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External Bus Interface (EBI)
MPC5566 Microcontroller Reference Manual, Rev. 2
12-50
Freescale Semiconductor
lists the patterns of the data transfer for write cycles when accesses are initiated by the MCU.
The bytes indicated as ‘—’ are not driven during that write cycle.
12.4.2.8
Arbitration
The external bus design provides for a single bus master at any one time, either the MCU or an external
device. One of the external devices on the bus has the capability of becoming bus master for the external
bus. Bus arbitration be handled either by an external central bus arbiter or by the internal on-chip arbiter.
The arbitration configuration (external or internal) is set via the EARB bit in the EBI_MCR.
Each bus master must have bus request, bus grant, and bus busy
signals. The signals are described in detail
Section 12.2.1, “Detailed Signal Descriptions
.” The device that needs the bus asserts the bus request
(BR) signal. The device then waits for the arbiter to assert the bus grant (BG) signal. In addition, the new
master must sample the bus busy (BB) signal to ensure that no other master is driving the bus before it can
assert bus busy to assume ownership of the bus. The new master must sample bus busy negated for two
cycles before asserting bus busy, to avoid any potential conflicts. Any time the arbiter has taken the bus
grant away from the master, and the master wants to execute a new cycle, the master must re-arbitrate
before a new cycle can begin. The EBI, however, whether the internal or external arbiter is used,
guarantees data coherency for access to a small port size and for decomposed bursts. This means that the
EBI does not release the bus before the completion of the transactions which are considered as atomic.
describes the basic protocol for bus arbitration.
Table 12-21. Data Bus Contents for Write Cycles
Transfer
Size
TSIZ[0:1]
Address
32-Bit Port Size
16-Bit Port Size
1
1
Also applies when DBM = 1 for 16-bit data bus mode.
A[30]
A[31]
D[0:7]
D[8:15]
D[16:23]
D[24:31]
D[0:7]
D[8:15]
Byte
01
0
0
OP0
—
—
—
OP0
—
01
0
1
OP1
OP1
—
—
—
OP1
01
1
0
OP2
—
OP2
—
OP2
—
01
1
1
OP3
OP3
—
OP3
—
OP3
16-bit
10
0
0
OP0
OP1
—
—
OP0
OP1
10
1
0
OP2
OP3
OP2
OP3
OP2
OP3
32-bit
00
0
0
OP0
OP1
OP2
OP3
OP0 or OP2
2
2
This case consists of two 16-bit external transactions, the first writing OP0 and OP1, the second writing OP2 and OP3.
OP1 or OP3
Содержание MPC5566
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