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EOF cleared, it indicates an intermediary buffer, and the transmit process attempts to
acquire the next Descriptor. If the EOF is set, it indicates the last buffer of the frame.
After the last buffer of the frame has been transmitted to MII, the MAC writes back the final
status information to the transmit Descriptor. At this time, if any interrupt condition is met, the
transmit interrupt is set, the next Descriptor is fetched, and the process repeats.
Actual frame transmission begins after the TxFIFO has reached either a programmable
threshold or a full frame is contained in the TxFIFO.
12.5.1.2.2
Transmit Polling Suspended
Transmit polling can be suspended by either of the following conditions:
•
The MAC detects a Descriptor owned by the C790.
•
A frame transmission is aborted when a locally induced error or G-Bus error is
detected.
If either of the previous two conditions occur, an interrupt is set and the information is written
to the frame Descriptor, causing the suspension.
The MAC does not automatically poll the transmit Descriptor list; the driver must issue a
transmit poll demand command after rectifying the cause of the suspension.
12.5.1.2.3
Transmit Frame DMA request
The TxFIFO uses a two-signal handshake for request and acknowledge synchronization.
The first signal TxFrmReq is active when the TxFIFO is capable of having data written to it.
The amount of data is a count threshold (PBL) in the CCReg (Command and Configuration
Register). If the PBL is equal to 001 and there are at least 32 bytes empty in the TxFIFO.
The second signal, TxFrmRdy, is active to start a burst of the number of bytes represented
by PBL.
12.5.1.3 Receive
In the running state, the reception process polls the reception Descriptor list in the memory.
Incoming frames are processed and placed in acquired Descriptor data buffers. Status
information is written to the last received Descriptor of the frame.
12.5.1.3.1
Poll Descriptor
The MAC always attempts to acquire an extra Descriptor in anticipation of incoming frames.
Descriptor acquisition is attempted if any of the following conditions are satisfied:
•
When Start Receive gets set in RFCReg immediately after being placed in the
running state.
•
When the MAC begins writing frame data to a data buffer, and the buffer ends
TX7901 User's Manual (Rev. 6.30T – Nov, 2001)
Chapter 12: 10/100 IEEE802.3 Media Access Controller
12-43
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