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TNETX4090
ThunderSWITCH II
9-PORT 100-/1000-MBIT/S ETHERNET
SWITCH
SPWS044E – DECEMBER 1997 – REVISED AUGUST 1999
37
POST OFFICE BOX 655303 • DALLAS, TEXAS 75265
speed, duplex, and flow-control negotiation (continued)
In 100-Mbit/s mode, M08_RXD4 and M08_RXD5 are reconfigured as open-drain inputs, to allow the port to
negotiate with the PHY device for duplex and IEEE Std 802.3 pause frame support at power up via the EEPROM
contents. M08_RXD4 is used for duplex and M08_RXD5 is used for pause (see Table 8 and Table 9).
Each of these terminals:
Has an integral weak pullup resistor.
Has a strong open-drain pulldown transistor that is enabled by setting to 1 the appropriate bit in
Port8Control.
Is connected (via synchronization logic) to the appropriate bit in PortxStatus. These bits directly control the
configuration of the ports.
Each terminal is considered bidirectional when pulled low by either the TNETX4090 or by the PHY (or other
external connections). If neither pulls the terminal low, the pullup resistor maintains a value of 1 on the terminal.
When the PHY does not pull down a terminal, it can determine the desired option being requested by the
TNETX4090. The TNETX4090 observes the terminal to determine if its desired option has been granted.
The sense of these signals is such that the higher-performance option is represented by a value of 1, so if the
MAC does not require the higher performance or the PHY cannot supply it, either can pull the signal low, forcing
the port to use the lower-performance option.
The status of the link for this port is indicated on M08_LINK and is observable in Port8Status. M08_LINK plays
no part in the negotiation of pause or duplex or their recording in Port8Status.
Table 8. Port 8 Duplex Negotiation in MII Mode
Port8Control
reqhd
M08_RXD4
Port8Status
duplex
OUTCOME
0 → Floating 1 → 1 Full duplex
1 → Driven 0 (by the TNETX4090) → 0 Half duplex
X Driven 0 (by PHY) → 0 Half duplex
Table 9. Port 8 Pause Negotiation in MII Mode
Port8Control
reqnp
M08_RXD5
Port8Status
pause
OUTCOME
0 → Floating 1 → 1 Pause support
1 → Driven 0 (by the TNETX4090) → 0 No pause support
X Driven 0 (by PHY) → 0 No pause support
full-duplex hardware flow control
This port provides hardware-level full-duplex flow control via the M08_COL and FLOW terminals.
The port does not start transmitting a new frame if M08_COL is active, though the value of this terminal is
ignored at other times.
FLOW becomes active when the number of free buffers is fewer than the number specified in
FlowThreshold, provided that flow in SysControl is set.
These two capabilities allow full-duplex flow control without the use of IEEE Std 802.3 pause frames when
connecting the TNETX4090 to another TNETX4090, or to some other device that supports this capability.