COP87L88CF Datasheet, PDF (27/40 Pages) National Semiconductor (TI) – 8-Bit CMOS OTP Microcontrollers with 16k Memory and

WATCHDOG Operation (Continued)
Key
Data
Match
Don’t Care
Mismatch
Don’t Care
TABLE 6. WATCHDOG Service Actions
Window
Data
Match
Mismatch
Don’t Care
Don’t Care
Clock
Monitor
Match
Don’t Care
Don’t Care
Mismatch
Action
Valid Service: Restart Service Window
Error: Generate WATCHDOG Output
Error: Generate WATCHDOG Output
Error: Generate WATCHDOG Output
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 Monitor
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 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).
• With the single-pin R/C oscillator mask option selected
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.
• With the crystal oscillator mask option selected, or with
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.
• The IDLE timer T0 is not initialized with 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 thirteenth bit of
the IDLE counter toggles (every 4096 instruction 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 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.
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
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).
MICROWIRE/PLUS
MICROWIRE/PLUS is a serial synchronous communications
interface. The MICROWIRE/PLUS capability enables the de-
vice to interface with any of National Semiconductor’s MI-
CROWIRE peripherals (i.e. A/D converters, display drivers,
E²PROMs etc.) and with other microcontrollers which sup-
port the MICROWIRE interface. It consists of an 8-bit serial
shift register (SIO) with serial data input (SI), serial data out-
put (SO) and serial shift clock (SK). Figure 16 shows a block
diagram of the MICROWIRE/PLUS logic.
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