MC68HC812A4 Datasheet, PDF (126/342 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
Table 8-2. Example Register Settings (Continued)
Register Value
Meaning
CSCTL0
$xF Enables CS3, CS2, CS1, and CS0
CSCTL1
$x8
Makes CS3 follow EPAGE
MISC %0xxxxxxx Puts EPAGE at $0400–$07FF
EPAGE
Keeps the translated value of the upper addresses the
$01 same as it would have been before translation; not
necessary if all external devices use chip-selects
CS3 can be used with a 1-Kbyte space in systems not using memory
expansion. However, it must be made to appear as if memory expansion
is in use. One of many possible configurations is:
• Select the desired 1-Kbyte space for EPAGE (EWDIR in MISC in
the MMI).
• Write the EPAGE register with $0000, if EWDIR is one or $0001 if
EWDIR is 0.
• Designate all port G pins as I/O.
• Enable EPAGE and CS3.
• Make CS3 follow EPAGE.
8.3.2 Chip-Selects Used in Conjunction with Memory Expansion
Memory expansion and chip-select functions can work independently,
but systems requiring memory expansion perform better when
chip-selects are also used. For each memory expansion window there is
a chip-select (or two) designed to function with it.
Figure 8-2 shows a memory expansion and chip-select example using
three chip-selects. Table 8-3 shows the register settings that correspond
to the example. The program space consists of 128 Kbytes of
addressable memory in eight 16-Kbyte pages. Page 7 is always
accessible in the space from $C000 to $FFFF. The data space consists
of 64 Kbytes of addressable memory in 16, 4-Kbyte pages. Unless CSD
is used to select the external RAM, pages 0 through 6 appear in the
$0000 to $6FFF space wherever there is no higher priority resource. The
MC68HC812A4 — Rev. 3.0
126
Memory Expansion and Chip-Select
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