DSP to ASIC

Posted on Edited on In

Number Systems

Harris, David Money, and Sarah L. Harris. Digital Design and Computer Architecture. 2nd ed. Morgan Kaufmann, 2013. [pdf]

integers

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rational number

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2's Complement

[https://bpb-us-w2.wpmucdn.com/sites.coecis.cornell.edu/dist/4/81/files/2019/06/4740_lecture23ALU-circuits.pdf]

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Google AI Mode [https://share.google/aimode/KsxxgDF0vAdhAIgm0]

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2's complement negative number

Flip all bits then Add 1

N-bit signed number \[ A = -M_{N-1}2^{N-1}+\sum_{k=0}^{N-2}M_k2^k \] Flip all bits \[\begin{align} A_{flip} &= -(1-M_{N-1})2^{N-1} +\sum_{k=0}^{N-2}(1-M_k)2^k \\ &= M_{N-1}2^{N-1}-\sum_{k=0}^{N-2}M_k2^k -2^{N-1}+\sum_{k=0}^{N-2}2^k \\ &= M_{N-1}2^{N-1}-\sum_{k=0}^{N-2}M_k2^k -1 \end{align}\]

Add 1 \[ A_- = A_{flip}+1 = M_{N-1}2^{N-1}-\sum_{k=0}^{N-2}M_k2^k = -A \]

Fixed Point Number

image-20260207095601437

Floating Point Number

Dennis Forbes. Understanding Floating-Point Numbers [https://dennisforbes.ca/blog/features/floating_point/understanding-floating-point-numbers/]

IEEE Standard for Floating-Point Arithmetic [https://www-users.cse.umn.edu/~vinals/tspot_files/phys4041/2020/IEEE%20Standard%20754-2019.pdf]

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32-bit floating-point version 1 store implicit leading one image-20260207102121844
32-bit floating-point version 2 discard implicit leading one image-20260207102147689
IEEE 754 floating point notation biased exponent image-20260207102210468

[https://share.google/aimode/yY2R2EQCTsP9BlMNx]

Format Exponent Bits Bias (Decimal) Representable Range
Single Precision (32-bit) 8 127 -126 to +127
Double Precision (64-bit) 11 1023 -1022 to +1023

image-20260207103557952

VLSI Arithmetic

TODO 📅

Word-Length Effects

Tianshuang Qiu; Ying Guo, "7. Finite-Precision Numerical Effects in Digital Signal Processing," in Signal Processing and Data Analysis , De Gruyter, 2018, pp.236-248

Antoniou, Andreas. “Digital Signal Processing: Signals, Systems, and Filters.” (2005). [pdf]

TODO 📅

DFE in digital

Synopsys Italia, Tech Talk: Introduction to DSP-based SerDes [https://youtu.be/puEP0DlVZGI]

Chen, Kuan-Chang (2022) Energy-Efficient Receiver Design for High-Speed Interconnects. Dissertation (Ph.D.), California Institute of Technology. [https://thesis.library.caltech.edu/14318/9/chen_kuan-chang_2022_thesis_final.pdf]

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Parallel implementation

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Loop-Unrolling DFE

image-20250525105017605

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Corresponding to the three distinct voltage thresholds in the PAM4 systems, it would need 12 slicers, 3 multiplexers, and one thermometer-to-binary decoder in each deserialized data path, even if only one tap of the DFE is unrolled

Look-Ahead Multiplexing DFE

image-20250525151918214

The look-ahead multiplexing technique brings the key benefit that the timing constraint can be significantly relaxed, as the iteration bound is doubled at the expense of extra hardware

image-20250525192228275

RTL module

MakerCode RTL Challenge [https://github.com/Weiyet/MakerCode_RTLChallenge]

decimation_filter

Filter Equation

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filtered[n] = (1*x[n] + 3*x[n-1] + 3*x[n-2] + 1*x[n-3]) / 8
y[k] = filtered[k*DECIMATION_FACTOR]
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iverilog -g2012 -o sim.vvp solution.sv tb.sv

vvp sim.vvp +VCDFILE=output.vcd


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module decimation_filter #(
    parameter DATA_WIDTH = 8,
    parameter DECIMATION_FACTOR = 4
)(
    input  wire                       clk,
    input  wire                       reset,
    input  wire signed [DATA_WIDTH-1:0]    data_in,
    input  wire                       data_valid_in,
    output wire signed [DATA_WIDTH-1:0]    data_out,
    output wire                       data_valid_out
);

    localparam COUNTER_WIDTH = $clog2(DECIMATION_FACTOR);
    
    // FIR filter coefficients: [1, 3, 3, 1] (normalized by 8)
    reg signed [DATA_WIDTH-1:0] x1, x2, x3;
    reg signed [DATA_WIDTH+3:0] filtered;
    reg [COUNTER_WIDTH-1:0] sample_counter;
    reg valid_out_reg;
    reg signed [DATA_WIDTH-1:0] output_reg;
    
    always @(posedge clk) begin
        if (reset) begin
            x1 <= {DATA_WIDTH{1'b0}};
            x2 <= {DATA_WIDTH{1'b0}};
            x3 <= {DATA_WIDTH{1'b0}};
            sample_counter <= {COUNTER_WIDTH{1'b0}};
            valid_out_reg <= 1'b0;
            output_reg <= {DATA_WIDTH{1'b0}};
        end else if (data_valid_in) begin
            // Update delay line
            x3 <= x2;
            x2 <= x1;
            x1 <= data_in;
            
