DM9620 DAVICOM [Davicom Semiconductor, Inc.], DM9620 Datasheet - Page 53

• Manufacturer Part Number: DM9620
• Description: USB2.0 to 10/100M Fast Ethernet Controller
• Manufacturer: DAVICOM [Davicom Semiconductor, Inc.]

Table 1. This conversion is required for control and
packet data to be combined in code groups. The
4B5B encoder substitutes the first 8 bits of the MAC
preamble with a J/K code-group pair (11000 10001)
upon transmit. The 4B5B encoder continues to
replace subsequent 4B preamble and data nibbles
with corresponding 5B code-groups. At the end of the
transmit packet, upon the deassertion of the Transmit
Enable signal from the MAC Reconciliation layer, the
4B5B encoder injects the T/R code-group pair (01101
00111) indicating end of frame. After the T/R
code-group pair, the 4B5B encoder continuously
injects IDLEs into the transmit data stream until
Transmit Enable is asserted and the next transmit
packet is detected.
The DM9620 includes a Bypass 4B5B conversion
option within the 100Base-TX Transmitter for support
of applications like 100 Mbps repeaters which do not
require 4B5B conversion.
7.2.4 Scrambler
The scrambler is required to control the radiated
emissions (EMI) by spreading the transmit energy
across the frequency spectrum at the media
connector and on the twisted pair cable in
100Base-TX operation.
By scrambling the data, the total energy presented to
the cable is randomly distributed over a wide
frequency range. Without the scrambler, energy
levels on the cable could peak beyond FCC
limitations at frequencies related to repeated 5B
sequences like continuous transmission of IDLE
symbols. The scrambler output is combined with the
NRZ 5B data from the code-group encoder via an
Preliminary
Version: DM9620 -15-DS-P02
February 20, 2012
XOR logic function. The result is a scrambled data
stream with sufficient randomization to decrease
radiated emissions at critical frequencies.
7.2.5 Parallel to Serial Converter
The Parallel to Serial Converter receives parallel 5B
scrambled data from the scrambler and serializes it
(converts it from a parallel to a serial data stream).
The serialized data stream is then presented to the
NRZ to NRZI Encoder block
7.2.6 NRZ to NRZI Encoder
After the transmit data stream has been scrambled
and serialized, the data must be NRZI encoded for
compatibility with the TP-PMD standard for
100Base-TX transmission over Category-5
unshielded twisted pair cable.
7.2.7 MLT-3 Converter
The MLT-3 conversion is accomplished by converting
the data stream output from the NRZI encoder into
two binary data streams with alternately phased logic
one events.
7.2.8 MLT-3 Driver
The two binary data streams created at the MLT-3
converter are fed to the twisted pair output driver
which converts these streams to current sources and
alternately drives either side of the transmit
transformer primary winding resulting in a minimal
current MLT-3 signal. Refer to figure 4 for the block
diagram of the MLT-3 converter.
USB2.0 to Fast Ethernet Controller
DM9620
53