VITESSE

SEMICONDUCTOR CORPORATION

Data Sheet

VSC870

High Performance Serial

Backplane Transceiver

Page 22

G52190-0, Rev 4.1

01/05/01

© VITESSE SEMICONDUCTOR CORPORATION • 741 Calle Plano • Camarillo, CA 93012

Tel: (800) VITESSE • FAX: (805) 987-5896 • Email: prodinfo@vitesse.com

Internet: www.vitesse.com

Figure 5: Multicast Recast Functional Timing (Beginning of Multicast)

Figure 6: Multicast Recast Functional Timing (At the End of Packet then Recast)

2.3.7 Unicast Multi Queue Mode (MD[1:0] = 10)

Muti Queue mode assumes there are several unicast data queues on each port card such as virtual output queues

or priority queues, and allows the port card to make multiple connection requests at the same time. The switch chip

performs two levels of arbitration in two word clock cycles. The first level determines which of the requested outputs

are available and holds these outputs. The second level chooses one winner from the available outputs then releases

the rest. Because outputs can be blocked during the first level of arbitration, all Muti Queue CRQ commands are held

at the switch chip and continue to request outputs until one is granted. As in section 2.3.5, if an ACK is not received

before a header word is detected at the parallel interface, a repeated sequence of CRQs are sent to the switch chip

until an output is granted. This sequence depends on the value of CT[2:0]. The port number of the granted output

(P[3:0]) is returned to the port card following the ACK pulse on the ACK/RCLK output. The functional timing

diagram for this mode is shown in Figure 7.

WCLK

TXIN[31:0]

TXTYP[1:0]

REN

CRQ

D0

D1

D2

D3

3

RTM/TCLK

ACK/RCLK

Min 9 word clocks

HDR

21

RFM

6 word clocks

WCLK

TXIN[31:0]

TXTYP[1:0]

REN

CRQ-N

D1-0

D2-0

3

RTM/TCLK

ACK/RCLK

HDR-N

2

1

RFM

D0-N

D0-0

D3-0

D4-0

1

NOTE: CRQ-N, HDR-N, D0-N: CRQ, header and first data word of the next packet;

these bytes are ignored by the transceiver during recast.

D0-0, D1-0, D2-0...:

Data words of the recasted packet.