Verilog code to implement a 4-bit shift register circuit with a parallel in/serial out.

Here’s an example Verilog code to implement a 4-bit shift register circuit with a parallel in/serial out:

module shift_register_parallel (output serial_out, input [3:0] parallel_in, input shift);

    reg [3:0] shift_reg;

    always @(posedge shift) begin
        shift_reg <= parallel_in;
    end

    assign serial_out = shift_reg[0];

    always @(posedge shift) begin
        shift_reg <= {shift_reg[2:0], 1'b0};
    end

endmodule

This code defines a module called "shift_register_parallel" that implements a 4-bit shift register with a parallel in/serial out. The output "serial_out" is a registered output that represents the serial output of the shift register. The input "parallel_in" is the parallel input to the shift register, and "shift" is the clock input that controls the shift operation.

The "reg [3:0] shift_reg;" statement declares a 4-bit register called "shift_reg" that stores the current state of the shift register.

The first "always @(posedge shift)" block is a sequential logic block that updates the "shift_reg" register based on the clock and parallel input signal. If the "shift" signal is high, then the shift register loads the value of "parallel_in" into "shift_reg".

The "assign serial_out = shift_reg[0];" statement is a combinational logic statement that assigns the least significant bit of "shift_reg" to the "serial_out" output.

The second "always @(posedge shift)" block is a sequential logic block that updates the "shift_reg" register based on the clock signal. If the "shift" signal is high, then the shift register shifts the current contents of "shift_reg" left by one bit and inserts a 0 at the least significant bit.

Note that this Verilog code assumes that the "serial_out" output and the "shift" input are 1-bit signals, and the "parallel_in" input is a 4-bit signal. If you want to use different signal widths, you would need to modify the code accordingly. Additionally, this code implements a rising-edge triggered shift register. If you wanted to implement a falling-edge triggered shift register, you would need to modify the code accordingly.