VITESSE
SEMICONDUCTOR CORPORATION
Data Sheet
High Performance Serial
Backplane Transceiver
VSC870
2.2.1 Packet Format
At both the transmit and receive sides, a start of packet is identified using a header word with TXTYP[1:0] and
RXTYP[1:0] set to “10”. Following this header word are the rest of the data words with TXTYP[1:0] and
RXTYP[1:0] set to “01”. This is shown in the table below.
TXTYP[1:0]
RXTYP[1:0]
Transmit
B[1:0]
Receive
B[1:0]
Word Type
SCRAM = 1 SCRAM = 0 Error Check
Command Word
Data Word
No Scramble
Scramble
No Scramble
No Scramble
No Scramble
No Scramble
IDLE words
0 0
0 1
1 0
1 1
0 0
01 or 10
1 1
0 0
01,10,11
0 0
No
No
No
Header Word
No Scramble
No Scramble
Connection Request
0 0
0 0
Two general ways of transmitting a data packets are shown in the figure below. In the first example, when the
transceiver reads a CRQ word (which signals the start of a new packet) it waits for the ACK signal before
transmitting the header word. In the second example, the CRQ word is sent D words before the end of the packet in
order to utilize early arbitration and optimize the throughput. If there is no available packet for the next transmission,
a null CRQ word and a null header word are used as end of packet. Command words such as IDLEs can be
embedded inside the packet. On the receive side, the packets are delineated by the header word at the beginning and a
CRQ word at the end. The word sequence loaded into the VSC870 parallel interface is also shown in the figure below.
More details can be found in Application Note 31.
Figure 2: Packet Transmission Format
Example 2
Example 1
Start of Packet
10
01
Header
D0
Start of Packet
10
01
01
Header
D0
D1
11
01
CCRRQQ
D words before EOP
D(N-D)
01
11
DN
CRQ
CRQ
End of Packet
Start of Packet
End of Packet
Start of Packet
01
DN
10
01
Header
D0
10
01
Header
D0
© VITESSE SEMICONDUCTOR CORPORATION • 741 Calle Plano • Camarillo, CA 93012
Tel: (800) VITESSE • FAX: (805) 987-5896 • Email: prodinfo@vitesse.com
Internet: www.vitesse.com
G52190-0, Rev 4.1
01/05/01
Page 13
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