Cirrus Logic CS8900A - Manual (Page 39)

DS271F5 39
CS8900A
Crystal LAN™ Ethernet Controller
CIRRUS LOGIC PRODUCT DATASHEET
at least four frames in a row with negative po-
larity after the EOF, the receive pair is consid-
ered reversed. Any data received before the
correction of the reversal is ignored.
3.11.6 Collision Detection
If half-duplex operation is selected (Register
19, Bit E, FDX), the CS8900A detects a
10BASE-T collision whenever the receiver and
transmitter are active simultaneously. When a
collision is present, the Collision Detection cir-
cuit informs the MAC by asserting the internal
Collision signal (see Section 3.9 on page 29
for collision handling).
3.12 Attachment Unit Interface (AUI)
The CS8900A Attachment Unit Interface (AUI)
provides a direct interface to external
10BASE2, 10BASE5, and 10BASE-FL Ether-
net transceivers. It is fully compliant with Sec-
tion 7 of the Ethernet standard (ISO/IEC 8802-
3), and as such, is capable of driving a full 50-
meter AUI cable.
The AUI consists of three pairs of signals: Data
Out (DO+/DO-), Data In (DI+/DI-), and Colli-
sion In (CI+/CI-). To select the AUI, the host
should set the AUI bit (Register 13, LineCTL,
Bit 8). The AUI can also be selected automati-
cally as described in the previous section
(Section 3.10.4 on page 36). Figure 15 pro-
vides a block diagram of the AUI. (For a con-
nection diagram, see Section 7.6 on
page 122).
3.12.1 AUI Transmitter
The AUI transmitter is a differential driver de-
signed to drive a 78
Ω cable. It accepts data
from the ENDEC and transmits it directly on
the DO+/DO- pins. After transmission has
started, the CS8900A expects to see the pack-
et “looped-back” (or echoed) to the receiver,
causing the Carrier Sense signal to be assert-
ed. This Carrier Sense presence indicates that
the transmit signal is getting through to the
transceiver. If the Carrier Sense signal re-
mains deasserted throughout the transmis-
sion, or if the Carrier Sense signal is
deasserted before the end of the transmission,
there is a Loss-of-Carrier error and the Loss-
of-CRS bit (Register 8, TxEvent, Bit 6) is set.
3.12.2 AUI Receiver
The AUI receiver is a differential pair circuit
that connects directly to the DI+/DI- pins. It is
designed to distinguish between transient
noise pulses and incoming Ethernet packets.
Incoming packets with proper amplitude and
pulse width are passed on to the ENDEC sec-
tion, while unwanted noise is rejected.
3.12.3 Collision Detection
The AUI collision circuit is a differential pair re-
ceiver that detects the presence of collision
signals on the CI+/CI- pins. The collision signal
is generated by an external Ethernet trans-
ceiver whenever a collision is detected on the
Ethernet segment. (Section 7.3.1.2 of ISO/IEC
8802-3, 1993, defines the collision signal as a
10 MHz ± 15% signal with a duty cycle no
worse than 60/40). When a collision is present,
the AUI Collision circuit informs the MAC by
asserting the internal Collision signal.
DI+
DI-
DO+
DO-
ENDEC
CL+
CL-
Collision
Detect
AUICol (to MAC)
AUIRX
AUISQL
AUITX
AUI
- +
- +
Figure 15. AUI