これがこう↓なるはず。

// moter_drive01

module moter_drive01(ck,pwm,cw_ccw,Lp,Ln,Rp,Rn);

input ck,pwm,cw_ccw;
output Lp,Ln,Rp,Rn;

reg [3:0] shift_pwm;

//4clock shift
always@(posedge ck) begin
shift_pwm[3] <= shift_pwm[2];
shift_pwm[2] <= shift_pwm[1];
shift_pwm[1] <= shift_pwm[0];
shift_pwm[0] <= pwm;
end

function [3:0] outport;
input [2:0] outport_in;
case(outport_in) //pn pn
3’b 000: outport = 4’b11_11; //XO XO
3’b 001: outport = 4’b10_11; //XX XO
3’b 011: outport = 4’b00_11; //OX XO
3’b 010: outport = 4’b10_11; //XX XO

3’b 100: outport = 4’b11_11;
3’b 101: outport = 4’b11_10;
3’b 111: outport = 4’b11_00;
3’b 110: outport = 4’b11_10;

default: outport = 4’bxx_xx;
endcase
endfunction

assign {Lp,Ln,Rp,Rn} = outport({cw_ccw,shift_pwm[3],pwm});

endmodule

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