External Bus Interface (EBI)
MPC5565 Microcontroller Reference Manual, Rev. 1.0
12-46
Freescale Semiconductor
12.4.2.7
Size, Alignment, and Packaging on Transfers
shows the allowed sizes that an internal or external master can request from the EBI. The
behavior of the EBI for request sizes not shown below is undefined. No error signal is asserted for these
erroneous cases.
Even though misaligned non-burst transfers from internal masters are supported, the EBI naturally aligns
the accesses when it sends them out to the external bus, splitting them into multiple aligned accesses if
necessary.
Natural alignment for the EBI means:
•
Byte access can have any address.
•
16-bit access, address bit 31 must be 0.
•
32-bit access, address bits 30–31 must be 0.
•
For burst accesses of any size, address bits 29–31 must be 0.
The EBI never generates a misaligned external access, so a multi-master system with two MCUs can never
have a misaligned external access. In the erroneous case that an externally-initiated misaligned access does
occur, the EBI errors the access (by asserting TEA externally) and does not initiate the access on the
internal bus.
The EBI requires that the portion of the data bus used for a transfer to/from a particular port size be fixed.
A 32-bit port must reside on data bus bits 0–31, and a 16-bit port must reside on bits 0–15.
In the following figures and tables the following convention is adopted:
•
The most significant byte of a 32-bit operand is OP0, and OP3 is the least significant byte.
•
The two bytes of a 16-bit operand are OP0 (most significant) and OP1, or OP2 (most significant)
and OP3, depending on the address of the access.
•
The single byte of a byte-length operand is OP0, OP1, OP2, or OP3, depending on the address of
the access.
Table 12-17. Transaction Sizes Supported by EBI
No. Bytes (Internal Master)
No. Bytes (External Master)
1
1
2
2
4
4
3
1
1
1
Some misaligned access cases may result in 3-byte writes. These
cases are treated as power-of-2 sized requests by the EBI, using
WE/BE[0:3] to make sure only the appropriate 3 bytes get written.
8
32
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