
M68HC11
REFERENCE MANUAL
CONFIGURATION AND MODES OF OPERATION
MOTOROLA
3-3
192-byte, mask-programmed ROM contains the firmware required to load a user’s
program through the SCI into the internal RAM and jump to the loaded program. In all
modes other than the special bootstrap mode, this ROM is disabled and does not oc-
cupy any space in the 64 Kbyte memory map. Although it is zero when the MCU
comes out of reset in test mode, the RBOOT bit may be written to one while in special
test mode.
SMOD — Special Mode
May be written to zero but not back to one
1 = Special mode variation in effect
0 = Normal mode variation in effect
MDA — Mode A Select
Can be written only while SMOD equals one
1 = Normal expanded or special test mode in effect
0 = Normal single-chip or special bootstrap mode in effect
IRV — Internal Read Visibility
Can be written only while SMOD equals one; forced to zero if SMOD equals zero
1 = Data driven onto external bus during internal reads
0 = Data from internal reads not visible on expansion bus (levels on bus ignored)
The IRV control bit is used during factory testing and sometimes during emulation to
allow internal read accesses to be visible on the external data bus. Care is required to
avoid data bus contention while IRV is active because the bidirectional data bus is driv-
en out during reads of internal addresses, even though the R/W line suggests the data
bus is in the high-impedance read mode. In normal modes, this function is disabled;
thus, complex decode logic is not required to protect against accidental bus conflicts.
3.2 EEPROM-Based CONFIG Register
The nonvolatile configuration (CONFIG) register allows additional flexibility in the MCU
that would otherwise be provided by a more complex hardware mode select structure.
By using EEPROM to implement the CONFIG register, these system controls are re-
tained even when no power is applied to the MCU. The functions controlled by this reg-
ister are characteristics that must be inherently known to the MCU system as it comes
out of the reset state. Ordinary software-accessible control bits would not effectively
regulate these controls.
3.2.1 Operation of CONFIG Mechanism
The CONFIG register actually consists of an EEPROM byte (separate from the 512-
byte EEPROM array), a static register that holds the configuration information during
operation, and the associated logic, which controls the transfer of information from the
EEPROM byte to the working static register. Programming and erasure of this register
use the same logic used for programming and erasure of the 512-byte EEPROM ar-
ray. Reads of this register return the contents of the static working register, not the EE-
PROM byte. During any reset, the contents of the EEPROM byte are transferred to the
working static register over the data bus. Due to this mechanism, changes to the EE-
PROM CONFIG location are not visible and do not alter the operation of the MCU until