Hello I am trying to implement a parallel in serial out shift register based on the data sheet for the 74LV165A 8 bit parallel in serial out shift register74LV165A shift register schematic

the spec sheet may be found at datasheet for 74LV165A

If I make my test bench begin by shifting in serially, then try to latch data on the parallel input the data does not appear. If I begin the test bench in with parallel latch enabled then the data appears on the Q and will clock through once shifting is enabled.

My goal is to shift several sets of data through this register however currently this behavior is preventing this. Any help would be very very appreciated, event if its just a hint :)

Currently I am monitoring the set and reset information and the Q lines.

module nand3(input one, input two, input three, output out);
    assign out = !(one&two&three);

module nand2(input one, input two, output out);
    assign out = !(one&two);

module DFlipFlop(input clk, input reset, input D, input set, output Q);

reg Q;


always @(posedge clk or negedge reset or negedge set)
    else if(!set)



module control_mux(input [7:0]set_bits, input CP, input CE, input PL, output [16:0]bus); // bit 16 is clk, bits 15 -> 8 are set, bits 7 -> 0 of bus are reset

assign VCC = 1;

nand3 control(!CP, !CE, PL, bus[16]);

nand2 set1(!PL, set_bits[7], bus[15]);
nand2 set2(!PL, set_bits[6], bus[14]);
nand2 set3(!PL, set_bits[5], bus[13]);
nand2 set4(!PL, set_bits[4], bus[12]);
nand2 set5(!PL, set_bits[3], bus[11]);
nand2 set6(!PL, set_bits[2], bus[10]);
nand2 set7(!PL, set_bits[1], bus[9]);
nand2 set8(!PL, set_bits[0], bus[8]);

nand2 reset1(!PL, bus[15], bus[7]);
nand2 reset2(!PL, bus[14], bus[6]);
nand2 reset3(!PL, bus[13], bus[5]);
nand2 reset4(!PL, bus[12], bus[4]);
nand2 reset5(!PL, bus[11], bus[3]);
nand2 reset6(!PL, bus[10], bus[2]);
nand2 reset7(!PL, bus[9],   bus[1]);
nand2 reset8(!PL, bus[8],   bus[0]);


module testbench;

    reg [7:0]set_bits;
                  reg [7:0]set_bits2;
    reg CE, PL, CP, D;

    wire [16:0]bus; 
    wire [7:0]Q;

        #5 CP = !CP;

        CP = 0;
        D = 0;
        set_bits = 8'b11001011;

                                     PL = 0; CE = 1;
         #10 PL = 1;  CE = 0;

        #500 $finish;

        $display("CP,      CE,        PL,       b16        b15        b14       b13        b12        b11        b10        b9        b8       b7       b6      b5      b4      b3      b2     b1     b0  Q7  Q6  Q5 Q4 Q3 Q2 Q1 Q0");
        $monitor("%b            %b            %b           %b            %b            %b            %b            %b            %b            %b            %b          %b        %b         %b         %b         %b        %b       %b       %b       %b      %b    %b    %b    %b    %b    %b    %b    %b", CP, CE, PL, bus[16], bus[15], bus[14], bus[13], bus[12], bus[11], bus[10], bus[9], bus[8], bus[7], bus[6], bus[5], bus[4], bus[3], bus[2], bus[1], bus[0], Q[7], Q[6],Q[5],Q[4],Q[3],Q[2],Q[1],Q[0]);

    control_mux CM(set_bits, CP, CE, PL, bus);
    DFlipFlop FF(bus[16], bus[7], D, bus[15], Q[7]);
    DFlipFlop FF2(bus[16], bus[6], Q[7], bus[14], Q[6]);
    DFlipFlop FF3(bus[16], bus[5], Q[6], bus[13], Q[5]);
    DFlipFlop FF4(bus[16], bus[4], Q[5], bus[12], Q[4]);
    DFlipFlop FF5(bus[16], bus[3], Q[4], bus[11], Q[3]);
    DFlipFlop FF6(bus[16], bus[2], Q[3], bus[10], Q[2]);
    DFlipFlop FF7(bus[16], bus[1], Q[2], bus[9], Q[1]);
    DFlipFlop FF8(bus[16], bus[0], Q[1], bus[8], Q[0]);


1 Answer 1


You have misinterpreted the schematic. This line:

nand3 control(!CP, !CE, PL, bus[16]);

is in contradiction with:

  • CP is active high,
  • CE is active low (note the bar above the "CE" on the schematic).
  • yet you use two inverted signals in the instantiation,


  • the bus you think a bus is just a wire (note the connecting dots!)
  • and I don't understand the control_mux concept entirely...

Now, checking out the NXP documentation, the schematic of theirs differs from what you attached.

And finally, why are you hand-instantiating gates instead of just coding up the functionality?


module sr_74LV165A (
  input DS, CP, CE_n, PL_n,
  input D0, D1, D2, D3, D4, D5, D6, D7,
  output Q7, Q7_n

reg [0:7] shr;
wire      clk;

assign clk = CP | CE_n | !PL_n;

always @(posedge clk or negedge PL_n)
  if   (!PL_n) shr <= { D0, D1, D2, D3, D4, D5, D6, D7 };
  else         shr <= { DS, shr[0:6] };

assign Q7   =  shr[7];
assign Q7_n = !shr[7];


After all, this is just a shift register with parallel async load.


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