Chapter 12: 10/100 IEEE802.3 Media Access Controller
TX7901 User’s Manual (Rev. 6.30T – Nov, 2001)
12-42
12.5 Functional Description
12.5.1 DMA
The MAC provides a master DMA interface that is capable of reading or writing data at high
speed. When the MAC wants to transfer data to/from memory, it follows the 64-bit G-Bus
conventions. All internal control signals are synchronous to the gbsBusClk. Each frame,
whether transmit or receive, must be delineated by two signals, SOF (Start of Frame) and
EOF(End of Frame).
12.5.1.1 DMA
Arbitration
The DMA arbitration scheme depends on the BAR bit setting in the CCReg. When it is reset,
the MAC grants precedence to the receive process instead of the transmit process. When
BAR is set or the MAC is in the full-duplex mode, a round-robin arbitration scheme is applied.
There are 4 different DMA requests.
The DMA requests possible from the transmit process are:
1. TxDesReq : This is used for DMA to fetch or to close a Descriptor
2. TxFrmReq : This is used for DMA to transfer data from memory to a TxFIFO
The DMA requests possible from the receive process are:
1. RxDesReq : This is used for DMA to fetch or to close a Descriptor
2. RxFrmReq : This is used for DMA to transfer data to memory from a RxFIFO.
12.5.1.2 Transmit
In the running state, the transmit process polls the transmit Descriptor list for frames
requiring transmission. After polling starts, it continues in either the sequential Descriptor
ring order or in the chained order. When frame transmission is complete, status information
is written into the Transmit Descriptor and the Descriptor is closed.
If either the MAC detects a Descriptor flagged as owned by the C790 or an error condition
occurs, the transmit process is suspended and an interrupt bit is asserted.
While in the running state, the transmit process can simultaneously acquire two frames. As
the transmit process completes copying the first frame, it immediately polls the transmit
Descriptor list for the second frame. If the second frame is valid, the transmit process copies
the frame without waiting for the status information of the first frame.
12.5.1.2.1
Transmit Frame Process
Frames can be data-chained and span several buffers. Frames must be delimited by SOF
(Start of Frame) and EOF (End of Frame) in the frame Descriptor.
SOF must be set before the transmit process is initiated. When this occurs, frame data are
transferred from the host buffer to the TxFIFO. Concurrently, if the current frame has the
Summary of Contents for TMPR7901
Page 1: ...TX System RISC TX79 Family TMPR7901 Symmetric 2 way superscalar 64 bit CPU ...
Page 14: ...Handling Precautions ...
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Page 41: ...4 Precautions and Usage Considerations 4 2 ...
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