AREA OPTIMIZATION IN FPGA

Optimization is a fundamental aspect of digital design, typically targeting three main objectives: minimizing area, increasing speed, and reducing power consumption. In this article, it is concentrated that the strategies is used to optimize area.

• Resource Sharing

While contemporary synthesis tools provide built-in mechanisms for resource sharing, these mechanisms are generally confined to intra-module optimization. Resource sharing across module boundaries is typically not performed by the synthesis engine.

For example :

module resource_sharing_example_0
(
  input           clk,
  input           mux,
  input           data_0,
  input           data_1,
  input           data_2,
  output reg      data_out
);

always @(posedge clk)
  begin
    if (mux)
    begin
      data_out   <= data_1 * data_0;
    end
    else
    begin
      data_out   <= data_2 * data_0;
    end
  end
endmodule

Ideally, this code block requires only a single multiplier, with input selection handled through a multiplexer. However, if the synthesis tool does not support resource sharing, it will instantiate two separate multipliers and then select between their outputs using a mux. This optimization — sometimes intentionally disabled to achieve higher performance — is normally a built-in capability of synthesis tools. Resource sharing can also be implemented manually by the designer, allowing a single functional unit to be used by multiple logic blocks. In this way, area optimization is achieved by reducing redundant hardware.

For example:

module resource_sharing_example_1
(
  input             clk,
  input             rst,
  output            PWM0,
  output            PWM1
);

reg [7:0]           counter;
always @(posedge clk)
begin
  if (rst)
  begin
    counter         <= 0;
  end
  else
  begin
    counter         <= counter + 1;
  end
end
assign PWM0 = counter[3];
assign PWM1 = counter[7];
endmodule

In this code block, a single counter is reused to generate two distinct PWM signals by selecting different bit positions from the counter vector. By exploiting this property, the design intentionally shares the same hardware resource — the counter logic — to perform multiple functions. This form of resource sharing is explicitly implemented at the architectural level by the designer, rather than relying on automated optimizations provided by the synthesis tool. In other words, the efficient reuse of logic is achieved through deliberate design choices rather than synthesis-driven inference.

• Impact of Reset

The reset methodology used in an FPGA design has a significant impact on resource utilization due to the fine-grained architecture of modern FPGA fabrics. In general, reset strategies can be categorized into three types: power-up reset, asynchronous reset, and synchronous reset. Each type interacts differently with FPGA primitives such as flip-flops, LUTs, and routing resources. The architectural implications and utilization effects of these reset schemes are examined below.

• Power-up reset (No reset logic)

Due to the fine-grained architecture of FPGAs, the choice of reset strategy in a digital design can significantly influence hardware utilization. In modern FPGA families, the Look-Up Table (LUT) is the fundamental logic element and can be configured not only to implement arbitrary combinational logic but also to function as a shift register (SRL) or a small distributed memory.

An important architectural constraint is that an LUT-based shift register does not support a reset mechanism after device configuration. Consequently, if a designer specifies a reset for logic that could otherwise be mapped to an SRL, the synthesis tool is forced to abandon the LUT-based implementation. Instead, it must construct the shift-register functionality using discrete flip-flops with reset capability, which leads to increased resource consumption and potentially longer routing paths.

In other words, when a shift register is coded without a reset, the synthesis tool can efficiently map it to LUT-based SRLs, achieving high density and performance. However, when a reset is explicitly included, the design becomes incompatible with SRL primitives, and the tool must fall back to flip-flop–based implementations, increasing logical utilization.

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For example:

module shift_register_without_reset
(
  input        clk,
  input        data_in,
  output       data_out
);

reg [15:0]    shift_register;
always @(posedge clk)
begin
  shift_register        <= {shift_register[14:0], data_in};
end
assign data_out         = shift_register[15];
endmodule 

module shift_register_without_reset
(
  input                  clk,
  input                  rst, 
  input                  data_in,
  output                 data_out
);

reg [15:0]                shift_register;
always @(posedge clk)
begin
  if (rst)
  begin
    shift_register        <= 0;
  end
  else
  begin
    shift_register        <= {shift_register[14:0], data_in};
  end
end
assign data_out           = shift_register[15];
endmodule

Shift Register with reset

Shift Register without reset

Utilization Report

• Utilization Set/Reset Flip-Flop pins for logical-function

The Flip-Flop in FPGA can vary vendor to vendor. Generally, most of Flip-Flop FPGA includes the set/reset and data pin ideally. The set/reset pin also can be used to implement logical-function instead of keeping them unused. In other words, the designer can implement her/his logic to help synthesis tool more are optimized design.

For example:

module ff_without_logic_function 
(
  input                 clk,
  input                 data_in0,
  input                 data_in1,
  output   reg          data_out
);

always @(posedge clk)
begin
  data_out             <= data_in0 || data_in1;
end
endmodule

module ff_with_logic_function 
(
  input                 clk,
  input                 data_in0,
  input                 data_in1,
  output   reg          data_out
);

always @(posedge clk)
begin
  if (data_in0)
  begin
    data_out           <= 1'b1;
  end
  else  
  begin
    data_out           <= data_in1;
  end
end
endmodule

Set/Reset without Optimization

Set/Reset with Optimization

Utilization Report

Area optimization in FPGA design often comes from making deliberate choices about how logic is structured. Modern synthesis tools already perform many optimizations — such as automatic resource sharing — but designers can push this even further by intentionally reusing hardware blocks themselves. A simple example is using a single counter to generate two different PWM signals instead of instantiating two separate counters.

Similarly, understanding the effect of reset usage in shift registers can lead to major savings. When a shift register does not require a reset, the tool can map it into an SRL, dramatically reducing the number of flip-flops used. Finally, even the set/reset pins of flip-flops can be used as part of the logic, offloading simple conditions from LUTs and helping reduce overall resource usage.

In the end, area optimization is not only about what the tools can do — it’s about how the designer structures the logic. By reusing hardware where possible, avoiding unnecessary resets, and taking advantage of built-in flip-flop features, an FPGA design can become noticeably more compact and efficient.