Digi NS9750 - Manual (Page 529)

w w w . d i g i e m b e d d e d . c o m 5 0 5
B B u s D M A C o n t r o l l e r
Figure 84: DMA buffer descriptor
Field Description
Source address Identifies the starting location of the source data buffer.
For transmit buffers. The source address can start on any byte boundary.
For receive buffers. The source address must be word-aligned.
Be sure the source address field points to an existing memory location.
Buffer length Indicates, in fly-by peripheral-to-memory operations, the maximum
number of bytes available in the receive buffer pointed to by the source
buffer pointer. After filling a receive buffer with peripheral data, the DMA
controller updates this field with the initial buffer length less the actual
receive data byte count.
Note: The buffer length must be a multiple of four bytes.
Indicates, in fly-by memory-to-peripheral operations, the number of bytes
to move from the source address pointer to the peripheral device. After
completing a transmit buffer descriptor, the DMA controller updates this
field with the initial buffer length less the actual transmit data byte count
(useful for error conditions).
In either mode, this field is limited to 16 bits, which supports a maximum
transfer size of 65535 bytes.
Destination address This field is not used in BBus DMA transfers. The field is updated with all
zeroes when the descriptor is retired.
W The wrap bit. When set, this bit informs the DMA controller that this is the last
buffer descriptor within the continuous list of descriptors for the channel. The
next buffer descriptor is found using the initial DMA channel buffer descriptor
pointer.
When the WRAP bit is not set, the next buffer descriptor is found using a 16
byte offset from the current buffer descriptor.
Table 303: DMA buffer descriptor definition
Destination Address
Buffer Length
Status
Source Address
0
OFFSET + 0
OFFSET + 4
OFFSET + 8
OFFSET + C
F I L W Reserved
31 30 29 28 16 15
Reserved