
Operating Modes and On-Chip Memory
Data Sheet
M68HC11E Family — Rev. 5
44
Operating Modes and On-Chip Memory
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MOTOROLA
2.3.1  RAM and Input/Output Mapping
Hardware priority is built into RAM and I/O mapping. Registers have priority over 
RAM and RAM has priority over ROM. When a lower priority resource is mapped 
at the same location as a higher priority resource, a read/write of a location results 
in a read/write of the higher priority resource only. For example, if both the register 
block and the RAM are mapped to the same location, only the register block will be 
accessed. If RAM and ROM are located at the same position, RAM has priority. 
The fully static RAM can be used to store instructions, variables, and temporary 
data. The direct addressing mode can access RAM locations using a 1-byte 
address operand, saving program memory space and execution time, depending 
on the application. 
RAM contents can be preserved during periods of processor inactivity by two 
methods, both of which reduce power consumption. They are:
1.
In the software-based stop mode, the clocks are stopped while V
DD
 powers 
the MCU. Because power supply current is directly related to operating 
frequency in CMOS integrated circuits, only a very small amount of leakage 
exists when the clocks are stopped. 
2.
In the second method, the MODB/V
STBY
 pin can supply RAM power from a 
battery backup or from a second power supply. 
Figure 2-8
 shows a typical 
standby voltage circuit for a standard 5-volt device. Adjustments to the 
circuit must be made for devices that operate at lower voltages. Using the 
MODB/V
STBY
 pin may require external hardware, but can be justified when 
a significant amount of external circuitry is operating from V
DD
. If V
STBY
 is 
used to maintain RAM contents, reset must be held low whenever V
DD
 is 
below normal operating level. Refer to 
Section 5. Resets and Interrupts
. 
Figure 2-8. RAM Standby MODB/V
STBY
 Connections
The bootloader program is contained in the internal bootstrap ROM. This ROM, 
which appears as internal memory space at locations $BF00–$BFFF, is enabled 
only if the MCU is reset in special bootstrap mode. 
4.7 k
MAX
690
V
BATT
+
4.8-V
NiCd
V
DD
V
DD
V
OUT
TO MODB/V
STBY
OF M68HC11
F
Freescale Semiconductor, Inc.
n
.