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LAN8820I Datasheet(PDF) 16 Page - Microchip Technology |
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LAN8820I Datasheet(HTML) 16 Page - Microchip Technology |
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16 / 83 page  LAN8820/LAN8820I DS00001871B-page 16 2009-2015 Microchip Technology Inc. Note 3-1 The internal CRS signal operates in two modes: Active and Low. When in Active mode, the internal CRS will transition high and low upon line activity, where a high value indicates a carrier has been detected. In Low mode, the internal CRS stays low and does not indicate carrier detection. 3.2 HP Auto-MDIX HP Auto-MDIX facilitates the use of CAT-5 (100BASE-T) media UTP interconnect cable without consideration of inter- face wiring scheme. If a user plugs in either a direct connect LAN cable, or a cross-over patch cable, as shown in Figure 3-1, the Auto-MDIX PHY is capable of configuring the twisted pair pins for correct transceiver operation. The internal logic of the device detects the TX and RX pins of the connecting device. It can automatically re-assign chan- nel 0 and 1 if required to establish a link. In 1000BASE-T mode, it can re-assign channel 2 and 3. Crossover resolution precedes the actual auto-negotiation process that involves exchange of FLPs to advertise capabilities. Automatic MDI/ MDIX is described in IEEE 802.3ab Clause 40, section 40.8.2. Since the RX and TX line pairs are interchangeable, spe- cial PCB design considerations are needed to accommodate the symmetrical magnetics and termination of an Auto- MDIX design. Auto-MDIX is enabled by default, and can be disabled by the Auto MDIX Disable bit in the 10/100 Mode Control/Status Register. When Auto-MDIX is disabled, the TX and RX pins can be configured manually by the MDI/MDI-X 0:1 and MDI/ MDI-X 2:3 bits in the Extended Mode Control/Status Register. The device includes an advanced crossover resolution capability called Semi Crossover. This is an extension to HP Auto-MDIX that corrects for a cable with only two pairs crossed. If Semi Crossover is enabled, after the device has attempted to establish a link with all four signal pairs normal or crossed, it will attempt to establish a link with pairs 2/3 switched and 0/1 straight, and then with pairs 0/1 switched and pairs 2/3 straight. The Semi Crossover is enabled by default, and can be disabled by the Semi Crossover Enable bit in the 10/100 Mode Control/Status Register. After resolution of crossed pairs is complete, using either HP Auto-MDIX or the Semi Crossover function, the MDI/MDI- X status is reported through the XOVER Resolution 0:1 and XOVER Resolution 2:3 bits of the User Status 2 Register. Auto-Negotiation 10 Mbps Full-Duplex Transmitting Low Auto-Negotiation 10 Mbps Full-Duplex Receiving Active Auto-Negotiation 100 Mbps Half-Duplex Transmitting Active Auto-Negotiation 100 Mbps Half-Duplex Receiving Active Auto-Negotiation 100 Mbps Full-Duplex Transmitting Low Auto-Negotiation 100 Mbps Full-Duplex Receiving Active TABLE 3-2: CRS BEHAVIOR (CONTINUED) Mode Speed Duplex Activity CRS Behavior ( Note 3-1) |
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