            // Apply FIR filter: (1*x[n] + 3*x[n-1] + 3*x[n-2] + 1*x[n-3]) / 8
            filtered = data_in + (x1 << 1) + x1 + (x2 << 1) + x2 + x3;
            
            // Increment sample counter
            if (sample_counter == DECIMATION_FACTOR - 1) begin
                sample_counter <= {COUNTER_WIDTH{1'b0}};
                valid_out_reg <= 1'b1;
                output_reg <= filtered >>> 3;  // Divide by 8
            end else begin
                sample_counter <= sample_counter + 1'b1;
                valid_out_reg <= 1'b0;
            end
        end else begin
            valid_out_reg <= 1'b0;
        end
    end
    
    assign data_out = output_reg;
    assign data_valid_out = valid_out_reg;

endmodule

Line 34 uses non-blocking assignment:

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x1 <= data_in;   // line 34: scheduled, NOT applied yet
...
filtered = data_in + (x1<<1) + x1 + ...;  // line 37: x1 still holds OLD value

Non-blocking assignments don't take effect until the end of the current time step (after all the blocking statements in the block have run). So at line 37:

  • x1 still holds its previous value — i.e. the sample from the last data_valid_in cycle, which is x[n-1].
  • data_in is the current sample, x[n].

They are different values. That's deliberate and necessary for the FIR math to be correct:

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filtered = 1*x[n]   + 3*x[n-1] + 3*x[n-2] + 1*x[n-3]
         = data_in  + 3*x1     + 3*x2     + 1*x3
Symbol Value at line 37 Filter tap
data_in x[n] (current) coeff 1
x1 x[n-1] coeff 3
x2 x[n-2] coeff 3
x3 x[n-3] coeff 1

image-20260619230018566

reference

Jabbour, Chadi, etc.. "Digitally enhanced mixed signal systems." IEEE International Symposium on Circuits and Systems (ISCAS). 2019.

Sen M. Kuo. Real-Time Digital Signal Processing: Fundamentals, Implementations and Applications, 3rd Edition. John Wiley & Sons 2013

Taylor, Fred. Digital filters: principles and applications with MATLAB. John Wiley & Sons, 2011

Kuo, Sen-Maw. (2013) Real-Time Digital Signal Processing: Implementations and Applications 3rd [pdf]

D. Markovic and R. W. Brodersen, DSP Architecture Design Essentials, Springer, 2012.

X. Yang, Integrated Circuit Design: IC Design Flow and Project-Based Learning, 1st edition. Boca Raton: CRC Press, 2024 [repo]

Bevan Baas, EEC281 VLSI Digital Signal Processing, [https://www.ece.ucdavis.edu/~bbaas/281/]

Mark Horowitz. EE371: Advanced VLSI Circuit Design Spring 2006-2007 [https://web.stanford.edu/class/archive/ee/ee371/ee371.1066/]

Tinoosh Mohsenin. CMPE 691: Digital Signal Processing Hardware Implementation [https://userpages.cs.umbc.edu/tinoosh/cmpe691/]

Keshab K. Parhi [http://www.ece.umn.edu/users/parhi/]

謝秉璇. 2019 積體電路設計導論 [link]

Jason Sachs. Understanding and Preventing Overflow (I Had Too Much to Add Last Night) [https://www.embeddedrelated.com/showarticle/532.php]

—. Round Round Get Around: Why Fixed-Point Right-Shifts Are Just Fine [https://www.embeddedrelated.com/showarticle/1015.php]

—. How to Build a Fixed-Point PI Controller That Just Works: Part I [https://www.embeddedrelated.com/showarticle/121.php]

—. How to Build a Fixed-Point PI Controller That Just Works: Part II [https://www.embeddedrelated.com/showarticle/123.php]

AHMED SHAHEIN, Fixed-Point Simulation in GNU Octave—Without MATLAB [https://www.dsprelated.com/showarticle/1786.php]

A. Antoniou, "On the roots of digital signal processing. Part I," in IEEE Circuits and Systems Magazine, vol. 7, no. 1, pp. 8-18, First Quarter 2007

—, "Feature - On the roots of digital signal processing - Part II," in IEEE Circuits and Systems Magazine, vol. 7, no. 4, pp. 8-19, Fourth Quarter 2007

Hideo Okawara's Mixed Signal Lecture Series [https://tomverbeure.github.io/2024/01/06/Hideo-Okawara-Mixed-Signal-Lecture-Series.html]

Jeffrey Walling, DSP to ASIC Block [https://youtube.com/playlist?list=PLP4ZmM6GPueNEdnLhgkdr8_X8dSizUwMs]