Datasheet

Memory Expansion and Chip-Select
MC68HC812A4 Data Sheet, Rev. 7
92 Freescale Semiconductor
8.7 Priority
Only one module or chip-select may be selected at a time. If more than one module shares a space, only
the highest priority module is selected.
Only one chip-select is active at any address. In the event that two or more chip-selects cover the same
address, only the highest priority chip-select is active.
Chip-selects have this order of priority:
Table 8-5. Module Priorities
Priority Module or Space
Highest
On-chip register space — 512 bytes fully blocked for registers although some of
this space is unused
BDM space (internal) — When BDM is active, this 256-byte block of registers
and ROM appear at $FFxx; cannot overlap RAM or registers
On-chip RAM
On-chip EEPROM (if enabled, EEON = 1)
E space (external)
(1)
— 1 Kbyte at either $0000 to $03FF or $0400 to $07FF;
may be used with “extra” memory expansion and CS3
1. External spaces can be accessed only if the MCU is in expanded mode. Priorities of differ-
ent external spaces affect chip-selects and memory expansion.
CS space (external)
(1)
— 512 bytes following the 512-byte register space; may
be used with CS3–CS0
P space (external)
(1)
— 16 Kbytes fixed at $8000 to $BFFF; may be used with
program memory expansion and CSP0 and/or CSP1
D space (external)
(1)
— 4 Kbytes fixed at $7000 to $7FFF; may be used with
data memory expansion and CSD or CSP1 (if set for full memory space) or the
entire half of memory space $0000–$7FFF
Lowest
Remaining external
(1)
Highest Lowest
CS3 CS2 CS1 CS0 CSP0 CSD CSP1