MPC8540 datasheet(674/1302 Pages) FREESCALE

674 / 1302 page

MPC8540 PowerQUICC III Integrated Host Processor Reference Manual, Rev. 1

14-48

Freescale Semiconductor

Three-Speed Ethernet Controllers

the packet, the end occurs around the 56th byte of the frame data. Slot time in 1-Gbps mode is not

supported.

Full-duplex flow control is provided for in IEEE 802.3x. Currently the standard does not address

flow control in half-duplex environments. Common in the industry, however, is the concept of

back pressure. The TSEC implements the optional back pressure mechanism using the raise carrier

method. If the system receive logic wishes to stop the reception of packets in a network-friendly

way, transmit half-duplex flow control (THDF) is set (TCTRL[THDF]). If the medium is idle, the

TSEC raises carrier by transmitting preamble. Other stations on the half-duplex network then defer

to the carrier.

In the event the preamble transmission happens to cause a collision, TSEC ensures the minimum

96-bit presence on the wire, then drops preamble and waits a back-off time depending on the value

of the configuration bit, back pressure no back-off (half-duplex) [BPNB]. These

transmitting-preamble-for-back pressure collisions are not counted. If BPNB is set, the TSEC

waits an inter-packet gap before resuming the transmission of preamble following the collision and

does not defer. If cleared, the TSEC adheres to the truncated BEB algorithm that allows the

possibility of packets being received. This also can be detrimental in that packets can 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 TSEC drops carrier (cease transmitting preamble) periodically to avoid excessive defer

conditions in other stations on the shared network. If, while applying back pressure, the TSEC is

requested to send a packet, it stops sending preamble, and waits one IPG before sending the packet.

BPNB applies for any collision that occurs during the sending of this packet. TSEC does not defer

while attempting to send packets while in back pressure. Again, back pressure is non-standard, yet

it can be effective in reducing the flow of receive packets.

14.5.3.5.5 Controlling PHY Links

Control and status to and from the PHY is provided via the two-wire MII management interface

described in IEEE 802.3u. The MII management registers (MII management configuration,

command, address, control, status, and indicator registers) are used to exercise this interface

between a host processor and one or more PHY devices (including the TBI). External PHYs may

only be configured using the MII management interface of TSEC1 since the interface signals

(EC_MDC and EC_MDIO) are only driven by TSEC1.

The TSEC MII’s registers provide the ability to perform continuous read cycles, called a scan

cycle. If requested (by setting MIIMCOM[Scan Cycle]), the part performs repetitive read cycles

of the PHY status register, for example. In this way, link characteristics may be monitored more

efficiently. The different fields in the MII management indicator register (scan, not valid and busy)

are used to indicate availability of each read of the scan cycle to the host from MIIMSTAT[PHY

Status].

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