VT6516 datasheet(31/73 Pages) ETC

31 / 73 page

VIA Technologies, Inc.

Preliminary VT6516 Datarsheet

-31-

3.1.4.2 MII interface

The VT6516 communicates with the external 10/100M Ethernet transceiver through standard MII

interface, in this mode the VT6516 became 12 ports MII port due to the MII signal multiplexed with RMII

signal. But the ports number of internal remained as 16 ports. The signals of MII interface are described in

Table-3-2:

Table 3-2 MII interface signals

Name

Type

Description

TCLK

Transmit Clock

TXD[3:0]

Transmit Data for.

TXEN

Transmit

COL

Collision Detected

CRS

Carrier Sense

RXD[3:0]

Receive Data

RCLK

Receive Clock

RXDV

Receive Data

Figure 3-2 MII timing diagram

(omitted)

3.1.4.3 Flow control

Under full-duplex mode operation, if the buffer utilization of whole switch has exceeded the upper threshold

and the permanent buffer has been used up, a pause frame with a pause time interval will be send to the sending

port to stop it from sending new frame. If register- FMFCT not enable at this switch, the public buffer will used

until no more buffers. Then further incoming frames will be dropped. The unit in pause time field of the flow

control frame is slot time (512 bits).

The max possible waiting time should be the max packet memory size divided by lowest port speed, for example

if 512MB is the max packet buffer size and 10Mb is the lowest speed, the 512M * 8 bits * 100ns = 409.6 seconds

(8M slot time) is the max possible waiting time. The congestion factor is the max possible waiting time at current

link load. The pause timer value is half of the max possible waiting time. If it is greater than the feasible max

pause time, use all 1’s in pause time value.

If the utilization of the public buffer of the switch drops below the lower threshold, a pause-frame with minimum

frame interval of 0 will be sent to the linking ports the enable new frame transmission.

Under half duplex operation, if the buffer utilization of whole switch has exceeded the upper threshold and the

permanent buffer has been used up, the port will perform back-pressure based flow control by sending a jam

pattern on each incoming frame. If backpressure flow control of the port is not enable, the frame will be dropped.

The flow control pause time is calculated by maintained the configuration of port speed of each port and the

buffer size. With input of the free memory block count and congestion factor, it determines flow control on or

off on an output port. If flow control is on, any new queue request from a input port to this output port will

trigger a flow control frame sent to that request port by the output MAC that is notified by the packet flow

control unit.

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