M68HC11
REFERENCE MANUAL
CONFIGURATION AND MODES OF OPERATION
MOTOROLA
3-11
tailed information on the use of the expansion bus, including a discussion of an ex-
panded-system example.
For emulation purposes, there is a special companion chip called the MC68HC24 port
replacement unit (PRU). This device reconstructs the parallel I/O functions that are
lost to the 18 expansion bus lines. Software developed on an expanded system, which
includes an MC68HC24, can later be submitted as a masked ROM pattern. The result-
ing custom-ROM part can then be operated in the single-chip mode, and all parallel I/
O functions will work as they did in the expanded system. Usually, the MC68HC24
companion chip would not be used as a general-purpose, peripheral I/O chip because
cheaper ways exist to add general-purpose I/O to an expanded system.
3.5 Special MCU Operating Modes
The special mode variations are selected by having a logic zero on the MODB pin dur-
ing reset. In the special mode variations, the reset and interrupt vectors are located at
$BFC0–$BFFF, and software has access to special test features. One of these special
test features (the disable resets (DISR) control bit in the TEST1 control register) tem-
porarily disables the COP watchdog and clock monitor reset functions. All the special
functions and privileges are available in the special test mode and special bootstrap
mode.
Since the reset vectors are located at $BFFE,BFFF, the internal 8-Kbyte ROM cannot
interfere with the vectors. The expanded special test mode assures that the reset vec-
tor is fetched from external memory even if the internal 8-Kbyte ROM is enabled. In
special bootstrap mode, an on-chip bootloader firmware ROM is enabled at addresses
$BF40–$BFFF so the reset vector is fetched from this internal ROM.
The SMOD control bit is latched as logic one when the MCU is reset in the special
modes. While SMOD is a one, special test functions and privileges are available.
RBOOT and MDA can be turned on or off, and SMOD and IRV can be turned off but
not back on. Thus, the operating mode of the MCU can be changed, but once the
mode is changed to a normal mode (SMOD = 0), the privileges are revoked. An impor-
tant, often overlooked application of this privilege is the ability to reset the MCU in
bootstrap mode, which is a single-chip mode, then change the MDA bit to one to en-
able the multiplexed expansion bus.
On present mask sets of the MC68HC11A8 (B96D and newer), the SMOD bit must be
set to one to allow programming of the EEPROM-based CONFIG register. In some
M68HC11 Family members, the EEPROM-based CONFIG register can be written in
special modes as if it were an ordinary static register. This privilege is not available in
the original MC68HC11A8 but is present in the MC68HC811A2.
Another group of control bits in the MCU have special protection mechanisms to pre-
vent accidental writes while operating in normal modes. These protections include
write permission only within the first 64 E-clock cycles after reset and/or the ability to
write these bits only one time. While in either special mode, these protections are over-
ridden, and these control bits may be written as if they were ordinary control bits. For
a detailed description of these protection mechanisms, see
3.3 Protected Control