TC1796
System Units (Vol. 1 of 2)
LMB External Bus Unit
User’s Manual
13-37
V2.0, 2007-07
EBU, V2.0
13.5.6
PLMB Bus Width Translation
If the internal access width is wider than the external bus width specified for the selected
external region (programmed via the EBU_BUSCONx.PORTW bit field), the internal
access is split in the EBU into several external accesses. For example, if the PLMB
requests to read a 64-bit word and the external device is only 16-bit wide, the EBU will
automatically perform four external 16-bit accesses. When multiple accesses are
generated in this way, external bus arbitration is blocked until the multiple access is
complete. This means that the EBU remains the owner of the external bus for the
duration of the access sequence. The external accesses are performed in ascending
PLMB address order.
To allow proper bus width translation, the EBU has the capability to re-align data
between the external bus and the PLMB as shown in
Figure 13-13 PLMB to External Bus Data Re-Alignment
•
During an access to a 32-bit wide external region, either Buffer 1 or Buffer 2 is
enabled (according to bit 2 of the PLMB address being accessed) to perform the
required 64-bit (PLMB) data to 32-bit bus alignment (signified by Data32[31:0]
above). To generate a 32-bit access to the external data bus D[31:0], Buffer 3 and
Buffer 5 are enabled together.
PLMB Data
Buffers
(most sig.
32 bits)
PLMB Data
Buffers
(least sig.
32 bits)
Buffer 1
Data64
[63:32]
Data3
2
[31:0]
Buffer 3
MCA05724
Buffer 2
Buffer 4
Buffer 5
Data64
[31:0]
Data32
[31:16]
Data32
[31:16]
Data32
[15:0]
DataMS16
[31:16]
DataLS16
[15:0]
EBU
Data Bus
Pins
D[31:16]
EBU
Data Bus
Pins
D[15:0]