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LAN8820 Datasheet(PDF) 28 Page - Microchip Technology |
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LAN8820 Datasheet(HTML) 28 Page - Microchip Technology |
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28 / 83 page  LAN8820/LAN8820I DS00001871B-page 28 2009-2015 Microchip Technology Inc. 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. |
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