LAN8740A Datasheet, PDF (27/136 Pages) Microchip Technology – Small Footprint MII/RMII 10/100 Energy Efficient Ethernet Transceiver with HP Auto-MDIX and flexPWR® Technology

LAN8740A/LAN8740Ai
3.4.2.1 CRS_DV - Carrier Sense/Receive Data Valid
The CRS_DV is asserted by the device when the receive medium is non-idle. CRS_DV is asserted asynchronously on
detection of carrier due to the criteria relevant to the operating mode. In 10BASE-T mode when squelch is passed, or
in 100BASE-TX mode when 2 non-contiguous zeroes in 10 bits are detected, the carrier is said to be detected.
Loss of carrier shall result in the deassertion of CRS_DV synchronous to the cycle of REF_CLK which presents the first
di-bit of a nibble onto RXD[1:0] (i.e., CRS_DV is deasserted only on nibble boundaries). If the device has additional bits
to be presented on RXD[1:0] following the initial deassertion of CRS_DV, then the device shall assert CRS_DV on cycles
of REF_CLK which present the second di-bit of each nibble and de-assert CRS_DV on cycles of REF_CLK which pres-
ent the first di-bit of a nibble. The result is, starting on nibble boundaries, CRS_DV toggles at 25 MHz in 100 Mbps mode
and 2.5 MHz in 10 Mbps mode when CRS ends before RXDV (i.e., the FIFO still has bits to transfer when the carrier
event ends). Therefore, the MAC can accurately recover RXDV and CRS.
During a false carrier event, CRS_DV shall remain asserted for the duration of carrier activity. The data on RXD[1:0] is
considered valid once CRS_DV is asserted. However, since the assertion of CRS_DV is asynchronous relative to
REF_CLK, the data on RXD[1:0] shall be “00” until proper receive signal decoding takes place.
3.4.2.2 Reference Clock (REF_CLK)
The RMII REF_CLK is a continuous clock that provides the timing reference for CRS_DV, RXD[1:0], TXEN, TXD[1:0]
and RXER. The device uses REF_CLK as the network clock such that no buffering is required on the transmit data path.
However, on the receive data path, the receiver recovers the clock from the incoming data stream, and the device uses
elasticity buffering to accommodate for differences between the recovered clock and the local REF_CLK.
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DS00001987A-page 27