Logic block

In computing, a logic block or configurable logic block (CLB) is a fundamental building block of field-programmable gate array (FPGA) technology. Logic blocks can be configured by the engineer to provide reconfigurable logic gates.

Logic blocks are the most common FPGA architecture, and are usually laid out within a logic block array. Logic blocks require I/O pads (to interface with external signals), and routing channels (to interconnect logic blocks).

Programmable logic blocks were invented by David W. Page and LuVerne R. Peterson, and defined within their 1985 patents.

An application circuit must be mapped into an FPGA with adequate resources. While the number of logic blocks and I/Os required is easily determined from the design, the number of routing tracks needed may vary considerably even among designs with the same amount of logic.

For example, a crossbar switch requires much more routing than a systolic array with the same gate count. Since unused routing tracks increase the cost (and decrease the performance) of the part without providing any benefit, FPGA manufacturers try to provide just enough tracks so that most designs that will fit in terms of Lookup tables (LUTs) and I/Os can be routed. This is determined by estimates such as those derived from Rent's rule or by experiments with existing designs.

FPGAs are also widely used for systems validation including pre-silicon validation, post-silicon validation, and firmware development. This allows chip companies to validate their design before the chip is produced in the factory, reducing the time-to-market.

In general, a logic block consists of a few logical cells (called ALM, LE, Slice etc.). A typical cell consists of a 4-input LUT, a Full adder (FA) and a D-type flip-flop, as shown to the right. The LUTs are in this figure split into two 3-input LUTs. In normal mode those are combined into a 4-input LUT through the left mux. In arithmetic mode, their outputs are fed to the FA. The selection of mode is programmed into the middle multiplexer. The output can be either synchronous or asynchronous, depending on the programming of the mux to the right, in the figure example. In practice, entire or parts of the FA are put as functions into the LUTs in order to save space.

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