VITESSE

SEMICONDUCTOR CORPORATION

Data Sheet

VSC870

High Performance Serial

Backplane Transceiver

G52190-0, Rev 4.1

Page 21

01/05/01

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as shown in Figure 3. If DLYEN/CCKIN is HIGH, it waits for N more cycles before it sets REN HIGH. If the counter

set by the CT[2:0] bits has expired and the transceiver has not received the ACK signal, it sends out a CRQ word with

AOA bit set LOW so that an ACK is returned if any output is granted. The transceiver keeps sending this CRQ word

until it receives the ACK signal. When the ACK is received, the transceiver sets the REN signal HIGH and sends the

header word and data to the switch for any output that was granted. This occurs right after receiving ACK if DLYEN/

CCKIN is LOW or N cycles later if DLYEN/CCKIN is HIGH. The table below shows the number of CRQ words sent

for each value of CT[2:0].

Recast Operation

If an ACK is not received before the CT[2:0] counter expires, the transceiver converts to recast mode. In this

mode, the memory system or FIFO should support retransmit capability. When the transceiver first reads the Header

word from the user interface, it sets the signal RTM/TCLK HIGH (Figure 5). This signal can be used to mark the

FIFO location of the first data word in this packet and not allow the user logic to overwrite this data packet in the

FIFO (see Application Note 31). The signal RTM/TCLK goes LOW when data has been sent to all requested

destinations or if ABORT signal is asserted HIGH. If during a multicast, the CT[2:0] counter expires and some

connections were not granted, an IDLE word or CRQ word will be returned to the transceiver from the switch chip

describing the current connection status of the port. Upon detecting the CRQ word for the next packet at the parallel

transmit interface, the transceiver will use this information from the switch to automatically form a new CRQ word

requesting the outputs that were not granted. This CRQ word will replace the CRQ word at the parallel interface and

is sent to the switch followed by remaining data if early arbitration is used. The process is repeated as if this is a new

multicast CRQ for the current Packet. Also in early arbitration mode, the CRQ for the next packet remains in the

external FIFO or memory system, and it is used after the last recast for the current packet. The ACK will toggle every

time the transceiver performs recast.

When the transceiver reads the header of the next packet, the RFM signal is toggled to set the FIFO pointer back

to the earlier location marked by RTM/TCLK in order to recast this packet to the new locations (Figure 6). Note that

the RTM/TCLK signal is set after the transceiver loads the Header word for the current packet. Therefor the header

word is lost and not retransmitted. During recast, this word will not be sent to the new locations if RTM/TCLK is

used to mark the retransmit location. Users can externally adjust the RTM/TCLK to account for this latency or use an

extra header word which will be lost during recast (see Application Note 31).

If the end of packet is reached before the connection information is returned from the switch, IDLE words will be

sent to the switch while waiting for this response word. The retransmission process will continue until the data has

been sent to all the requested destinations or ABORT is set HIGH. If ABORT is set HIGH (transmission is aborted),

all of the following data words will not be retransmitted and will be ignored. The transceiver will then look for the

next CRQ word at the parallel interface.

CT[2:0]

Number of

CRQ Words

CT[2:0]

Number of

CRQ Words

0 0 0

1

1 0 0

64

0 0 1

8

1 0 1

128

0 1 0

16

1 1 0

256

0 1 1

32

1 1 1

512