
MOTOROLA
3-6
CONFIGURATION AND MODES OF OPERATION
M68HC11
REFERENCE MANUAL
of an MC68HC11A1 or MC68HC11A0 device is erased to $0F, the internal ROM and
EEPROM memories become enabled but are not necessarily useful. The ROM of an
MC68HC11A1 or MC68HC11A0 part may contain a customer’s program (with their
permission) or a defective program. The EEPROM of an MC68HC11A0 part could be
partially/completely broken and should not be used because the error could be related
to temperature or voltage. Therefore, the EEPROM might check as flawless but later
fail when least expected. The upper four bits are not implemented in the working static
register and always read zero. Although the corresponding bits in the EEPROM byte
are implemented, they are not visible to the user.
The erased state of the CONFIG register in the MC68HC811A2 version is $FF, which
means the 2-Kbyte EEPROM is enabled in the area from $F800–$FFFF when the part
comes from the Motorola factory. To use the part, the user must have a meaningful
reset vector at $FFFE,FFFF or must connect the mode pins so the system will come
out of reset in one of the special modes. The reset vector can be programmed into the
internal EEPROM before installing the part into a finished system, or the EEPROM can
be moved out of the way (by programming the CONFIG register) so an external mem-
ory in the end system can provide the reset vector.
3.3 Protected Control Register Bits
In the MC68HC11A8, several sensitive control registers and bits are protected against
writes except under special circumstances. The protect mechanisms include the ability
to write these bits only within the first 64 bus cycles after any reset and/or the ability to
write them only one time after each reset. These bits control the basic configuration of
the MCU where an accidental write could cause serious system problems — that is,
these protections make it practical to include software-controlled features that might
otherwise be excluded. As new members of the M68HC11 Family are developed, ad-
ditional control bits could fall into this category, but in the MC68HC11A8, only three
control registers are involved (INIT, TMSK2, and OPTION). Some users have ex-
pressed concern about being able to write all of these control bits within 64 cycles,
which will not be a problem since only three writes are required.
Because these protect mechanisms are overridden in the special operating modes,
these bits may be changed repeatedly during testing without going through a reset se-
quence. If the MCU is going to be changed to a normal mode variation after being reset
in a special mode, write to the protected registers before writing the SMOD control bit
to zero.
3.3.1 RAM and I/O Mapping Register (INIT)
RAM[3:0] — RAM Map Position
These four bits, which specify the upper hexadecimal digit of the RAM address, control
INIT —
RAM and I/O Mapping Register
$103D
BIT 7
RAM3
0
6
5
4
3
2
1
BIT 0
REG0
1
RAM2
0
RAM1
0
RAM0
0
REG3
0
REG2
0
REG1
0
RESET: