Minimum in array

Question

I am trying to build a module for practice which gets as input:

clk, rst.
an array of 3 numbers each one is 4 bits.

The final purpose is to return in the output the minimum value in the array but I can't get it really working and I don't understand why.

module minVal(
input logic rstN, clk,
input logic unsigned [2:0][3:0] resArray,
output logic [3:0] minVal
);

logic unsigned [3:0] currRes;

always_ff @(posedge clk, posedge rstN) begin
    if(rstN == 1'b1) begin
        currRes <= 4'b1111;
    end
    else begin
        for(int i = 0; i < 3; i++) begin
            if( resArray[i] < currRes) begin
                currRes <= resArray[i];
                minVal <= resArray[i];
            end
        end
    end
end

endmodule

I wrote the following test bench:

module minVal_tb();

logic rstN, clk;
logic unsigned [2:0][3:0] resArray;
logic [3:0] minVal;


minVal thisInst(.rstN(rstN), .clk(clk), .resArray(resArray), .minVal(minVal));

always begin
    #20 clk = ~clk;
end

initial begin
    clk = 1'b0;
    rstN = 1'b1;
    resArray[0] = 5;
    resArray[1] = 1;
    resArray[2] = 3;
    #5 rstN = 1'b0;
end

endmodule

I expect the output to be 1 right after the first clock cycle, but I get it only after 2 clock cycles. Why?

toolic · Accepted Answer · 2022-01-14 13:28:07Z

When you unroll the for loop, this is what it would look like:

always_ff @(posedge clk, posedge rstN) begin
    if(rstN == 1'b1) begin
        currRes <= 4'b1111;
    end
    else begin
        if( resArray[0] < currRes) begin
            currRes <= resArray[0];
            minVal  <= resArray[0];
        end
        if( resArray[1] < currRes) begin
            currRes <= resArray[1];
            minVal  <= resArray[1];
        end
        if( resArray[2] < currRes) begin
            currRes <= resArray[2];
            minVal  <= resArray[2];
        end
    end
end

You end up with multiple nonblocking assignments to the same register (currRes).

On the 1st posedge of the clock after reset, all 3 if clauses are true, and the last assignment wins:

            currRes <= resArray[2];

So, currRes is assigned the value 3, not 1.

The same is true of minVal.

You need to sort the 3 input values, then compare the minimum of those values to the current minimum.

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