MC68HC812A4 Datasheet, PDF (124/342 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
8.3 Generation of Chip-Selects
To use chip-selects the MCU must be in one of the expanded modes.
Each of the seven chip-selects has an address space for which it is
active — that is, when the current CPU address is in the range of that
chip-select, it becomes active. Chip-selects are generally used to reduce
or eliminate external address decode logic. These active low signals
usually are connected directly to the chip-select pin of an external
device.
8.3.1 Chip-Selects Independent of Memory Expansion
Three types of chip-selects are program memory chip-selects, other
memory chip-selects and peripheral chip-selects. Memory chip-selects
cover a medium-to-large address space. Peripheral chip-selects
(CS3–CS0) cover a small address space. The program memory
chip-select includes the vector space and is generally used with
non-volatile memory. To start the user’s program, the program
chip-select is designed to be active out of reset. This is the only
chip-select which has a functional difference from the others, so a small
memory could use a peripheral chip-select and a peripheral could use a
memory chip-select.
Figure 8-1 shows peripheral chip-selects in an expanded portion of the
memory map. Table 8-2 shows the register settings that correspond to
the example. Chip-selects CS2–CS0 always map to the same 2-Kbyte
block as the internal register space. The internal registers cover the first
512 bytes and these chip-selects cover all or part of the 512 bytes
following the register space blocking out a full 1-Kbyte space. CS3 can
map with these other chip-selects or be used in a 1-Kbyte space by itself
which starts at either $0000 or $0400. CS3 can be used only for a
1-Kbyte space when it selects the E page of memory expansion and E
page is active.
MC68HC812A4 — Rev. 3.0
124
Memory Expansion and Chip-Select
Advance Information
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