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DP83255AVF Datasheet(PDF) 9 Page - National Semiconductor (TI) |
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DP83255AVF Datasheet(HTML) 9 Page - National Semiconductor (TI) |
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9 / 96 page  30 Functional Description (Continued) DATA REGISTERS Data from the Configuration Switch is stored in the Data Registers The 10-bit byte-wide data consists of a parity bit a control bit and two 4-bit symbols as shown in Figure 3-3 b9 b8 b7 b0 Parity Bit Control Bit Data Bits FIGURE 3-3 Byte-Wide Data PARITY CHECKER The Parity Checker verifies that the parity bit in the Data Register represents odd parity (ie odd number of 1s) The parity checking is enabled and disabled through bit 6 (PRDPE) of the Current Transmit State Register (CTSR) If a parity error occurs the Parity Checker will set bit 0 (DPE) in the Interrupt Condition Register (ICR) and report the error to the Repeat Filter 4B5B ENCODER The 4B5B Encoder converts the two 4-bit symbols from the Configuration Switch into their respective 5-bit codes See Table 3-2 for the Symbol Encoding list REPEAT FILTER The Repeat Filter is used to prevent the propagation of code violations in data frames to the downstream station Upon receiving violations in data frames the Repeat Filter replaces them with two Halt symbol pairs followed by Idle symbols Thus the code violations are isolated and recov- ered at each link and will not be propagated throughout the entire ring Details on Repeat Filter operation are described in Section 84 SMOOTHER The Smoother is used to keep the preamble length of a frame to a minimum of 6 Idle symbol pairs Idle symbols in the preamble of a frame may have been added or deleted by each station to compensate for the difference between the Receive Clock and its Local Clock The preamble needs to be maintained at a minimum length to allow stations enough time to complete processing of one frame and prepare to receive another Without the Smooth- er function the minimum preamble length (6 Idle symbol pairs) may not be maintained as several stations may con- secutively delete Idle symbols The Smoother attempts to keep the number of Idle symbol pairs in the preamble at 7 by Deleting an Idle symbol pair in preambles which have more than 7 Idle symbol pairs andor Inserting an Idle symbol pair in preambles which have less than 7 idle symbol pairs (ie Extend State) The Smoother Counter starts counting upon detecting an Idle symbol pair It stops counting upon detecting a JK sym- bol pair More details on the operation of the Smoother can be found in Section 85 LINE STATE GENERATOR The Line State Generator allows the transmission of the PHY Request data and can also generate and transmit Idle Master Halt or Quiet symbol pairs which can be used to implement the Connection Management procedures as specified in the FDDI Station Management (SMT) docu- ment The Line State Generator is programmed through Transmit bits 0 to 2 (TMk20l) of the Current Transmit State Regis- ter (CTSR) Based on the setting of these bits the Transmitter Block operates in the Transmit Modes where the Line State Gen- erator overwrites the Repeat Filter and Smoother outputs See Table 3-3 for the listing of the Transmit Modes TABLE 3-2 4B5B Symbol Encoding Symbol 4B Code Outgoing 5B 0 0000 11110 1 0001 01001 2 0010 10100 3 0011 10101 4 0100 01010 5 0101 01011 6 0110 01110 7 0111 01111 8 1000 10010 9 1001 10011 A 1010 10110 B 1011 10111 C 1100 11010 D 1101 11011 E 1110 11100 F 1111 11101 N 0000 11110 or 11111 JK (Starting 1101 11000 and Delimiter) 10001 T (Ending 0100 or 01101 Delimiter) 0101 R (Reset) 0110 00111 S (Set) 0111 11001 TABLE 3-3 Transmit Modes Active Transmit Mode Normal Transmission Mode Off Transmit Mode Transmit Quiet symbol pairs and disable the Fiber Optic Transmitter Idle Transmit Mode Transmit Idle symbol pairs Master Transmit Mode Transmit Halt-Quiet symbol pairs Quiet Transmit Mode Transmit Quiet symbol pairs Reserved Transmit Reserved for future use If Mode selected Quiet symbol pairs will be transmitted Halt Transmit Mode Transmit Halt symbol pairs 9 |
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