MPC555
/
MPC556
EXTERNAL BUS INTERFACE
MOTOROLA
USER’S MANUAL
Rev. 15 October 2000
9-30
NOTE: “—” denotes a byte not required during that read cycle.
lists the patterns of the data transfer for write cycles when the MPC555
/
MPC556 initiates an access.
NOTE: “—” denotes a byte not driven during that write cycle.
9.5.6 Arbitration Phase
The external bus design provides for a single bus master at any one time, either the
MPC555
/ MPC556 or an external device. One or more of the external devices on the
bus can have the capability of becoming bus master for the external bus. Bus arbitra-
tion may be handled either by an external central bus arbiter or by the internal on-chip
arbiter. In the latter case, the system is optimized for one external bus master besides
the MPC555
/ MPC556. The arbitration configuration (external or internal) is set at sys-
tem reset.
Each bus master must have bus request (BR), bus grant (BG), and bus busy
(BB)
sig-
nals. The device that needs the bus asserts BR. The device then waits for the arbiter
to assert BG. In addition, the new master must look at BB to ensure that no other mas-
ter is driving the bus before it can assert BB to assume ownership of the bus. Any time
the arbiter has taken the bus grant away from the master and the master wants to ex-
Table 9-2 Data Bus Requirements For Read Cycles
Transfer
Size
TSIZE
[0:1]
Address
32-bit Port Size
16-bit Port Size
8-bit
Port
Size
ADDR
[30:31]
DATA
[0:7]
DATA
[8:15]
DATA
[16:23]
DATA
[24:31]
DATA
[0:7]
DATA
[8:15]
DATA
[0:7]
Byte
01
00
OP0
—
—
—
OP0
—
OP0
01
01
—
OP1
—
—
—
OP1
OP1
01
10
—
—
OP2
—
OP2
—
OP2
01
11
—
—
—
OP3
—
OP3
OP3
Half-word
10
00
OP0
OP1
—
—
OP0
OP1
OP0
10
10
—
—
OP2
OP3
OP2
OP3
OP2
Word
00
00
OP0
OP1
OP2
OP3
OP0
OP1
OP0
Table 9-3 Data Bus Contents for Write Cycles
Transfer
Size
TSIZE[0:1]
Address
External Data Bus Pattern
ADDR
[30:31]
DATA
[0:7]
DATA
[8:15]
DATA
[16:23]
DATA
[24:31]
Byte
01
00
OP0
—
—
—
01
01
OP1
OP1
—
—
01
10
OP2
—
OP2
—
01
11
OP3
OP3
—
OP3
Half-word
10
00
OP0
OP1
—
—
10
10
OP2
OP3
OP2
OP3
Word
00
00
OP0
OP1
OP2
OP3
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Freescale Semiconductor, Inc.
For More Information On This Product,
Go to: www.freescale.com
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