COP8SE datasheet(30/47 Pages) NSC

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9.0 WATCHDOG/Clock Monitor (Continued)

TABLE 7. WATCHDOG Service Actions

Key

Window

Clock

Action

Data

Monitor

Match

Valid Service: Restart Service Window

Don’t Care

Mismatch

Don’t Care

Error: Generate WATCHDOG Output

Mismatch

Don’t Care

Error: Generate WATCHDOG Output

Don’t Care

Mismatch

Error: Generate WATCHDOG Output

9.3 WATCHDOG AND CLOCK MONITOR SUMMARY

The following salient points regarding the WATCHDOG and

CLOCK MONITOR should be noted:

•

Both the WATCHDOG and CLOCK MONITOR detector

circuits are inhibited during RESET.

•

Following RESET, the WATCHDOG and CLOCK MONI-

TOR are both enabled, with the WATCHDOG having the

maximum service window selected.

•

The WATCHDOG service window and CLOCK MONI-

TOR enable/disable option can only be changed once,

during the initial WATCHDOG service following RESET.

•

The initial WATCHDOG service must match the key data

value in the WATCHDOG Service register WDSVR in or-

der to avoid a WATCHDOG error.

•

Subsequent WATCHDOG services must match all three

data fields in WDSVR in order to avoid WATCHDOG er-

rors.

•

The correct key data value cannot be read from the

WATCHDOG Service register WDSVR. Any attempt to

read this key data value of 01100 from WDSVR will read

as key data value of all 0’s.

•

The WATCHDOG detector circuit is inhibited during both

the HALT and IDLE modes.

•

The CLOCK MONITOR detector circuit is active during

both the HALT and IDLE modes. Consequently, the de-

vice inadvertently entering the HALT mode will be de-

tected as a CLOCK MONITOR error (provided that the

CLOCK MONITOR enable option has been selected by

the program).

•

With the single-pin R/C oscillator option selected and the

CLKDLY bit reset, the WATCHDOG service window will

resume following HALT mode from where it left off before

entering the HALT mode.

•

With the crystal oscillator option selected, or with the

single-pin R/C oscillator option selected and the CLKDLY

bit set, the WATCHDOG service window will be set to its

selected value from WDSVR following HALT. Conse-

quently, the WATCHDOG should not be serviced for at

least 2048 instruction cycles following HALT, but must be

serviced within the selected window to avoid a WATCH-

DOG error.

•

The IDLE timer T0 is not initialized with external RESET.

•

The user can sync in to the IDLE counter cycle with an

IDLE counter (T0) interrupt or by monitoring the T0PND

flag. The T0PND flag is set whenever the selected bit of

the IDLE counter toggles (every 4, 8, 16, 32 or 64k in-

struction cycles). The user is responsible for resetting the

T0PND flag.

•

A hardware WATCHDOG service occurs just as the de-

vice exits the IDLE mode. Consequently, the WATCH-

DOG should not be serviced for at least 2048 instruction

cycles following IDLE, but must be serviced within the se-

lected window to avoid a WATCHDOG error.

•

Following RESET, the initial WATCHDOG service (where

the service window and the CLOCK MONITOR enable/

disable must be selected) may be programmed any-

where within the maximum service window (65,536 in-

struction cycles) initialized by RESET. Note that this initial

WATCHDOG service may be programmed within the ini-

tial 2048 instruction cycles without causing a WATCH-

DOG error.

9.4 DETECTION OF ILLEGAL CONDITIONS

The device can detect various illegal conditions resulting

from coding errors, transient noise, power supply voltage

drops, runaway programs, etc.

Reading of undefined ROM gets zeroes. The opcode for

software interrupt is 00. If the program fetches instructions

from undefined ROM, this will force a software interrupt, thus

signaling that an illegal condition has occurred.

The subroutine stack grows down for each call (jump to sub-

routine), interrupt, or PUSH, and grows up for each return or

POP. The stack pointer is initialized to RAM location 06F Hex

during reset. Consequently, if there are more returns than

calls, the stack pointer will point to addresses 070 and 071

Hex (which are undefined RAM). Undefined RAM from ad-

dresses 070 to 07F (Segment 0), and all other segments

(i.e., Segments 4 … etc.) is read as all 1’s, which in turn will

cause the program to return to address 7FFF Hex. It is rec-

ommended that the user either leave this location unpro-

grammed or place an INTR instruction (all 0’s) in this location

to generate a software interrupt signaling an illegal condition.

Thus, the chip can detect the following illegal conditions:

Executing from undefined ROM.

Over “POP”ing the stack by having more returns than

calls.

When the software interrupt occurs, the user can re-initialize

the stack pointer and do a recovery procedure before restart-

ing (this recovery program is probably similar to that follow-

ing reset, but might not contain the same program initializa-

tion procedures). The recovery program should reset the

software interrupt pending bit using the RPND instruction.

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