FPGA.GP.01.03 Coregen Adders

After taking a first stab in the last blog entry, design modifications are to be considered to better utilize the FPGA resources. One way is to replace some of the addition rtl with adder instantiations available in the Coregen Libraries. The libraries will create instantiation templates of predefined adders that better utilize the FPGA fabric as compared to handing over the generic code to the Tools to map the adders. This can be done by using "fabric" adders and DSP48E adders if using the Virtex Series FPGA.

The uncommented code are the original generic verilog rtl adders. The commented code are the Fabric and DSP48E adder instatiations from Coregen Templates. Each piece of code is toggled back and forth with the comments for each build.

wire [31:0] t1 = rx_state[`IDX(7)] + e1_w + ch_w + rx_w[31:0] + k;
wire [31:0] t2 = e0_w + maj_w;
wire [31:0] new_w = s1_w + rx_w[319:288] + s0_w + rx_w[31:0];
wire [31:0] t3 = rx_state[`IDX(3)] + t1;
wire [31:0] t4 = t1 + t2;

// wire [31:0] t1, t2, new_w, t3, t4;
// wire cout0, cout1, cout2;
// wire gnd = 0;

// fabric_32bit_adder adder0 (.a(rx_state[`IDX(7)]),.b(e1_w),.c_in(gnd),.c_out(cout0) );
// fabric_32bit_adder adder1 (.a(ch_w),.b(rx_w[31:0]),.c_in(cout0),.c_out(cout1) );
// fabric_32bit_adder adder2 (.a(k),.b(gnd),.c_in(cout1),.s(t1) );
// fabric_32bit_adder adder3 (.a(e0_w),.b( maj_w),.c_in(gnd),.s(t2) );
// fabric_32bit_adder adder4 (.a(s1_w),.b(rx_w[319:288]),.c_in(gnd),.c_out(cout2) );
// fabric_32bit_adder adder5 (.a(s0_w),.b(rx_w[31:0]),.c_in(cout2),.s(new_w) );
// fabric_32bit_adder adder6 (.a(rx_state[`IDX(3)]),.b(t1),.c_in(gnd),.s(t3) );
// fabric_32bit_adder adder7 (.a(t1),.b(t2),.c_in(gnd),.s(t4) );

// dsp48_32bit_adder adder0 (.a(rx_state[`IDX(7)]),.b(e1_w),.c_in(gnd),.c_out(cout0) );
// dsp48_32bit_adder adder1 (.a(ch_w),.b(rx_w[31:0]),.c_in(cout0),.c_out(cout1) );
// dsp48_32bit_adder adder2 (.a(k),.b(gnd),.c_in(cout1),.s(t1) );
// dsp48_32bit_adder adder3 (.a(e0_w),.b( maj_w),.c_in(gnd),.s(t2) );
// dsp48_32bit_adder adder4 (.a(s1_w),.b(rx_w[319:288]),.c_in(gnd),.c_out(cout2) );
// dsp48_32bit_adder adder5 (.a(s0_w),.b(rx_w[31:0]),.c_in(cout2),.s(new_w) );
// dsp48_32bit_adder adder6 (.a(rx_state[`IDX(3)]),.b(t1),.c_in(gnd),.s(t3) );
// dsp48_32bit_adder adder7 (.a(t1),.b(t2),.c_in(gnd),.s(t4) );

4 target boards are used for the Coregen Adder insertion: Spartan 3e Starter Kit, Spartan 3a Starter Kit, ML505 and ML506. Identical source code was used for all four boards, with the Coregen Adders created for each board seperately.

By using the fabric adders, the LUT usage went down 16.9%, 16.9%, 6.4%, and 6.4% respectively. By using the DSP48E adders with the ML505 and ML506 boards, the LUT usage when down 20%.

ISE 13.2 Map Results

Since the sha256 transform LOOP_LOG2 parameter what set to 5 for all runs, further work will be done going forward to try to unroll the design further and fully use the FPGA fabric that is available.