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Moteur de recherche de fiches techniques de composants électroniques |
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IDT77V126L200 Fiches technique(PDF) 6 Page - Integrated Device Technology |
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IDT77V126L200 Fiches technique(HTML) 6 Page - Integrated Device Technology |
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6 / 30 page  6 of 30 December 2004 IDT77V126L200 Functional Description Transmission Convergence (TC) Sub Layer Introduction The TC sub layer defines the line coding, scrambling, data framing and synchronization. Under control of a switch interface or Segmenta- tion and Reassembly (SAR) unit, the 25.6Mbps ATM PHY accepts a 53- byte ATM cell, scrambles the data, appends a command byte to the beginning of the cell, and encodes the entire 53 bytes before transmis- sion. These data transformations ensure that the signal is evenly distrib- uted across the frequency spectrum. In addition, the serialized bit stream is NRZI coded. An 8kHz timing sync pulse may be used for isochronous communications. Data Structure and Framing Each 53-byte ATM cell is preceded with a command byte. This byte is distinguished by an escape symbol followed by one of 17 encoded symbols. Together, this byte forms one of seventeen possible command bytes. Three command bytes are defined: 1. X_X (read: 'escape' symbol followed by another 'escape'): Start- of-cell with scrambler/descrambler reset. 2. X_4 ('escape' followed by '4'): Start-of-cell without scrambler/ descrambler reset. 3. X_8 ('escape' followed by '8'): 8kHz timing marker. This command byte is generated when the 8kHz sync pulse is detected, and has priority over all line activity (data or command bytes). It is transmitted immediately when the sync pulse is detected. When this occurs during a cell transmission, the data transfer is temporarily interrupted on an octet boundary, and the X_8 command byte is inserted. This condition is the only allowed interrupt in an otherwise contiguous transfer. Below is an illustration of the cell structure and command byte usage: {X_X} {53-byte ATM cell} {X_4} {53-byte ATM {X_8} cell}... In the above example, the first ATM cell is preceded by the X_X start- of-cell command byte which resets both the transmitter-scrambler and receiver-descrambler pseudo-random nibble generators (PRNG) to their initial states. The following cell illustrates the insertion of a start-of-cell command without scrambler/descrambler reset. During this cell's trans- mission, an 8kHz timing sync pulse triggers insertion of the X_8 8kHz timing marker command byte. Transmission Description Refer to Figure 2. Cell transmission begins with the PHY-ATM Inter- face. An ATM layer device transfers a cell into the 77V126L200 across the Utopia transmit bus. This cell enters a 3-cell deep transmit FIFO. Once a complete cell is in the FIFO, transmission begins by passing the cell, four bits (MSB first) at a time to the 'Scrambler'. The ‘Scrambler’ takes each nibble of data and exclusive-ORs them against the 4 high order bits (X(t), X(t-1), X(t-3)) of a 10 bit pseudo- random nibble generator (PRING). Its function is to provide the appro- priate frequency distribution for the signal across the line. The PRNG is clocked every time a nibble is processed, regardless of whether the processed nibble is part of a data or command byte. Note however that only data nibbles are scrambled. The entire command byte (X _C) is NOT scrambled before it's encoded (see diagram for illustration). The PRNG is based upon the following polynomial: X10 + X7 + 1 With this polynomial, the four output data bits (D3, D2, D1, D0) will be generated from the following equations: D3 = d3 xor X(t-3) D2 = d2 xor X(t-2) D1 = d1 xor X(t-1) D0 = d0 xor X(t) The following nibble is scrambled with X(t+4), X(t+3), X(t+2), and X(t+1). A scrambler lock between the transmitter and receiver occurs each time an X_X command is sent. An X_X command is initiated only at the beginning of a cell transfer after the PRNG has cycled through all of its states (210 - 1 = 1023 states). The first valid ATM data cell transmitted after power on will also be accompanied with an X_X command byte. Each time an X_X command byte is sent, the first nibble after the last escape (X) nibble is XOR'd with 1111b (PRNG = 3FFx). Because a timing marker command (X_8) may occur at any time, the possibility of a reset PRNG start-of-cell command and a timing marker command occurring consecutively does exist (e.g. X_X_X_8). In this case, the detection of the last two consecutive escape (X) nibbles will cause the PRNG to reset to its initial 3FFx state. Therefore, the PRNG is clocked only after the first nibble of the second consecutive escape pair. Once the data nibbles have been scrambled using the PRNG, the nibbles are further encoded using a 4b/5b process. The 4b/5b scheme ensures that an appropriate number of signal transitions occur on the line. A total of seventeen 5-bit symbols are used to represent the sixteen 4-bit data nibbles and the one escape (X) nibble. The table below lists the 4-bit data with their corresponding 5-bit symbols: Data 0000 0100 1000 1100 Symbol 10101 00111 10010 10111 Symbol 01001 01101 11001 11101 Data 0001 0101 1001 1101 Symbol 01010 01110 11010 11110 Data 0010 0110 1010 1110 Data 0011 0111 1011 1111 ESC(X) = 00010 3505 drw 05a Symbol 01011 01111 11011 11111 . . |
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