS

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DS87C520/DS83C520

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by a “Timed Access” circuit. This prevents errant software from accidentally clearing the Watchdog.

Time-out values are precise since they are a function of the crystal frequency as shown in Table 8. For

reference, the time periods at 33 MHz also are shown.

The Watchdog also provides a useful option for systems that do not require a reset circuit. It will set an

interrupt flag 512 clocks before setting the reset flag. Software can optionally enable this interrupt source.

The interrupt is independent of the reset. A common use of the interrupt is during debug, to show

developers where the Watchdog times out. This indicates where the Watchdog must be restarted by

software. The interrupt also can serve as a convenient time-base generator or can wake-up the processor

from power saving modes.

The Watchdog function is controlled by the Clock Control (CKCON-8Eh), Watchdog Control (WDCON-

D8h), and Extended Interrupt Enable (EIE-E8h) SFRs. CKCON.7 and CKCON.6 are WD1 and WD0

respectively and they select the Watchdog time-out period as shown in Table 8.

WATCHDOG TIME-OUT VALUES Table 8

WD1

WD2

INTERRUPT

TIME-OUT

TIME (33 MHz)

RESET TIME-OUT

TIME (33 MHz)

17 clocks

3.9718 ms

17 + 512 clocks

3.9874 ms

20 clocks

31.77 ms

20 + 512 clocks

31.79 ms

23 clocks

254.20 ms

23 + 512 clocks

254.21 ms

26 clocks

2033.60 ms

26 + 512 clocks

2033.62 ms

As shown above, the Watchdog Timer uses the crystal frequency as a time base. A user selects one of

four counter values to determine the time-out. These clock counter lengths are 2

17 =131,072 clocks; 220

=1,048,576; 2

23=8,388,608 clocks; and 226 =67,108,864 clocks. The times shown in Table 8 above are

with a 33 MHz crystal frequency. Once the counter chain has completed a full interrupt count, hardware

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 interrup t 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 time-out. Both of these bits are protected by Timed

Access discussed below. As mentioned previously, WD1 and 0 (CKCON .7 and 6) select the time-out.

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 provides 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.

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