Reference Designer

Let us understand up-down counter and shift register using one verilog with parallel load capability.

Features:
- 16 bit parallel load
- 16 bit programmable up-down counter
- Synchronous reset
- Programmable left-right shift
SINGNAL NAME       IN/OUT DESCRIPTION
Din                                  16 bit In For parallel load
nReset                             In Synchronous reset, active low
Clk                                    In Clock
Load                                 In Enables synchronous parallel load
Count                               In Enables counting
Shift                                 In Enables shift operation
Up                                    In Control for up counting
Dn                                    In Control for down counting
Right                                In Control for Right shifting
Left                                   In Control for Left shifting
Qout                                 16 bit Out For parallel readout
Sout-lsb                           Out Serial out – LSB
Sout-msb                         Out Serial out - MSB

Verilog Code:-

module counter_shift_reg ( nReset,
 din,
 Clk,
 Load,
 Count,
 Shift,
 Up,
 Dn,
 Right,
 Left,
 Qout,
 Sout_lsb,
 Sout_msb
 );

input nReset,Clk;
input Load, Count, Shift, Up, Dn;
input Right, Left;
input [15:0] din;

output Sout_lsb, Sout_msb;
output reg [15:0] Qout;
reg [15:0] temp;
wire Sout_lsb, Sout_msb;

////16 bit Programable counter
always @(posedge Clk) begin
 if(nReset==1'b0)
 Qout <= 0;
 else if(Load)
 Qout <=din;
 else if((Count && Up) == 1'b1 )
 Qout <= Qout +1;
 else if((Count && Dn) == 1'b1)
 Qout <= Qout - 1;
 else Qout <= Qout;
end

///Left shift and Right Shift
always @(posedge Clk) begin
 if(nReset==1'b0) begin
 temp <= 0;
 end
 else if(Load)
 temp <= din;
 else if ((Shift && Left)==1'b1) begin
 temp <= temp<<1;
 end
 else if ((Shift && Right) == 1'b1)
 temp <= temp >> 1;
 else
 temp <= temp;
end
assign Sout_lsb = Right ? temp[0] : 0;
assign Sout_msb = Left ? temp[15] : 0;
endmodule
 

Testbench:-

module counter_shift_tb;
output reg nReset,Clk;
output reg Load,Count,Shift,Up,Dn;
output reg Right,Left;
output reg [15:0] din;
input Sout_lsb,Sout_msb;
input [15:0] Qout;
counter_shift_reg counter_shift(nReset,din,Clk,Load,Count,Shift,Up,Dn,Right,Left,Qout,Sout_lsb,Sout_msb);
initial
Clk=1'b0;
always
#5 Clk = ~Clk;
initial
 begin
 $monitor($time,"nReset=%b,din=%b,Clk=%b,Load=%b,Count=%b,Shift=%b,Up=%b,Dn=%b,Right=%b,Left=%b,Qout=%b,Sout_lsb=%b,Sout_msb=%b",nReset,din,Clk,Load,Count,Shift,Up,Dn,Right,Left,Qout,Sout_lsb,Sout_msb);
 nReset=1'b0;
 #15 nReset=1;Load=1;din=16'b0000000000000001;
 #30 Load=0;Count=1;Up=1;Dn=0;Shift=0;Right=0;Left=0;
 #40 Load=1;din=16'b0000000000001111;Up=0;Count=0;
 #45 Load=0;Count=1;Up=0;Dn=1;Shift=0;Right=0;Left=0;
 #65 Load=1;din=16'b1000000000001111;
 #75 Load=0;Shift=1;Right=1;Up=0;Dn=0;
 #85 Load=1;din=16'b1111000000000001;Right=0;
 #95 Load=0;Shift=1;Left=1;Up=0;Dn=0;
 #1000 $finish;
 end
initial
 begin
 $dumpfile("counter_shift_tb.vcd");
 $dumpvars(0,counter_shift_tb);
 end
endmodule

Please note that we have used $dumpfile and $dumpvars to view vcd file using gtkwave in testbench.Gtkwave is used to view waveform.

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