Texas Instruments TNETX4090 specifications Port 8 Duplex Negotiation in MII Mode, Outcome

TNETX4090

ThunderSWITCH II 9-PORT 100-/1000-MBIT/S ETHERNET SWITCH

SPWS044E ± DECEMBER 1997 ± REVISED AUGUST 1999

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:

DHas an integral weak pullup resistor.

DHas a strong open-drain pulldown transistor that is enabled by setting to 1 the appropriate bit in Port8Control.

DIs 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		M08_RXD4		Port8Status	OUTCOME
reqhd				duplex
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		M08_RXD5		Port8Status	OUTCOME
reqnp				pause
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.

DThe port does not start transmitting a new frame if M08_COL is active, though the value of this terminal is ignored at other times.

DFLOW 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.

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