NUC970 Technical Reference Manual
Publication Release Date: Dec. 15, 2015
- 583 -
Revision V1.30
NUC97
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CHNIC
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5.21.5 Functional Description
Arbiter
5.21.5.1
In the EMAC, there are two different bus requests, RXREQ and TXREQ respectively. Arbiter does
the arbitration between the RXREQ and TXREQ, and then decides which one can request the
AHB bus. The arbitration results are shown below:
RXREQ
TXREQ
Granted
0
0
Neither TXDMA nor RXDMA granted.
0
1
TXDMA granted.
1
0
RXDMA granted.
1
1
If TXFIFO valid data byte count is less than RXFIFO free space byte count,
TXDMA granted.
1
1
If RXFIFO free space byte count is less than or equal to TXFIFO valid space
byte count, RXDAM granted.
Table 5.21-1 Arbiter Arbitration Results
TXDMA State Machine
5.21.5.2
The TXDMA state machine transfers data from the system memory to the internal 256 bytes
transmit FIFO through the AHB master. Then, the TXDMA state machine will request the transmit
MAC to send the data out. During the transmission process, the TXDMA will fetch the transmit
descriptor first. Through the buffer address field of the transmit descriptor, the TXDMA fetch the
frame data from the system and store it into the internal 256 bytes transmit FIFO. Then, the
transmit MAC will read frame data from the transmit FIFO and send the frame out. After the finish
of the frame transmission, the TXDMA updates the transmit status of current frame and write the
transmit descriptor back to the system memory to indicate the frame transmission has finished.
RXDMA State Machine
5.21.5.3
The RXDMA state machine transfers data from the internal 256 bytes receiving FIFO to the
system memory through AHB master. During the receiving process, the RXDMA will fetch the
received descriptor first. Through the buffer address field of the received descriptor, the RXDMA
will know memory space which is allocated to store the incoming frame. After the received MAC
indicates there is a new incoming frame, the RXDMA starts to transfer the frame data from the
internal received FIFO to the system memory. After the receiving process has finished, the
RXDMA will update the receiving status of current frame and write the received descriptor back to
system memory to indicate a new incoming frame is in the system memory.
Flow Control
5.21.5.4
This block implements the flow control function while EMAC operates in the full duplex mode. The
flow control function is defined in the IEEE 802.3 Std. chapter 31. The type of flow control frame
defined in the IEEE 802.3 Std. is only the PAUSE frame at the moment. The control frame
transmission and reception is programmable through the control registers.
To receive a control frame, software must set the bit ACP (Accept Control Packet) of register
EMACn_MCMDR (MAC Command Register). While a PAUSE frame is received, the flow control