LAN8820 Datasheet, PDF (28/83 Pages) Microchip Technology – RGMII 10/100/1000 Ethernet Transceiver

LAN8820/LAN8820I
3.9.2 ISOLATE MODE
The device data paths may be electrically isolated from the RGMII interface by setting the Isolate bit of the Basic Control
Register to “1”. In isolation mode, the transceiver does not respond to the TXD and TXCTRL inputs, but does respond
to management transactions.
Isolation provides a means for multiple transceivers to be connected to the same RGMII interface without contention.
By default, the transceiver is not isolated (on power-up, Isolate=0).
3.9.3 CARRIER SENSE
The carrier sense signal is output on RXCTRL. Carrier sense operation is defined in the IEEE 802.3u standard. The
PHY asserts carrier sense based only on receive activity whenever the PHY is either in repeater mode or full-duplex
mode. Otherwise, the PHY asserts carrier sense based on either transmit or receive activity.
The carrier sense logic uses the encoded, unscrambled data to determine carrier activity status. It activates carrier
sense with the detection of 2 non-contiguous zeros within any 10 bit span. Carrier sense terminates if a span of 10 con-
secutive ones is detected before a /J/K/ Start-of Stream Delimiter pair. If an SSD pair is detected, carrier sense is
asserted until either /T/R/ End–of-Stream Delimiter pair or a pair of IDLE symbols is detected. Carrier is negated after
the /T/ symbol or the first IDLE. If /T/ is not followed by /R/, then carrier is maintained. Carrier is treated similarly for IDLE
followed by some non-IDLE symbol.
3.9.4 LINK INTEGRITY
This section details the establishment, maintenance and removal of links between the device and a link partner in
1000BASE-T, 100BASE-TX and 10BASE-T modes. Link status is reported in the Link Status bit of the Basic Status Reg-
ister. The link status is also used to drive the device LEDs as described in Section 3.9.1, "LEDs," on page 27.
3.9.4.1 Establishing and Maintaining a Link
Once a link state is determined via auto-negotiation, parallel detection, or forced operation, the device and the link part-
ner establish a link.
The completion of the auto-negotiation process is reported via the Auto-Negotiate Complete bit of the Basic Status Reg-
ister and issues an interrupt as described in Section 3.5, "Interrupt Management," on page 21. The speed of the link is
reported in the Speed Indication field of the PHY Special Control / Status Register. The speed is also reported on the
LED pins for any link.
Failure to complete the auto-negotiation process is reported through the following status bits:
• Parallel Detection Fault reported in the Auto Negotiation Expansion Register while operating in 10BASE-T or
100BASE-TX modes.
• Master/Slave Configuration Fault reported in the Master/Slave Status Register while operating in 1000BASE-T
mode.
A fault occurs if the Master/Slave configuration conditions do not allow master/slave resolution, as defined in the Master/
Slave Manual Config Enable and Master/Slave Manual Config Value bits in the Master/Slave Control Register of the
local and remote link partners.
3.9.4.2 1000BASE-T
For 1000BASE-T links, the device and its link partner enter a training phase after completion of the auto-negotiation
process. The links exchange idle symbols and use the information obtained from receiving this signal to set their adap-
tive filter coefficients.
These coefficients are used in the receiver to equalize the incoming signal, as well as eliminate signal impairments such
as echo and cross-talk. Each side indicates completion of the training phase to its link partner by changing the encoding
of the idle-symbols it transmits.
The link is established after both sides indicate completion of the training phase. Each side continues to send idle sym-
bols whenever it has no data to transmit. The link is maintained as long as valid idle, data, or carrier extension symbols
are received.
Status of both local and remote receivers is reported in the Local Receiver Status and Remote Receiver Status bits of
the Master/Slave Status Register.
DS00001871B-page 28
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