![](http://datasheet.mmic.net.cn/380000/-PD98405_datasheet_16745025/-PD98405_92.png)
CHAPTER 4 INTERFACES
92
(4) Byte alignment data transfer
The
μ
PD98405 supports a burst size select function for byte alignment data. The receive buffer may be
located on a byte boundary. Transmission data may start on a byte boundary. For DMA read/write
transfer in such a case, the
μ
PD98405 adds 1 word to the burst size. For example, the
μ
PD98405
performs 13-word burst for 12-word burst to transfer 1 cell.
Byte alignment transfer for transmit data
During a transmit data read transaction, the
μ
PD98405 reads the data in 32-bit (word) units regardless
of whether the transmit data starts on a word boundary, and internally ignores all unnecessary bytes.
For example, when 1-cell transmit data starts on a byte boundary, the
μ
PD98405 reads the data by
performing 13-word burst and internally ignores all unnecessary bytes.
Byte alignment transfer for receive data
During a receive data write transaction, when the receive buffer is located on a byte boundary, the
μ
PD98405 writes the data in word units and outputs signals indicating valid bytes on PCBE_B[3:0]. It
outputs an address in word units. For example, the
μ
PD98405 performs 13-word burst to write 1-cell
receive data into the receive buffer on a byte boundary.
This example is shown below.
Figure 4-24. Example of Storing Cell Data into a Buffer on a Byte Boundary
Buffer start address AD[1:0] = 10, 13-word burst
31
Buffer address[1:0]
01
02
03
Payload13
2423
1615
8 7
0
1
0
0
0
1
1
0
0
0
2
0
0
0
0
3
0
0
0
0
0
Payload5
Payload9
Payload0
Payload4
Payload8
Payload12
Payload1
Payload3
Payload7
Payload11
Payload2
Payload6
Payload10
0a
0b
0c
Payload41
Payload45
(don't care)
Payload40
Payload44
(don't care)
Payload39
Payload43
Payload47
Payload38
Payload42
Payload46
PCBE_B[3:0]
0
0
0
0
0
0
0
0
1
0
0
1
00
(don't care)
(don't care)
The function for adding 1 word to the burst size for byte alignment data is supported not only for 1-cell
transfer, but also for all burst sizes for transmission and reception. It is also supported for multi-cell
transfer. For example, to perform byte alignment transfer for 5-cell (60-word) transfer, the
μ
PD98405
transfers data with a 61-word burst size. In a mode in which the AD bit is set to 0 to consider cache
boundaries, however, the
μ
PD98405 does not add 1 word to the burst size when a cache boundary is
crossed.