Datasheet

 2016 Microchip Technology Inc. DS00002165B-page 23
LAN8720A/LAN8720AI
3.4.1.1 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-X mode when 2 non-contiguous zeros 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-b
it of a nibble onto RXD[1:0] (for example, CRS_DV is deasserted only on nibble boundaries). If the device has addi-
tional 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 present the first di-bit of a nibble. The result is, starting on nibble boundaries, CRS_DV toggles at 25
MHz in 100Mbps mode and 2.5 MHz in 10Mbps mode when CRS ends before RXDV (for example, 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 fo
r 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.
3.4.1.2 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.
3.5 Serial Management Interface (SMI)
The Serial Management Interface is used to control the device and obtain its status. This interface supports registers 0
through 6 as required by Clause 22 of the 802.3 standard, as well as “vendor-specific” registers 16 to 31 allowed by the
specification. Non-supported registers (such as 7 to 15) will be read as hexadecimal “FFFF”. Device registers are
detailed in Section 4.0, "Register Descriptions," on page 43.
At the system level, SMI provides 2 signals: MDIO and MDC. The MDC signal is
an aperiodic clock provided by the
station management controller (SMC). MDIO is a bi-directional data SMI input/output signal that receives serial data
(commands) from the controller SMC and sends serial data (status) to the SMC. The minimum time between edges of
the MDC is 160 ns. There is no maximum time between edges. The minimum cycle time (time between two consecutive
rising or two consecutive falling edges) is 400 ns. These modest timing requirements allow this interface to be easily
driven by the I/O port of a microcontroller.
The data on the MDIO line is latched on the rising edge of the MDC. Th
e frame structure and timing of the data is shown
in Figure 3-5 and Figure 3-6. The timing relationships of the MDIO signals are further described in Section 5.5.6, "SMI
Timing," on page 64.
FIGURE 3-5:
MDC
MDIO
Read Cycle
...
32 1's 0110A4A3A2A1A0R4R3R2R1R0
D1
...
D15 D14 D0
Preamble
Start of
Frame
OP
Code
PHY Address Register Address
Turn
Around
Data
Data From Phy
Data To Phy
MDIO TIMING AND FRAME STRUCTURE - READ CYCLE