COP888CL Datasheet, PDF (26/42 Pages) Texas Instruments – COP888CL 8-Bit Microcontroller

WATCHDOG Operation (Continued)
• 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 Watchdog
should not be serviced for at least 2048 instruction cycles
following IDLE, but must be serviced within the selected
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 WATCHDOG
service may be programmed within the initial 2048 in-
struction 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
software 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
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 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). The recovery program should reset the
software interrupt pending bit using the RPND instruction.
MICROWIRE/PLUS
MICROWIRE/PLUS is a serial synchronous communications
interface. The MICROWIRE/PLUS capability enables the
device to interface with any of National Semiconductor’s
MICROWIRE peripherals (i.e. A/D converters, display driv-
ers, E²PROMs etc.) and with other microcontrollers which
support the MICROWIRE 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 13 shows
a block diagram of the MICROWIRE 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 MI-
CROWIRE arrangement with an external shift clock is called
the Slave mode of operation.
DS009766-20
FIGURE 13. MICROWIRE/PLUS Block Diagram
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 IV details the
different clock rates that may be selected.
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 14 shows how
two COP888CL microcontrollers and several peripherals
may be interconnected using the MICROWIRE/PLUS ar-
rangements.
Warning:
The SIO register should only be loaded when the SK clock is
low. Loading the SIO register while the SK clock is high will
result in undefined data in the SIO register. The SK clock is
normally low when not shifting.
Setting the BUSY flag when the input SK clock is high in the
MICROWIRE/PLUS slave mode may cause the current SK
clock for the SIO shift register to be narrow. For safety, the
BUSY flag should only be set when the input SK clock is low.
MICROWIRE/PLUS Master Mode Operation
In the MICROWIRE/PLUS Master mode of operation the
shift clock (SK) is generated internally. The MICROWIRE
Master always initiates all data exchanges. The MSEL bit in
the CNTRL register must be set to enable the SO and SK
functions onto the G Port. The SO and SK pins must also be
selected as outputs by setting appropriate bits in the Port G
configuration register. Table V summarizes the bit settings
required for Master mode of operation.
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