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AM79C978VC/W 数据表(PDF) 76 Page - Advanced Micro Devices |
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AM79C978VC/W 数据表(HTML) 76 Page - Advanced Micro Devices |
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76 / 261 page  76 Am79C978 Figure 37. HomePNA PHY Framing HomePNA Symbol Waveform All HomePNA symbols are composed at the transmitter of a silence interval, and a pulse formed of an integer number of cycles (TX_PULSE_CYCLES_P/N in H P R29 ) of a squar e wa ve of f r e que ncy (CENTER_FREQUENCY TX_PULSE_WIDTH in HPR29) that has been filtered with a bandpass filter. Data is encoded in the time interval from the preceding pulse. Table 11. HomePNA PHY Pulse Parameters Time Interval Unit HomePNA PHY time intervals are expressed in Time Interval Clock (TIC) units. One TIC is defined as 1/60E6 seconds or approximately 116.7 ns. ACCESS ID Intervals A HomePNA frame begins with an Access ID (AID) in- terval which is composed of eight equally spaced sub- intervals termed AID symbols 0 through 7 as shown in Figure 37. An AID symbol is 129 TICs long. Transmit timing is shown in Figure 38; receive timing in Figure 39. Timing starts at the beginning of each AID symbol at TIC = 0 and ends at TIC = 129. These symbols are described in the following sections. Symbol 0 (SYNC interval) SYNC Transmit Timing The SYNC interval (AID symbol 0) delineates the be- ginning of a HomePNA frame and is composed of a SYNC_START pulse, followed by a SYNC_END pulse, after a fixed silence interval as shown in Figure 38. Timing for this (AID symbol 0) starts (TIC = 0) at the be- ginning of the SYNC_START pulse. The SYNC_END pulse starts at TIC = 126. At TIC = 129, this AID symbol 0 ends and the next AID symbol begins, with the symbol timing reference reset to TIC = 0. No information bits are coded in the SYNC (AID symbol 0 interval). SYNC Receive Timing As soon as the SYNC_START pulse is detected the re- ceiver disables (blanks) further detection until time TIC = 61, after which detection is re-enabled for the next received pulse. The receiver allows for jitter by es- tablishing a window around each legal pulse position. This window is -2 +1 TICS wide on either side of the po- sition. A SYNC_END pulse that arrives outside the window of the legal TIC = 126 is considered a noise event which is used in setting the adaptive squelch level, aborts the packet, and sets the receiver in search of a new SYNC_START pulse and SYNC interval. If it is a trans- mitting station, the COLLISION event is asserted as described in the Collisions section. SYNC interval Access ID Silence PCOM 4 bytes Source 6 Destination Length 2 ETHERNET MAC and DATA max 1500 CRC 4 32 bits PCOM Ethernet Packet Fixed 14.93 µs AID blanking interval AID blanking interval AID blanking interval AID blanking interval AID blanking interval AID blanking interval 01 11 10 00 01 00 60 tics 129 tics 129 tics 129 tics 129 tics 129 tics 129 tics 129 tics Data symbols 20 tics 66 tics Silence interval SYNC Symbol 0 ACCESS ID Symbol 1 ACCESS ID Symbol 2 ACCESS ID Symbol 3 ACCESS ID Symbol 4 ACCESS ID Symbol 5 ACCESS ID Symbol 6 ACCESS ID Symbol 7 30.75 µs @ 1 Mbps ACCESS ID interval Example Access ID of 01110100 and control word 0100 Fixed 119.44 µs HomePNA PHY Header 150.19 µs @ 1 Mbps 1 Tic = 116.6667 ns HomePNA Header Ethernet Packet pulse 129 tics = receiver blanking interval potential pulse position 6 22206B-41 Parameter Value Tolerance Unit CENTER_FREQUENCY 7.5 500 PPM MHz CYCLES_PER_PULSE 4 -- Cycles |
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