MC68HC812A4 Datasheet, PDF (122/342 Pages) Motorola, Inc – 16-bit device composed of standard on-chip peripheral modules connected by an intermodule bus. Modules include

Memory Expansion and Chip-Select
8.2 Introduction
To use memory expansion, the MCU must be operated in one of the
expanded modes. Sections of the standard 64-Kbyte address space
have memory expansion windows which allow an external address
space larger than 64 Kbytes. Memory expansion consists of three
memory expansion windows and six address lines which are used in
addition to the standard 16 address lines.
The memory expansion function reuses as many as six of the standard
16 address lines. To do this, some of the upper address lines of internal
addresses falling in an active window are overridden. Consequently, the
address viewed externally may not match the internal address. Usage of
chip-selects identify the source of the internal address for debugging and
selection of the proper external devices.
All memory expansion windows have a fixed size and two have a fixed
address location. The third has two selectable address locations. When
an internal address falls into one of these active windows, it is translated
as shown in Table 8-1.
Addresses ADDR9–ADDR0 are not affected by memory expansion and
are the same externally as they are internally. Addresses
ADDR21–ADDR16 are generated only by memory expansion and are
individually enabled by software-programmable control bits. If not
enabled, they may be used as general-purpose I/O (input/output).
Addresses ADDR15–ADDR10 can be the internal addresses or they can
be modified by the memory expansion module. These are not available
as general-purpose I/O in expanded modes.
MC68HC812A4 — Rev. 3.0
122
Memory Expansion and Chip-Select
Advance Information
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