![]() ![]() Both Xilinx and Altera have quite a bit of documentation on the specifics of the routing network and construction of the logic elements. I would highly recommend looking at an FPGA datasheet to get a feel for how these components are implemented on a commercial FPGA. Dedicated clock distribution networks also exist to distribute high speed, high fanout clock and reset signals. Closing the correct switches allows signals to be routed around the FPGA between the various components. Let us for now NOT to worry about how the 4-LUT is implemented internally. Typically an LE consists of a 4-input Look-up Table (LUT) and a D-flipflop. It consists of horizontal and vertical wires of various lengths and interconnecting switches. Consider the logic block shown in blue in the last slide (Altera calls their logic block a Logic Element (LE)). When an FPGA is configured, the bits of the LUT are loaded with ones or zeros based on what the desired truth table would be. The LUT inputs act as the address lines for a corresponding one-bit-wide RAM cell. Think of the LUT as a small scratchpad RAM. The routing matrix is what interconnects the reconfigurable components inside the FPGA. The LUT in an FPGA holds a custom truth table, which is loaded when the chip is powered up. The logical functionality is the easy part. Xilinx PROGRAM_B or Altera nCONFIG) and can load the bitstream from a number of different sources including JTAG, SPI or parallel flash chips, a microcontroller or microprocessor, another FPGA or CPLD, etc. Implementing a C-element on LUT-based FPGAs can be done, although it is not very simple. This configuration routine is triggered after any reset of the entire FPGA (i.e. If the delays of all the connections between the. This includes LUTs, block RAM, clock management components (PLL, DCM, MMCM, etc), and the routing matrix. Previous work reports a 3 improvement of the average critical path delay of a subset of VTR circuits 18, 26. Extra logic inside the FPGA (hard logic, not LUTs) reads the configuration bitstream (sof or bit file) from an external flash chip or from the JTAG interface and then stores it into the correct locations inside the FPGA. The LUT is loaded with data with the internal configuration logic. 4 inputs - 1 output OR LUT configuration example 10:19. ![]() The LUT is loaded with data with the internal configuration logic. Within this context, this module guides you through a simple example, which is abstracting the complexity of the underlying FPGA, starting from the description of the circuit you may be willing to implement to the bitstream used to configure the FPGA. ![]()
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