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ManualsBrandsMicrochip ManualsIntegrated Circuits (ICs)TX Ethernet Transceiver Ethernet/MII/RMII VQFN 114mA
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2013-2015 Microchip Technology Inc. DS00001987A-page 19
LAN8740A/LAN8740Ai
3.1.1.3 Scrambling
Repeated data patterns (especially the IDLE code-group) can have power spectral densities with large narrow-band
peaks. Scrambling the data helps eliminate these peaks and spread the signal power more uniformly over the entire
channel bandwidth. This uniform spectral density is required by FCC regulations to prevent excessive EMI from being
radiated by the physical wiring.
The seed for the scrambler is generated from the transceiver address, PHYAD, ensuring that in multiple-transceiver
applications, such as repeaters or switches, each transceiver will have its own scrambler sequence.
The scrambler also performs the Parallel In Serial Out conversion (PISO) of the data.
3.1.1.4 NRZI and MLT-3 Encoding
The scrambler block passes the 5-bit wide parallel data to the NRZI converter where it becomes a serial 125 MHz NRZI
data stream. The NRZI is encoded to MLT-3. MLT-3 is a tri-level code where a change in the logic level represents a
code bit “1” and the logic output remaining at the same level represents a code bit “0”.
3.1.1.5 100M Transmit Driver
The MLT3 data is then passed to the analog transmitter, which drives the differential MLT-3 signal, on outputs TXP and
TXN, to the twisted pair media across a 1:1 ratio isolation transformer. The 10BASE-T and 100BASE-TX signals pass
through the same transformer so that common “magnetics” can be used for both. The transmitter drives into the 100 Ω
impedance of the CAT-5 cable. Cable termination and impedance matching require external components.
3.1.1.6 100M Phase Lock Loop (PLL)
The 100M PLL locks onto reference clock and generates the 125 MHz clock used to drive the 125 MHz logic and the
100BASE-TX transmitter.
3.1.2 100BASE-TX RECEIVE
The 100BASE-TX receive data path is shown in Figure 3-2. Each major block is explained in the following subsections.
FIGURE 3-2: 100BASE-TX RECEIVE DATA PATH