LAN8740A Datasheet, PDF (22/136 Pages) Microchip Technology – Small Footprint MII/RMII 10/100 Energy Efficient Ethernet Transceiver with HP Auto-MDIX and flexPWR® Technology

LAN8740A/LAN8740Ai
3.1.3.1 10M Transmit Data Across the MII/RMII Interface
The MAC controller drives the transmit data onto the TXD bus. For MII, when the controller has driven TXEN high to
indicate valid data, the data is latched by the MII block on the rising edge of TXCLK. The data is in the form of 4-bit wide
2.5 MHz data. For RMII, TXD[1:0] shall transition synchronously with respect to REF_CLK. When TXEN is asserted,
TXD[1:0] are accepted for transmission by the device. TXD[1:0] shall be “00” to indicate idle when TXEN is deasserted.
Values of TXD[1:0] other than “00” when TXEN is deasserted are reserved for out-of-band signaling (to be defined).
Values other than “00” on TXD[1:0] while TXEN is deasserted shall be ignored by the device.TXD[1:0] shall provide valid
data for each REF_CLK period while TXEN is asserted.
In order to comply with legacy 10BASE-T MAC/Controllers, in half-duplex mode the transceiver loops back the trans-
mitted data, on the receive path. This does not confuse the MAC/Controller since the COL signal is not asserted during
this time. The transceiver also supports the SQE (Heartbeat) signal. See Section 3.8.9, "Collision Detect", for more
details.
3.1.3.2 Manchester Encoding
The 4-bit wide data is sent to the 10M TX block. The nibbles are converted to a 10 Mbps serial NRZI data stream. The
10M PLL locks onto the external clock or internal oscillator and produces a 20 MHz clock. This is used to Manchester
encode the NRZ data stream. When no data is being transmitted (TXEN is low), the 10M TX block outputs Normal Link
Pulses (NLPs) to maintain communications with the remote link partner.
3.1.3.3 10M Transmit Drivers
The Manchester-encoded data is sent to the analog transmitter where it is shaped and filtered before being driven out
as a differential signal across the TXP and TXN outputs.
3.1.4 10BASE-T RECEIVE
The 10BASE-T receiver gets the Manchester- encoded analog signal from the cable via the magnetics. It recovers the
receive clock from the signal and uses this clock to recover the NRZI data stream. This 10M serial data is converted to
4-bit data nibbles which are passed to the controller via MII at a rate of 2.5 MHz.
This 10M receiver uses the following blocks:
• Filter and SQUELCH (analog)
• 10M PLL (analog)
• RX 10M (digital)
• MII (digital)
3.1.4.1 10M Receive Input and Squelch
The Manchester signal from the cable is fed into the transceiver (on inputs RXP and RXN) via 1:1 ratio magnetics. It is
first filtered to reduce any out-of-band noise. It then passes through a SQUELCH circuit. The SQUELCH is a set of
amplitude and timing comparators that normally reject differential voltage levels below 300 mV and detect and recognize
differential voltages above 585 mV.
3.1.4.2 Manchester Decoding
The output of the SQUELCH goes to the 10M RX block where it is validated as Manchester encoded data. The polarity
of the signal is also checked. If the polarity is reversed (local RXP is connected to RXN of the remote partner and vice
versa), the condition is identified and corrected. The reversed condition is indicated by the XPOL bit of the Special Con-
trol/Status Indications Register. The 10M PLL is locked onto the received Manchester signal, from which the 20 MHz
cock is generated. Using this clock, the Manchester encoded data is extracted and converted to a 10 MHz NRZI data
stream. It is then converted from serial to 4-bit wide parallel data.
The 10M RX block also detects valid 10BASE-T IDLE signals - Normal Link Pulses (NLPs) - to maintain the link.
DS00001987A-page 22
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