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AM79C976 Datasheet(PDF) 82 Page - Advanced Micro Devices |
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AM79C976 Datasheet(HTML) 82 Page - Advanced Micro Devices |
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82 / 309 page  82 Am79C976 8/01/00 P R E L I M INARY )LJXUH0HGLD ,QGHSHQGHQW,QWHUIDFH The receive process starts when RX_DV is asserted. RX_DV must remain asserted until the end of the re- ceive frame. If the external PHY device detects errors in the currently received frame, it asserts the RX_ER signal. RX_ER can be used to signal special conditions out of band when RX_DV is not asserted. Two defined out-of-band conditions for this are the 100BASE-TX signaling of bad Start of Frame Delimiter and the 100BASE-T4 indication of illegal code group before the receiver has synchronized with the incoming data. The Am79C976 controller will not respond to these condi- tions. All out of band conditions are currently treated as NULL events. Certain in-band non-IEEE 802.3-compli- ant flow control sequences may cause erratic behavior for the Am79C976 controller. Consult the switch/ bridge/router/hub manual to disable the in-band flow control sequences if they are being used. 0,, 1HWZRUN6WDWXV,QWHUIDFH The MII also provides the CRS (Carrier Sense) and COL (Collision Sense) signals that are required for IEEE 802.3 operation. Carrier Sense is used to detect non-idle activity on the network for the purpose of inter- frame spacing timing in half-duplex mode. Collision Sense is used to indicate that simultaneous transmis- sion has occurred in a half-duplex network. 0,, 0DQDJHPHQW,QWHUIDFH The MII provides a two-wire management interface so that the Am79C976 controller can control external PHY devices and receive status from them. The Am79C976 controller offers direct hardware sup- port of the external PHY device without software inter- vention. The device automatically uses the MII Management Interface to read auto-negotiation infor- mation from the external PHY device and configures the MAC accordingly. The controller also provides the host CPU indirect access to the external PHY through the MII Control, Address, and Data registers (BCR32, 33, 34). With software support the Am79C976 controller can support up to 31 external PHYs attached to the MII Management Interface. Two independent state machines use the MII Manage- ment Interface to poll external PHY devices: the Net- work Port Manager and the Auto-poll State Machine. The Network Port Manager coordinates the auto-nego- tiation process, while the Auto-poll State Machine inter- rupts the host CPU when it detects changes in user- selected PHY registers. The Network Port Manager sends a management frame to the default PHY about once every 900 ms to determine auto-negotiation results and the current link status. The Network Port Manager uses the auto-nego- tiation results to set the MAC’s speed, duplex mode, and flow control ability. Changes detected by the Net- work Port Manager affect the operation of the MAC and MIB counters. For example, if link failure is detected, the transmitter will increment the XmtLossCarrier counter each time it attempts to transmit a frame. The Auto-poll State Machine periodically sends man- agement frames to poll the status register of the default PHY device plus up to 5 user-selected PHY registers and interrupts the host processor if it detects a change in any of these registers. The Auto-poll State Machine does not change the state of the MAC engine. 0,, 0DQDJHPHQW)UDPHV The format of an MII Management Frame is defined in Clause 22 of IEEE Std 802.3. The start of an MII Man- 4 RXD(3:0) RX_DV RX_ER RX_CLK 4 TXD(3:0) TX_EN Am79C976 COL CRS Receive Signals Transmit Signals Management Port Signals Network Status Signals TX_CLK MDIO MDC |
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