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Xilinx LCA (a trademark, denoting logic cell array) basic logic cells, configurable logic blocks or CLBs , are bigger and more complex than the Actel or QuickLogic cells. The Xilinx LCA basic logic cell is an example of a coarsegrain architecture . The Xilinx CLBs contain both combinational logic and flipflops.
The XC3000 CLB, shown in Figure 5.6 , has five logic inputs (A–E), a common clock input (K), an asynchronous directreset input (RD), and an enable (EC). Using programmable MUXes connected to the SRAM programming cells, you can independently connect each of the two CLB outputs (X and Y) to the output of the flipflops (QX and QY) or to the output of the combinational logic (F and G).
FIGURE 5.6 The Xilinx XC3000 CLB (configurable logic block). (Source: Xilinx.) 
A 32bit lookup table ( LUT ), stored in 32 bits of SRAM, provides the ability to implement combinational logic. Suppose you need to implement the function F = A · B · C · D · E (a fiveinput AND). You set the contents of LUT cell number 31 (with address '11111') in the 32bit SRAM to a '1'; all the other SRAM cells are set to '0'. When you apply the input variables as an address to the 32bit SRAM, only when ABCDE = '11111' will the output F be a '1'. This means that the CLB propagation delay is fixed, equal to the LUT access time, and independent of the logic function you implement.
There are seven inputs for the combinational logic in the XC3000 CLB: the five CLB inputs (A–E), and the flipflop outputs (QX and QY). There are two outputs from the LUT (F and G). Since a 32bit LUT requires only five variables to form a unique address (32 = 2^{ 5} ), there are several ways to use the LUT:
Figure 5.7 shows the CLB used in the XC4000 series of Xilinx FPGAs. This is a fairly complicated basic logic cell containing 2 fourinput LUTs that feed a threeinput LUT. The XC4000 CLB also has special fast carry logic hardwired between CLBs. MUX control logic maps four control inputs (C1–C4) into the four inputs: LUT input H1, direct in (DIN), enable clock (EC), and a set / reset control (S/R) for the flipflops. The control inputs (C1–C4) can also be used to control the use of the F' and G' LUTs as 32 bits of SRAM.

FIGURE 5.7 The Xilinx XC4000 family CLB (configurable logic block). ( Source: Xilinx.) 
Figure 5.8 shows the basic logic cell, a Logic Cell or LC, used in the XC5200 family of Xilinx LCA FPGAs.^{ 1 } The LC is similar to the CLBs in the XC2000/3000/4000 CLBs, but simpler. Xilinx retained the term CLB in the XC5200 to mean a group of four LCs (LC0–LC3).
The XC5200 LC contains a fourinput LUT, a flipflop, and MUXes to handle signal switching. The arithmetic carry logic is separate from the LUTs. A limited capability to cascade functions is provided (using the MUX labeled F5_MUX in logic cells LC0 and LC2 in Figure 5.8 ) to gang two LCs in parallel to provide the equivalent of a fiveinput LUT.

FIGURE 5.8 The Xilinx XC5200 family LC (Logic Cell) and CLB (configurable logic block). (Source: Xilinx.) 
The use of a LUT in a Xilinx CLB to implement combinational logic is both an advantage and a disadvantage. It means, for example, that an inverter is as slow as a fiveinput NAND. On the other hand a LUT simplifies timing of synchronous logic, simplifies the basic logic cell, and matches the Xilinx SRAM programming technology well. A LUT also provides the possibility, used in the XC4000, of using the LUT directly as SRAM. You can configure the XC4000 CLB as a memory—either two 16 ¥ 1 SRAMs or a 32 ¥ 1 SRAM, but this is expensive RAM.
Figure 5.9 shows the timing model for Xilinx LCA FPGAs.^{ 2 } Xilinx uses two speedgrade systems. The first uses the maximum guaranteed toggle rate of a CLB flipflop measured in MHz as a suffix—so higher is faster. For example a Xilinx XC3020125 has a toggle frequency of 125 MHz. The other Xilinx naming system (which supersedes the old scheme, since toggle frequency is rather meaningless) uses the approximate delay time of the combinational logic in a CLB in nanoseconds—so lower is faster in this case. Thus, for example, an XC40106 has t_{ ILO} = 6.0 ns (the correspondence between speed grade and t_{ ILO} is fairly accurate for the XC2000, XC4000, and XC5200 but is less accurate for the XC3000).
FIGURE 5.9 The Xilinx LCA timing model. The paths show different uses of CLBs (configurable logic blocks). The parameters shown are for an XC52106. ( Source: Xilinx.) 
The inclusion of flipflops and combinational logic inside the basic logic cell leads to efficient implementation of state machines, for example. The coarsegrain architecture of the Xilinx CLBs maximizes performance given the size of the SRAM programming technology element. As a result of the increased complexity of the basic logic cell we shall see (in Section 7.2, “Xilinx LCA”) that the routing between cells is more complex than other FPGAs that use a simpler basic logic cell.
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Copyright 1997 by Addison Wesley Longman, Inc. No part of this book may be printed, reproduced, stored, or transmitted without the prior written permission of the publisher. 