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ORT8850 Datasheet(PDF) 45 Page - Lattice Semiconductor |
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ORT8850 Datasheet(HTML) 45 Page - Lattice Semiconductor |
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45 / 105 page  Lattice Semiconductor ORCA ORT8850 Data Sheet 45 The pointer mover block can correctly process any length of concatenation of STS frames (multiple of three) as long as it begins on an STS-3 boundary (i.e., STS-1 number one, four, seven, ten, etc.) and is contained within the smaller of STS-3, 12, or 48. The pointer value can be adjusted to change the start of the SPE position and thereby adjust for frequency changes. The variations in frequency are taken care of using justification. When the incoming data rate exceeds the nominal rate, negative justification is used to compensate for the fre- quency difference. Data are transmitted in the pointer action byte, H3, and thereby adjust for the additional incom- ing data. The size of one SPE is still the same, but it is transmitted in less time than usual, and so a higher data rate is achieved for some time. Negative justification causes the start of SPE to move left by one column or by decreas- ing the pointer by one. The following frames contain the new pointer value. Negative justification is indicated by inverting the D bits (bits 8, 10, 12, 14, 16) of the pointer word. The receiver determines the occurrence of negative justification by examining these bits of the pointer word and applying a 5-bit majority logic on them. When the incoming data rate lags the nominal rate, positive justification is used to compensate for the frequency difference. Data are not transmitted in the byte following the pointer action byte, H3, and thereby adjust for the lack of incoming data. The size of one SPE is still the same, but it is transmitted in more time than usual, and hence a slower data rate is achieved for some time. Positive justification causes the start of SPE to move right by one col- umn or by increasing the pointer by one. The following frames contain the new pointer value. Positive justification is indicated by inverting the I bits (bits 7, 9, 11, 13, 15) of the pointer word. The receiver deter- mines the occurrence of positive justification by examining these bits of the pointer word and applying a 5-bit major- ity logic on them. The SPE signal to the FPGA logic must be high during negative stuff opportunity byte time slots (H3) for which valid data is carried (negative stuffing). SPE signal must be low during positive stuff opportunity byte time slots for which there is no valid data (positive stuffing). This behavior is shown in the following figure. Figure 25. SPE Signal During Justification In either justification, the pointer must remain unchanged for at least three consecutive frames before it can be jus- tified again. The pointer can jump randomly to a new position at any point of time. This can happen in conditions when the transmitting end has just recovered from an error condition. A sudden jump in the pointer value is indi- cated through NDF, New Data Flag. This information is carried in the four MS bits of the pointer word. A 3-bit major- ity logic is applied on the NDF bits to determine the status of the pointer jump. Pointer Generator The pointer generator maps the corresponding bytes into their appropriate location in the outgoing byte stream. The generator also creates offset pointers based on the location of the J1 byte as indicated by the pointer inter- preter. The generator will signal NDFs when the interpreter signals that it is coming out of AIS state. The pointer generator resets the pointer value and generates NDF every time a byte marked J1 is read from the elastic store that doesn't match the previous offset. Increment and decrement signals from the pointer interpreter are latched once per frame on either the F1 or E2 byte times (depending on collisions); this ensures constant values during the H1 through H3 times. The choice of which byte time to do the latching on is made once when the relative frame STS-12 TOH ROW # 4 SPE ROW # 4 H1 H1 H1 H1 H1 H1 H1 H1 H1 H1 H1 H1 H2 H2 H2 H2 H2 H2 H2 H2 H2 H2 H2 H2 H3 H3 H3 H3 H3 H3 H3 H3 H3 H3 H3 H3 STS-12 SPE POSITIVE STUFF OPPORTUNITY BYTES 12 34 56 7 8 9 10 11 12 NEGATIVE STUFF OPPORTUNITY BYTES SPE SIGNAL SHOWS NEGATIVE STUFFING FOR 2ND STS-1, AND POSITIVE STUFFING FOR 6TH STS-1 |
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