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FLEX8000 Datasheet(PDF) 8 Page - Altera Corporation |
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FLEX8000 Datasheet(HTML) 8 Page - Altera Corporation |
8 / 61 page 8 Altera Corporation FLEX 8000 Programmable Logic Device Family Data Sheet The FLEX 8000 architecture provides two dedicated high-speed data paths—carry chains and cascade chains—that connect adjacent LEs without using local interconnect paths. The carry chain supports high- speed counters and adders; the cascade chain implements wide-input functions with minimum delay. Carry and cascade chains connect all LEs in an LAB and all LABs in the same row. Heavy use of carry and cascade chains can reduce routing flexibility. Therefore, the use of carry and cascade chains should be limited to speed-critical portions of a design. Carry Chain The carry chain provides a very fast (less than 1 ns) carry-forward function between LEs. The carry-in signal from a lower-order bit moves forward into the higher-order bit via the carry chain, and feeds into both the LUT and the next portion of the carry chain. This feature allows the FLEX 8000 architecture to implement high-speed counters and adders of arbitrary width. The MAX+PLUS II Compiler can create carry chains automatically during design processing; designers can also insert carry chain logic manually during design entry. Figure 4 shows how an n-bit full adder can be implemented in n + 1 LEs with the carry chain. One portion of the LUT generates the sum of two bits using the input signals and the carry-in signal; the sum is routed to the output of the LE. The register is typically bypassed for simple adders, but can be used for an accumulator function. Another portion of the LUT and the carry chain logic generate the carry-out signal, which is routed directly to the carry-in signal of the next-higher-order bit. The final carry-out signal is routed to another LE, where it can be used as a general-purpose signal. In addition to mathematical functions, carry chain logic supports very fast counters and comparators. |
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