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Bit 15 / Indirect Access Busy (IAB). When an indirect read or write access is in progress, this read only
bit will be set to a one. During a read operation, this bit will be set to a one until the data is ready to be
read. It will be set to zero when the data is ready to be read. During a write operation, this bit will be set
to a one while the write is taking place. It will be set to zero once the write operation has completed.
Register Name:
RDMAC
Register Description: Receive DMA Channel Configuration
Register Address:
0774h
76
5432
10
D7
D6
D5
D4
D3
D2
D1
D0
15
14
13
12
11
10
9
8
D15
D14
D13
D12
D11
D10
D9
D8
Note: Bits that are underlined are read only, all other bits are read-write.
Bits 0 to 15 / Receive DMA Configuration RAM Data (D0 to D15). Data that is written to or read
from the Receive DMA Configuration RAM.
8.2 TRANSMIT SIDE
8.2.1 OVERVIEW
The Transmit DMA uses a scatter gather technique to read packet data from main memory. The Host will
keep track of and decide where (and when) the DMA should grab the outgoing packet data from. There
are a set of descriptors that is handed back and forth between the Host and the DMA. Via the descriptors
the Host can inform the DMA where to obtain the packet data from and the DMA can tell the Host when
the data has been transmitted.
The operation of the Transmit DMA has three main areas as shown in Figures 8.2.1A and 8.2.1B and
Table 8.2.1A. The Host will write to the Pending Queue informing the DMA which channels have packet
data that is ready to be transmitted. Associated with each Pending Queue Descriptor is a data buffer that
contains the actual data payload of the HDLC packet. The data buffers can be between 1 and 8191 bytes
in length (inclusive). If an outgoing packet requires more than memory than a data buffer contains, then
the Host can link the data buffers to handle packets of any size.
The Done Queue Descriptors contain information that the DMA wishes to pass to the Host. The DMA
will write to the Done Queue when it has completed transmitting either a complete packet or data buffer
(see the discussion on DMA Update to the Done Queue below). Via the Done Queue Descriptors, the
DMA informs the Host about the status of the outgoing packet data.
If an error occurs in the
transmission, the Done Queue can be used by the Host to recover the packet data that did not get
transmitted and the Host can then re-queue the packets for transmission.
If enabled, the DMA can burst read the Pending Queue Descriptors and burst writes the Done Queue
Descriptors. This helps minimize PCI Bus accesses, freeing the PCI Bus up to do more time critical
functions. See Sections 8.2.3 and 8.2.4 for more details on this feature.