MSC8113 Reference Manual, Rev. 0
25-22
Freescale Semiconductor
Ethernet Controller
25.7.2 Controlling Packet Flow
Packet flow can be handled in several ways. A default retransmit attempt limit of 15 can be
reduced via the HAFDUPR. The slot time or collision window can gate the retry window and
possibly reduce the transmit buffering within the system. The slot time for 10/100 Mbps is 512
bit times. Because the slot time starts at the beginning of the packet, the end occurs around byte
56 of the frame data. Full-duplex flow control is covered in IEEE Std. 802.3x™. The standard
does not address flow control in half-duplex environments. However, the concept of back
pressure is common in the industry. The Ethernet controller implements the optional back
pressure mechanism using the raise carrier method, if the system receive logic can stop the
reception of packets in a network-friendly way by setting the transmit half-duplex flow control
(TCTRL[THDF]). If the medium is idle, the Ethernet controller raises the carrier by transmitting
the preamble. Other stations on the half-duplex network then defer to the carrier.
If the preamble transmission causes a collision, the Ethernet controller ensures the minimum
96-bit presence on the wire, drops the preamble, and waits a back-off time depending on the
value of the (half-duplex) BPNB bit. These transmitting-preamble-for-back-pressure collisions
are not counted. If BPNB is set, the Ethernet controller waits an inter-packet gap before resuming
the preamble transmission and does not defer. If BPNB is cleared, the Ethernet controller adheres
to the truncated BEB algorithm so that packets can be received. This also can be detrimental in
that packets may now experience excessive collisions, causing them to be dropped in the stations
from which they originate. To reduce the likelihood of lost packets and packets leaking through
the back pressure mechanism, BPNB must be set. The Ethernet controller periodically drops the
carrier (ceases transmitting preamble) to avoid excessive defer conditions in other stations on the
shared network. It does not defer when attempting to send packets in back pressure. Back
pressure is nonstandard, yet it can be effective in reducing the receive packet flow.
25.7.3 Controlling PHY Links
Control and status to and from the PHY is provided via the two-wire MII management interface
described in IEEE Std. 802.3u™. The MII management registers exercise this interface between
a host processor and one or more PHY devices The Ethernet Controller MII registers provide the
ability to perform continuous read cycles (called a scan cycle) even though scan cycles are not
explicitly defined in the standard. When requested (by setting MIIMCOMR[SCYC]), the
MSC8113 performs repetitive read cycles of the PHY status register, for example. This method
allows link characteristics to be monitored more efficiently. The different fields in the MII
management indicator register (scan, not valid and busy) indicate availability of each read of the
scan cycle to the host from MIIMSTATR[PHYS]. The length of the MII management interface
preamble can also be modified through the MII registers. After establishing that a PHY supports
preamble suppression, the host may configure the Ethernet controller accordingly. While
enabled, the length of MII management frames are reduced from 64 clocks to 32 clocks. This
effectively doubles the efficiency of the interface. See Section 25.17.6, MII Management
Registers, on page 25-92 for details.
Содержание MSC8113
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