VITESSE
SEMICONDUCTOR CORPORATION
Data Sheet
High Performance Serial
Backplane Transceiver
VSC870
2), it stops reading from the parallel interface by setting REN LOW, sets RTM/TCLK HIGH and starts sending a
repeated sequence of CRQ words to the switch which are arbitrated only on the cycle that they arrive. During this
operation, the CRQ words are not stored at the switch. If all connections are granted, the transceiver will receive an
ACK from the switch. If DLYEN/CCKIN is LOW, it sets the REN signal HIGH when it receives the ACK, and sends
the header word and data to the switch. If DLYEN/CCKIN is HIGH, it waits for N more cycles before it sets REN
HIGH and then sends header word and data to the switch.
During the repeated sequence of CRQs described above, different priorities for connections can be established by
the number of the IDLE words sent between each CRQ words (determined by the CT[2:0]). The number of IDLEs
after each CRQ word can range from 0-7 as shown in the table below. Generally, the more often a CRQ is received by
the switch, the higher the probability that this connection will be granted.
Number of
IDLE Words
Number of
IDLE Words
CT[2:0]
CT[2:0]
0 0 0
0 0 1
0 1 0
0 1 1
0
1
2
3
1 0 0
1 0 1
1 1 0
1 1 1
4
5
6
7
The functional timing diagram for camp-on mode with DLYEN/CCKIN set LOW is shown in figure 3. There are
a minimum of 9 word clock cycles between loading the CRQ into the transceiver and the signal REN going HIGH (to
start sending the data packet D0, D1, D2 etc.). This time will be longer if the requested output is busy or the port card
is located further from the switch card. For multicast, the switch will reserve any available outputs and accumulate
them as more CRQ commands are processed. This continues until all connections are reserved. At this point an ACK
signal is sent back to the transceiver.
Figure 3: Camp-on with Priority Transmitter Functional Timing (no early arbitration)
WCLK
Minimum of 9 clock cycles
TXIN[31:0]
TXTYP[1:0]
CRQ
3
D1 D2
D3 D4 D5
HDR
2
D0
1
ACK/RCLK
REN
RTM/TCLK
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Internet: www.vitesse.com
G52190-0, Rev 4.1
01/05/01
Page 19
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