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AM79C976 Scheda tecnica(PDF) 73 Page - Advanced Micro Devices |
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AM79C976 Scheda tecnica(HTML) 73 Page - Advanced Micro Devices |
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73 / 309 page  8/01/00 Am79C976 73 P R E L I M INARY )LJXUH ,((( )UDPHDQG/HQJWK)LHOG7UDQVPLVVLRQ2UGHU Since any valid Ethernet Type field value will always be greater than a normal IEEE 802.3 Length field (Š46), the Am79C976 controller will not attempt to strip valid Ethernet frames. Note that for some network protocols, the value passed in the Ethernet Type and/or IEEE 802.3 Length field is not compliant with either standard and may cause problems if pad stripping is enabled. 5HFHLYH )&6&KHFNLQJ Reception and checking of the received FCS is per- formed automatically by the Am79C976 controller. Note that if the Automatic Pad Stripping feature is en- abled, the FCS for padded frames will be verified against the value computed for the incoming bit stream including pad characters, but the FCS value for a pad- ded frame will not be passed to the host. If an FCS error is detected in any frame, the error will be reported in the CRC bit in the Receive Descriptor. 5HFHLYH ([FHSWLRQ&RQGLWLRQV Exception conditions for frame reception fall into two categories, i.e., those conditions which are the result of normal network operation, and those which occur due to abnormal network and/or host related events. Normal exception events are caused by collisions, which can distor t and truncate received frames. Frames shorter than 64 bytes will, by default, be dis- carded. These fragments will be discarded regardless of whether the receive frame was the first (or only) frame in the FIFO or if the receive frame was queued behind a previously received message. There are two control bits that can be used to cause the MAC to override normal behavior and accept all frames that pass address match, regardless of the frame length. Setting the Runt Packet Accept (RPA) bit (CMD2, bit 19) causes the MAC to accept runt packets when the device is operating in either half- or full-du- plex mode. Setting Full-Duplex Runt Packet Accept (FDRPA, CMD2, bit 20) causes the MAC to accept runt packets when the device is operating in full-duplex mode. (When the value of RPA is 1, runt packets are accepted regardless of the duplex mode or the value of FDRPA.) In either case, there is a minimum frame size of 16 bytes. Frames shorter than this may not be ac- cepted, regardless of the value of RPA or FDRPA. Abnormal network conditions include: s )&6 HUURUV s )UDPLQJ HUURUV s 5HFHLYHU RYHUIORZ HYHQWV These error conditions are reported in the correspond- ing receive descriptors. The RcvFCSErrors, RcvAlign- m ent Erro rs , or R cvMi ssPkt s cou n t e r i s a l so incremented when one of these events occurs. Statistics Counters In order to provide network management information with minimum host CPU overhead, the Am79C976 de- vice automatically maintains a set of 32-bit controller statistics counters. These counters are mapped di- Preamble 1010....1010 SFD 10101011 Destination Address Source Address Length LLC Data Pad FCS 4 Bytes 46 – 1500 Bytes 2 Bytes 6 Bytes 6 Bytes 8 Bits 56 Bits Start of Frame at Time = 0 Increasing Time Bit 0 Bit 7 Bit 0 Bit 7 Most Significant Byte Least Significant Byte 1 – 1500 Bytes 45 – 0 Bytes |
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