
Technical Data
MC68HC11E Family
—
Rev. 4
80
Operating Modes and On-Chip Memory
MOTOROLA
Operating Modes and On-Chip Memory
4.4.1 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 4-8
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
.