cop8tab5 National Semiconductor Corporation, cop8tab5 Datasheet - Page 33

• Manufacturer Part Number: cop8tab5
• Description: 8-bit Cmos Rom Microcontroller With 2k Or 4k Memory
• Manufacturer: National Semiconductor Corporation

14.0 WATCHDOG/Clock Monitor
14.4 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
subroutine), 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 re-
turns than calls, the stack pointer will point to addresses 070
and 071 Hex (which are undefined RAM). Undefined RAM
from addresses 070 to 07F (Segment 0), and all other seg-
ments (i.e., Segments 4 … etc.) is read as all 1’s, which in
turn will cause the program to return to address 7FFF Hex.
• The CLOCK MONITOR detector circuit is active during
• With the single-pin R/C oscillator option selected and the
• With the crystal oscillator option selected, or with the
• The IDLE timer T0 is not initialized with external RESET.
• The user can sync in to the IDLE counter cycle with an
• A hardware WATCHDOG service occurs just as the de-
• Following RESET, the initial WATCHDOG service (where
• In order to RESET the device on the occurrence of a
(Continued)
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).
CLKDLY bit reset, the WATCHDOG service window will
resume following HALT mode from where it left off before
entering the HALT mode.
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 256 instruction cycles following HALT, but must be
serviced within the selected window to avoid a WATCH-
DOG error.
IDLE counter (T0) interrupt or by monitoring the T0PND
flag. The T0PND flag is set whenever the twelfth bit of the
IDLE counter toggles (every 4096 instruction cycles). The
user is responsible for resetting the T0PND flag.
vice exits the IDLE mode. Consequently, the WATCH-
DOG should not be serviced for at least 256 instruction
cycles following IDLE, but must be serviced within the
selected window to avoid a WATCHDOG error.
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 256 instruction cycles without causing a WATCHDOG
error.
WATCH event, the user must connect the WDOUT pin
(G1) pin to the RESET external to the device. The weak
pull-up on the WDOUT pin is sufficient to provide the
RESET connection to V
Power On Reset and WATCHDOG.
CC
for devices which use both
33
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). The recovery program should reset the
software interrupt pending bit using the RPND instruction.
15.0 MICROWIRE/PLUS
MICROWIRE/PLUS is a serial SPI compatible synchronous
communications interface. The MICROWIRE/PLUS capabil-
ity enables the device to interface with MICROWIRE/PLUS
or SPI peripherals (i.e. A/D converters, display drivers, EE-
PROMs etc.) and with other microcontrollers which support
the MICROWIRE/PLUS or SPI interface. It consists of an
8-bit serial shift register (SIO) with serial data input (SI),
serial data output (SO) and serial shift clock (SK). Figure 24
shows a block diagram of the MICROWIRE/PLUS logic.
The shift clock can be selected from either an internal source
or an external source. Operating the MICROWIRE/PLUS
arrangement with the internal clock source is called the
Master mode of operation. Similarly, operating the
MICROWIRE/PLUS arrangement with an external shift clock
is called the Slave mode of operation.
The CNTRL register is used to configure and control the
MICROWIRE/PLUS mode. To use the MICROWIRE/PLUS,
the MSEL bit in the CNTRL register is set to one. In the
master mode, the SK clock rate is selected by the two bits,
SL0 and SL1, in the CNTRL register. Table 11 details the
different clock rates that may be selected.
Where t
15.1 MICROWIRE/PLUS OPERATION
Setting the BUSY bit in the PSW register causes the
MICROWIRE/PLUS to start shifting the data. It gets reset
when eight data bits have been shifted. The user may reset
the BUSY bit by software to allow less than 8 bits to shift. If
enabled, an interrupt is generated when eight data bits have
been shifted. The device may enter the MICROWIRE/PLUS
mode either as a Master or as a Slave. Figure 24 shows how
two microcontroller devices and several peripherals may be
interconnected using the MICROWIRE/PLUS arrangements.
The SIO register should only be loaded when the SK clock is
in the idle phase. Loading the SIO register while the SK clock
is in the active phase, will result in undefined data in the SIO
register.
Setting the BUSY flag when the input SK clock is in the
active phase while in the MICROWIRE/PLUS is in the slave
calls.
C
SL1
is the instruction cycle clock
0
0
1
TABLE 11. MICROWIRE/PLUS
Master Mode Clock Select
SL0
WARNING
0
1
x
2 x t
4 x t
8 x t
C
C
C
SK Period
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