
M68HC11
REFERENCE MANUAL
PINS AND CONNECTIONS
MOTOROLA
2-37
2.7 System Development and Debug Features
The designers of the M68HC11 carefully considered the system development needs
of the user. Since smaller users cannot afford thousands of dollars for a development
system, the M68HC11 was specifically designed to accommodate low-cost develop-
ment tools. The M68HC11 EVB evaluation board and M68HC11 EVM evaluation mod-
ule are two examples of such low-cost tools. Several customers have also built small
plug-in modules that emulate the MC68HC11A8 for product development purposes.
The small size of these plug-in emulators is possible because of the development fea-
tures designed into the M68HC11.
2.7.1 Load Instruction Register (LIR)
The LIR signal is intended as a debugging aid. This signal is driven to active low for
the first bus cycle of each new instruction, making it easy to reverse assemble (disas-
semble) instructions from the display of a logic analyzer.
2.7.2 Internal Read Visibility (IRV)
During debugging of an application, it is useful to see what is being read from internal
registers and memory locations. The IRV feature provides this capability. This feature
should usually be disabled during normal operation of the system due to the possibility
of bus conflicts.
The IRV feature is controlled by the IRV bit in the HPRIO register. When the IRV bit is
one, the data from a read of an internal register or memory location is driven out on
the data bus so it can be monitored by a logic analyzer. If the IRV bit is zero, the IRV
function is disabled, and the data bus is undriven during reads of an internal address.
Special restrictions apply to the use of the IRV bit and function. When the MCU is reset
in normal modes, the IRV bit is initially zero. In all but the newest derivatives in the
M68HC11 Family, the IRV bit may not be written to one in the normal modes. In special
test and bootstrap modes, the IRV bit is initially one and may be written to zero after
which it becomes a read-only bit.
Care should be used if the IRV function is enabled. During reads of an internal ad-
dress, the data bus is driven out even though the R/W line indicates that the bus direc-
tion is toward the MCU. Some external device may also be trying to drive the data
lines, which leads to an undesirable bus contention. In a test or debugging situation,
special address decode logic can be used to prevent such contention. It would be ex-
pensive and inappropriate to have this additional decode logic on all normal mode sys-
tems; thus, the IRV function was only provided in the special test and bootstrap
modes. Due to several customer requests for the IRV function in normal modes, the
logic was changed to allow the function to be enabled in normal modes on new ver-
sions of the M68HC11. The default condition in normal modes is still IRV equals zero,
which disables the function. If a user specifically wants the IRV function, IRV may be
written to one, and the user becomes responsible for avoiding bus contentions. IRV
can be written to one at any time unless it has previously been written to zero. If the
IRV bit is written to zero, the function becomes disabled until the next reset sequence.