Datasheet

Memory Expansion and Chip-Select

MC68HC812A4 Data Sheet, Rev. 7

82 Freescale Semiconductor

Figure 8-1. Chip-Selects CS3–CS0 Partial Memory Map

Table 8-2. Example Register Settings

INITRM $00 Assigns internal RAM to $0000–$0FFF

INITRG $08

Assigns register block to $0800–$09FF and register-following

chip-selects at $0A00–$0BFF

WINDEF $20 Enable EPAGE

MXAR $00 No port G lines assigned as extended address

CSCTL0 $xF Enables CS3, CS2, CS1, and CS0

CSCTL1 $x8 Makes CS3 follow EPAGE

MISC %0xxxxxxx Puts EPAGE at $0400–$07FF

EPAGE $01

Keeps the translated value of the upper addresses the same as it

would have been before translation; not necessary if all external

devices use chip-selects

$0000

$0100

$0200

$0300

$0400

$0500

$0600

$0700

$0800

$0900

$0A00

$0B00

$0C00

$0D00

$0E00

$0F00

$0FFF

INTERNAL SPACE

RAM

1 KBYTE

REGISTERS

EXTERNAL SPACE

CS0

CS1

256 BYTES

128 BYTES

1 KBYTE

CS3

CS2