CS8904 Datasheet, PDF (13/34 Pages) Cirrus Logic – ETHERNET TRANSCEIVER

CS8904
Crystal LAN™ Quad Ethernet Transceiver
3.2.1 Encoder
The encoder converts NRZ data from the digital
controller and a 10 MHz Transmit Clock signal into
a serial stream of Manchester data. The Transmit
Clock is produced by an on-chip oscillator circuit
that is driven by either an external 20 MHz quartz
crystal or a TTL-level CMOS clock input. The
encoded signal is routed to the 10BASE-T
transceiver.
3.2.2 Carrier Detection
The internal Carrier Detection circuit informs the
digital controller that valid receive data is present
by asserting the Carrier Detect (CD) signal as soon
it detects a valid Ethernet preamble. During normal
packet reception, Carrier Detect remains asserted
while the frame is being received, and is de-
asserted after the last low-to-high transition of the
End-of-Frame (EOF) sequence. Whenever the
receiver is idle (no receive activity), Carrier Detect
remains de-asserted.
3.2.3 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 incoming
data. The receive clock is then compared to the
incoming data at the bit cell center and any phase
difference is corrected. The PLL remains locked as
long as the receiver input signal is valid. Once the
PLL has locked on the incoming data, the ENDEC
converts the Manchester data to NRZ and passes
the decoded data and the recovered clock to the
digital controller for further processing.
3.3 10BASE-T Transceiver
The CS8904 includes integral 10BASE-T
transceivers that are compliant with the relevant
portions of clause 14 of the Ethernet standard
(ISO/IEC 8802-3:1996). It includes all analog and
digital circuitry needed to interface the CS8904
directly to a simple isolation transformer (see
Figure 2 for a connection diagram). Figure 5
provides a block diagram of one of the 10BASE-T
transceivers.
Auto-Negotiation and Link
Control and Status
10BASE-T Transceiver
Auto-Negotiation
and Link Pulse
RX
ENDEC
RX
Comparator
RX Filters
Rx+
Rx-
TX
TX Pre-
Distortion
TX Filters
TX Drivers
Tx+
Tx-
Filter Tuning
Figure 5. 10BASE-T Transceiver
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