Cirrus Logic CS8900A - Manual (Page 35)

DS271F5 35
CS8900A
Crystal LAN™ Ethernet Controller
CIRRUS LOGIC PRODUCT DATASHEET
bit times), and k is the smaller of n or 10, where
n is the number of retransmission attempts.
3.9.5.9 Modified Backoff
The Modified Backoff is described by the
equation:
0
≤ r ≤ 2 k
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 retransmission at-
tempts.
The advantage of the Modified Backoff algo-
rithm over the Standard Backoff algorithm is
that it reduces the possibility of multiple colli-
sions on the first three retries. The disadvan-
tage is that it extends the maximum time
needed to gain access to the network for the
first three retries.
The host may choose to disable the Backoff al-
gorithm altogether by setting the DisableBack-
off bit (Register 19, TestCTL, Bit B). When
disabled, the CS8900A only waits the 9.6 µs
IPG time before starting transmission.
3.9.5.10 SQE Test
If the CS8900A is transmitting on the AUI, the
external transceiver should generate an SQE
Test signal on the CI+/CI- pair following each
transmission. The SQE Test is a 10 MHz sig-
nal lasting 5 to 15 bit times and starting within
0.6 to 1.6 µs after the end of transmission.
During this period, the CS8900A ignores re-
ceive carrier activity (see SQE Error in this
section for more information).
3.10 Encoder/Decoder (ENDEC)
The CS8900A’s integrated encoder/decoder
(ENDEC) circuit is compliant with the relevant
portions of section 7 of the Ethernet standard
(ISO/IEC 8802-3, 1993). Its primary functions
include: Manchester encoding of transmit da-
ta; informing the MAC when valid receive data
is present (Carrier Detection); and, recovering
the clock and NRZ data from incoming Man-
chester-encoded data.
Figure 12 provides a block diagram of the EN-
DEC and how it interfaces to the MAC, AUI
and 10BASE-T transceiver.
3.10.1 Encoder
The encoder converts NRZ data from the MAC
and a 20 MHz Transmit Clock signal into a se-
rial stream of Manchester data. The Transmit
Clock is produced by an on-chip oscillator cir-
cuit that is driven by either an external 20 MHz
quartz crystal or a TTL-level CMOS clock in-
put. If a CMOS input is used, the clock should
be 20 MHz
±0.01% with a duty cycle between
Encoder
Carrier
Detector
Decoder
& PLL
RX
MUX
TX
MUX
RXSQL
AUISQL
RX
TX
AUIRX
AUITX
AUICol
Clock
Carrier Sense
RX CLK
RX NRZ
TXCLK
TX NRZ
TEN
Port Select
Collision
MAC
ENDEC
10BASE-T
Transceiver
AUI
Figure 12. ENDEC