Cirrus Logic CS8900A - Manual (Page 36)

36 DS271F5
CS8900A
Crystal LAN™ Ethernet Controller
CIRRUS LOGIC PRODUCT DATASHEET
40% and 60%. The specifications for the crys-
tal are described in Section 7.7 on page 122.
The encoded signal is routed to either the
10BASE-T transceiver or AUI, depending on
configuration.
3.10.2 Carrier Detection
The internal Carrier Detection circuit informs
the MAC that valid receive data is present by
asserting the internal Carrier Sense signal as
soon it detects a valid bit pattern (1010b or
0101b for 10BASE-T, and 1b or 0b for AUI).
During normal packet reception, Carrier Sense
remains asserted while the frame is being re-
ceived, and is deasserted 1.3 to 2.3 bit times
after the last low-to-high transition of the End-
of-Frame (EOF) sequence. Whenever the re-
ceiver is idle (no receive activity), Carrier
Sense is deasserted. The CRS bit (Register
14, LineST, Bit E) reports the state of the Car-
rier Sense signal.
3.10.3 Clock and Data Recovery
When the receiver is idle, the phase-lock loop
(PLL) is locked to the internal clock signal. The
assertion of the Carrier Sense signal interrupts
the PLL. When it restarts, it locks on the in-
coming data. The receive clock is then com-
pared to the incoming data at the bit cell center
and any phase difference is corrected. The
PLL remains locked as long as the receiver in-
put signal is valid. Once the PLL has locked on
the incoming data, the ENDEC converts the
Manchester data to NRZ and passes the de-
coded data and the recovered clock to the
MAC for further processing.
3.10.4 Interface Selection
Physical interface selection is determined by
AUIonly bit (Bit 8) and the AutoAUI/10BT (Bit
9) in the LineCTL register (Register 13). Table
12 describes the possible configurations.
3.10.4.1 10BASE-T Only
When configured for 10BASE-T only opera-
tion, the 10BASE-T transceiver and its inter-
face to the ENDEC are active, and the AUI is
powered down.
3.10.4.2 AUI Only
When configured for AUI-only operation, the
AUI and its interface to the ENDEC are active,
and the 10BASE-T transceiver is powered
down.
3.10.4.3 Auto-Select
In Auto-Select mode, the CS8900A automati-
cally selects the 10BASE-T interface and pow-
ers down the AUI if valid packets or link pulses
are detected by the 10BASE-T receiver. If val-
id packets and link pulses are not detected, the
CS8900A selects the AUI. Whenever the AUI
is selected, the 10BASE-T receiver remains
active to listen for link pulses or packets. If
10BASE-T activity is detected, the CS8900A
switches back to 10BASE-T.
3.11 10BASE-T Transceiver
The CS8900A includes an integrated
10BASE-T transceiver that is compliant with
the relevant portions of section 14 of the Ether-
net standard (ISO/IEC 8802-3, 1993). It in-
cludes all analog and digital circuitry needed to
interface the CS8900A directly to a simple iso-
lation transformer (see Section 7.5 on
page 121 for a connection diagram). Figure 13
provides a block diagram of the 10BASE-T
transceiver.
AUIonly
(Bit 8)
AutoAUI/10BT
(Bit 9)
Physical
Interface
0 0 10BASE-T Only
1 N/A AUI Only
0 1 Auto-Select
Table 12. Interface Selection