cop888cf National Semiconductor Corporation, cop888cf Datasheet - Page 31

• Manufacturer Part Number: cop888cf
• Description: 8-bit Cmos Rom Based Microcontrollers With 4k Memory And A/d Converter
• Manufacturer: National Semiconductor Corporation

WATCHDOG Operation
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
• Following RESET, the WATCHDOG and CLOCK MONI-
• The WATCHDOG service window and Clock Monitor
• The initial WATCHDOG service must match the key data
• Subsequent WATCHDOG services must match all three
• The correct key data value cannot be read from the
• The WATCHDOG detector circuit is inhibited during both
• The Clock Monitor detector circuit is active during both
• With the single-pin R/C oscillator mask option selected
• With the crystal oscillator mask option selected, or with
• The IDLE timer T0 is not initialized with RESET.
• The user can sync in to the IDLE counter cycle with an
are inhibited during RESET.
TOR are both enabled, with the WATCHDOG having the
maximum service window selected.
enable/disable option can only be changed once, during
the initial WATCHDOG service following RESET.
value in the WATCHDOG Service register WDSVR in or-
der to avoid a WATCHDOG error.
data fields in WDSVR in order to avoid WATCHDOG er-
rors.
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 HALT and IDLE modes.
the HALT and IDLE modes. Consequently, the device in-
advertently entering the HALT mode will be detected as a
Clock Monitor error (provided that the Clock Monitor en-
able option has been selected by the program).
and the CLKDLY bit reset, the WATCHDOG service win-
dow will resume following HALT mode from where it left
off before entering the HALT mode.
the single-pin R/C oscillator mask option selected and the
CLKDLY bit set, the WATCHDOG service window will be
set to its selected value from WDSVR following HALT.
Consequently, 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
WATCHDOG error.
IDLE counter (T0) interrupt or by monitoring the T0PND
flag. The T0PND flag is set whenever the thirteenth bit of
the IDLE counter toggles (every 4096 instruction cycles).
The user is responsible for resetting the T0PND flag.
Don’t Care
Don’t Care
Mismatch
Match
Data
Key
Don’t Care
Don’t Care
Mismatch
Window
Match
Data
(Continued)
TABLE 6. WATCHDOG Service Actions
Don’t Care
Don’t Care
Mismatch
Monitor
Clock
Match
31
Valid Service: Restart Service Window
Error: Generate WATCHDOG Output
Error: Generate WATCHDOG Output
Error: Generate WATCHDOG Output
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 zeros. The opcode for soft-
ware interrupt is zero. 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 Hex is read as all 1’s, which in turn will
cause the program to return to address 7FFF Hex. This is an
undefined ROM location and the instruction fetched (all 0’s)
from this location will generate a software interrupt signaling
an illegal condition.
Thus, the chip can detect the following illegal conditions:
1. Executing from undefined ROM
2. Over “POP”ing the stack by having more returns than
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).
• A hardware WATCHDOG service occurs just as the de-
• Following RESET, the initial WATCHDOG service (where
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.
the service window and the CLOCK MONITOR enable/
disable must be selected) may be programmed anywhere
within the maximum service window (65,536 instruction
cycles) initialized by RESET. Note that this initial WATCH-
DOG service may be programmed within the initial 2048
instruction cycles without causing a WATCHDOG error.
calls.
Action
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