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Home > FPGA Technical Tutorials > Design Recipes for FPGAs Using Verilog and VHDL > Design Optimization Example: DES > Initial Synthesis

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Initial Synthesis

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The design was synthesized by MOODS with delay prioritized first and area prioritized second. The target technology was the Xilinx Virtex library. Figure 19.2 shows the control state machine of the synthesized design. The whole state sequence represents the process, which is a loop as shown by the state transition from the last state (c11) back to the first (c1).

The first two states c1 and c2 implement the input handshake on signal go to trigger the process. The DES core is implemented by the remaining states, namely states c3 to c11, which

Control state machine for initial synthesispng

Figure 19.2 Control state machine for initial synthesis.

are in the main loop as shown by the state transition back from c11 to c3, so are executed 16 times. There are nine states in this inner loop, giving a total algorithm length of 146 cycles, including the two states required for the input handshake and 144 for the DES core itself. However, an inspection of the original structure shown in Figure 19.1 suggests that a reasonable target for the inner loop is 2 cycles per round with an optimistic target of 1 cycle. Clearly there is a problem with this design. The synthesis software predicts that this design has the area and delay characteristics shown in Table 19.1 in the line labeled (1).

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