M68HC11
REFERENCE MANUAL
ON-CHIP MEMORY
MOTOROLA
4-1
SECTION 4
ON-CHIP MEMORY
The MC68HC11A8 includes on-chip random-access memory (RAM), read-only mem-
ory (ROM), and electrically erasable programmable ROM (EEPROM) memories. The
on-chip RAM is a fully static read-write memory used for storage of variable and tem-
porary information. The MC68HC11A8 has 256 bytes of RAM; whereas other mem-
bers of the M68HC11 Family include more or less RAM (MC68HC11E9 has 512 bytes
of RAM and MC68HC11D3 has 192 bytes of RAM). Members of the M68HC11 Family
include various amounts of on-chip mask-programmed ROM. The MC68HC11A8 has
8 Kbytes of user ROM, the MC68HC11E9 has 12 Kbytes, and the MC68HC11D3 has
4 Kbytes. This ROM is used for storage of user program instructions and fixed data.
Some members of the M68HC11 Family have this internal ROM disabled, and the user
programs reside in external memories. The last major memory system on the
M68HC11 is the EEPROM. The MC68HC11A8 includes 512 bytes of EEPROM;
whereas other members of the M68HC11 Family include as much as 8.5 Kbytes of EE-
PROM. Data can be programmed into the EEPROM or erased from the EEPROM un-
der software control. No power supplies other than the normal V
DD
(5 Vdc) supply are
needed for programming or erasure of the 512 bytes of on-chip EEPROM in the
MC68HC11A8. No power supplies are required to maintain the contents of this mem-
ory. This memory is commonly used for semipermanent information such as calibra-
tion tables, personality data, or product history information. The EEPROM can also be
used for program memory; furthermore, the non-volatile nature of this EEPROM sup-
ports programs that can adapt to changing conditions.
4.1 ROM
The primary use for on-chip ROM is to hold the user’s application program instructions.
Since these instructions are programmed into the microcontroller unit (MCU) when it
is manufactured, they cannot be changed. A user develops the application program
and debugs it before ordering production MCUs. The user places an order for produc-
tion units with the pattern of instructions and data to be programmed into the on-chip
ROM. Motorola then translates this pattern into a photographic mask to be used during
processing of silicon wafers. Motorola then produces a small batch of these parts and
returns them to the customer for verification. These units are called ROM verification
units (RVUs). After customer approval of these RVUs, Motorola begins full production
of these customized MCUs. The RVUs, processed on a quick turnaround basis, are
not tested to environmental extremes because their sole purpose is to demonstrate
that the customer-requested ROM pattern was properly implemented.
The on-chip program ROM can be disabled by an EEPROM-based control bit in the
configuration control (CONFIG) register. When the program ROM is disabled, it uses
up no space in the 64-Kbyte memory space, and an external memory is used for pro-
gram instructions. ROM-less versions of the M68HC11 Family, such as the
MC68HC11A1, actually have on-chip ROM, but the ROM is disabled by the enable on-