MC68HC812A4 Datasheet, PDF (140/342 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
8.8 Priority
Table 8-4. Stretch Bit Definition
Stretch Bit
SxxxA
0
0
1
1
Stretch Bit
SxxxB
0
1
0
1
Number of E-Clocks
Stretched
0
1
2
3
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.
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
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⁾
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.
MC68HC812A4 — Rev. 3.0
140
Memory Expansion and Chip-Select
Advance Information
MOTOROLA