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DP83849C Datasheet(PDF) 33 Page - National Semiconductor (TI) |
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DP83849C Datasheet(HTML) 33 Page - National Semiconductor (TI) |
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33 / 96 page  33 www.national.com The signal at the start of a packet is checked by the smart squelch and any pulses not exceeding the squelch level (either positive or negative, depending upon polarity) will be rejected. Once this first squelch level is overcome cor- rectly, the opposite squelch level must then be exceeded within 150 ns. Finally the signal must again exceed the original squelch level within 150 ns to ensure that the input waveform will not be rejected. This checking procedure results in the loss of typically three preamble bits at the beginning of each packet. Only after all these conditions have been satisfied will a control signal be generated to indicate to the remainder of the circuitry that valid data is present. At this time, the smart squelch circuitry is reset. Valid data is considered to be present until the squelch level has not been generated for a time longer than 150 ns, indicating the End of Packet. Once good data has been detected, the squelch levels are reduced to minimize the effect of noise causing premature End of Packet detection. end of packet start of packet VSQ-(reduced) VSQ- VSQ+(reduced) VSQ+ <150 ns <150 ns >150 ns Figure 11. 10BASE-T Twisted Pair Smart Squelch Operation |
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