CS8920A View Datasheet(PDF) - Cirrus Logic
Part Name
Description
MFG CO.
'CS8920A' PDF : 144 Pages View PDF
CS8920A
Onecoll bit is set, the MAC discards the packet
without attempting any re-transmission.
Late Collisions: If a collision is detected after
the first 512 bits have been transmitted, the
MAC immediately terminates transmission,
transmits the jam sequence, discards the packet,
and sets the Out-of-window bit (Register 8,
TxEvent, Bit 9). The CS8920A does not initiate
backoff or attempt to re-transmit the frame. For
additional information about Late Collisions, see
Out-of-Window Error in this section.
Backoff: After the MAC has completed transmit-
ting the jam sequence, it must wait, or "Back
off", before attempting to transmit again. The
amount of time it must wait is determined by
one of two Backoff algorithms: the Standard
Backoff algorithm (ISO/IEC 4.2.3.2.5) or the
Modified Backoff algorithm. The host selects
which algorithm through the ModBackoffE bit
(Register 13, LineCTL, Bit B).
Standard Backoff: The Standard Backoff algo-
rithm, also called the "Truncated Binary
Exponential Backoff", is described by the equa-
tion:
0 ≤ r ≤ 2k
where r (a random integer) is the number of slot
times the MAC must wait (1 slot time = 512 bit
times), and k is the smaller of n or 10, where n
is the number of re-transmission attempts.
Modified Backoff: The Modified Backoff is de-
scribed by the equation:
0 ≤ r ≤ 2k
where r (a random integer) is the number of slot
times the MAC must wait, and k is 3 for n <3
and k is the smaller of n or 10 for n>3, where n
is the number of re-transmission attempts.
The advantage of the Modified Backoff algo-
rithm over the Standard Backoff algorithm is that
it reduces the possibility of multiple collisions
on the first three re-tries. The disadvantage is
that it extends the maximum time needed to gain
access to the network for the first three re-tries.
The host may choose to disable the Backoff al-
gorithm altogether by setting the DisableBackoff
bit (Register 19, TestCTL, Bit B). When dis-
abled, the CS8920A only waits the 9.6 µs IPG
time before starting transmission.
Carrier Sense
RX CLK
MAC
RX NRZ
TXCLK
TX NRZ
TEN
Port Select
Collision
Figure 3.9. ENDEC
ENDEC
Carrier
Detector
Decoder
& PLL
RX
MUX
Encoder
TX
MUX
Clock
RXSQL
10BASE-T
RX
Transceiver
TX
AUISQL
AUIRX
AUITX
AUI
AUICol
32
DS238PP2
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