MC68HC812A4 Datasheet, PDF (135/342 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
Chip-Selects
8.6 Chip-Selects
The chip-selects are all active low. All pins in the associated port are
pulled up when they are inputs and the PUPF bit in PUCR is set.
If memory expansion is used, usually chip-selects should be used as
well, since some translated addresses can be confused with
untranslated addresses that are not in an expansion window.
In single-chip modes, enabling the chip-select function does not affect
the associated pins.
The block of register-following chip-selects CS3–CS0 allows many
combinations including:
• 512-byte CS0
• 256-byte CS0 and 256-byte CS1
• 256-byte CS0, 128-byte CS1, and 128-byte CS2
• 128-byte CS0, 128-byte CS1, 128-byte CS2, and 128-byte CS3
These register-following chip-selects are available in the 512-byte space
next to and higher in address than the 512-byte space which includes
the registers. For example, if the registers are located at $0800 to
$09FF, then these register-following chip-selects are available in the
space from $0A00 to $0BFF.
Advance Information
MOTOROLA
Memory Expansion and Chip-Select
MC68HC812A4 — Rev. 3.0
135