
MOTOROLA
3-8
SYSTEM RESOURCES
CPU16
REFERENCE MANUAL
3.4 Intermodule Bus
The intermodule bus is a standardized bus developed to facilitate design of modular
microcontrollers. Bus protocols are based on the MC68020 bus. The IMB contains cir-
cuitry to support exception processing, address space partitioning, multiple interrupt
levels, and vectored interrupts.
Modular microcontroller family modules communicate with one another via the IMB.
Although the full IMB supports 24 address and 16 data lines, CPU16 uses only 16 data
lines and 20 address lines — ADDR[23:20] are tied to ADDR19 when processor driv-
en.
3.5 External Bus Interface
The external bus interface (EBI) is contained in the system integration module of the
modular microcontroller. This section provides a general discussion of EBI capabili-
ties. Refer to the appropriate microcontroller user's manual for detailed information
about the bus interface.
The external bus is essentially an extension of the IMB. There are 24 address lines
and 16 data lines. ADDR[19:0] are normal address outputs, ADDR[23:20] follow the
output state of ADDR19. It provides dynamic sizing between 8- and 16-bit data ac-
cesses. A three-line handshaking interface performs bus arbitration.
Table 3-1 Operations that Cross Bank Boundaries
Type of Operation
Extension Field Used
Extension Field
Affected
PK
None
Effect on
ADDR[19:16]
Equals new PK
Used for
Effective Address
Used for
Effective Address
Used for
Effective Address
Used for
Effective Address
Used for
Effective Address
Normal PC Increments
Operand Read Using
Indexed Addressing Mode
Operand Write Using
Indexed Addressing Mode
Operand Read Using
Extended Addressing Mode
Operand Write Using
Extended Addressing Mode
Post-modified Indexed Addressing
(XK is modified after use as effective
address)
JMP, JSR Instruction
Branch Instructions
(Including BSR and LBSR)
Stack Access
AIX, AIY, AIZ, or AIS Instruction
TSX, TSY, or TSZ Instruction
TXS, TYS, or TZS Instruction
TXY or TXZ Instruction
TYX or TYZ Instruction
TZX or TZY Instruction
PK
XK, YK, ZK
XK, YK, ZK
None
EK
None
EK
None
XK
XK
None
PK
PK
PK
Equals new PK
Equals new PK
SK
SK
Stack at new SK
None
None
None
None
None
None
XK, YK, ZK, or SK
SK
XK, YK, or ZK
XK
YK
ZK
XK, YK, ZK, or SK
XK, YK, or ZK
SK
YK, ZK
XK, ZK
XK, YK
F
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