Foreword:

This column aims to record high-frequency pen interview hand-torn code questions for digital front-end autumn recruits. All articles in this column provide principle analysis, codes and waveforms, and all codes have been verified by myself.

The directory is as follows:

...Continually updated

For more hand-tearing code questions, you can go to the digital IC hand-tearing code--question bank

Table of contents

topic description

problem solving ideas

the code

testbench

waveform

topic description

It is known that a section of data stream data is input, and when datain_ena is high, please select the largest and second largest value of this data stream, and give the corresponding result max, submax and result enable dataout_ena after a section of datain_ena ends.

Input clk； 
Input [4:0] datain; 
Input datain_ena; 
Output [4:0] max; 
Output [4:0] submax; 
Output dataout_ena；

problem solving ideas

This question is relatively simple and relatively large. There are only two points to pay attention to:

① When the input datain does not exceed two, dataout_ena cannot be pulled high. Only when at least two numbers are input, both max and submax are meaningful.

② Considering all possible situations, when the input data is larger than max, replace max with datain and submax with max; when the input data is equal to max, replace submax data with datain, so that max=submax (see the meaning of the question) Understand, you can not do this); when the input data is less than max but greater than or equal to submax, replace submax with datain; when the input data is less than submax, do nothing.

the code

module find_max(
    input               clk         ,
    input    [4:0]      datain      ,
    input               datain_ena  ,
    input               rstn        ,

    output  reg [4:0]   max         ,  
    output  reg [4:0]   submax      ,
    output              dataout_ena
);

always @(posedge clk)begin
    if(!rstn)begin
        max <= 5'd0;
        submax <= 5'd0;
    end
    else if(datain_ena == 1'b1)begin
        if(datain > max)begin
            max <= datain;
            submax <= max;
        end
        else if(datain == max)begin
            submax <= datain;
        end
        else if(datain >=submax && datain < max)begin
            submax <= datain;
        end
    end
end

reg [1:0] count;
always @(posedge clk)begin
    if(!rstn)begin
       count <= 2'd0; 
    end
    else if(datain_ena && count < 2'd3)begin
        count <= count + 1'b1;
    end
end

assign dataout_ena = (count >= 2) ? 1:0;  //if input number >= 2 , dataout_ena = 1

endmodule

testbench

module find_max_tb();

reg clk,rstn;
reg [4:0] datain;
reg datain_ena;

wire [4:0] max,submax;
wire dataout_ena;

always #5 clk <= ~clk;

initial begin
    clk <= 1'b0;
    rstn <= 1'b0;
    datain_ena <= 1'b0;
    #16
    rstn <= 1'b1;
    #20
    datain_ena <= 1'b1;
    datain <= 5'd2; 
    #10
    datain <= 5'd3;
    #10
    datain <= 5'd7;
    #10
    datain <= 5'd4;
    #10
    datain <= 5'd9;
    #20
    datain_ena <= 1'b0;
    #200
    $finish();
end

//dump fsdb
initial begin
    $fsdbDumpfile("find_max.fsdb");
    $fsdbDumpvars(0);
end

find_max u_find_max(
    .clk        (clk)           ,
    .rstn       (rstn)          , 
    .datain     (datain)        ,
    .datain_ena (datain_ena)    ,
    
    .max        (max)           ,
    .submax     (submax)        ,
    .dataout_ena(dataout_ena)
);

endmodule

waveform

As can be seen from the waveform, dataout_ena will be pulled high after inputting two datain. All considerations are met.

For more hand-tearing code questions, you can go to the digital IC hand-tearing code--question bank

Digital IC hand tearing code-XX company written test questions (maximum data flow)

topic description

problem solving ideas

the code

testbench

waveform

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