justhomework/Quartus/v4/jyh_4490_4_counter.v

module jyh_4490_4_counter(Q,clk,clr,load,in,en,upd,co);

input[3:0] in;
input en,clk,clr,load,upd;
output reg [3:0] Q;
output reg co;

reg co_flag;

always@(posedge clk,negedge clr)
begin

	//异步清零
	if(!clr)
		Q<=0;
	
	else if(en)
	begin	
		//同步置数
		if(load)
			begin
			Q<=in;
			co<=0;
			end
		else
		begin
			if(co_flag)
			begin
				co<=1;
				co_flag=0;
			end
			else if(!co_flag)
				co<=0;
			//正反计数
			 if(upd)
				begin
					if(Q>=4'd9)
						begin
						Q<=4'd0;
						co_flag=1;
						end
					else
						begin
						Q <= Q+1;
						end
				end
			else 
				begin
					if(Q<=4'd0)
						begin
						Q<=4'd9;
						end
					else if(Q==4'd1)
						begin
						Q <= Q-1;
						co_flag=1;
						end
					else
						begin
						Q <= Q-1;
						end
				end
			end
	end
end
endmodule