数字逻辑设计基础-加/减法计数器

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module   versat_updown_counter
(
clk,
restb,
new_cntr_perset;
new_cntr_perset_value,
enable_cnt_up,
enable_cnt_dn,
pause_counting,
ctr_expired
)

//*********************************************************
input		clk;
input		restb;
input		new_cntr_perset;
input [7:0] new_cntr_perset_value;
input		enable_cnt_dn;
input		enable_cnt_up;
input		pause_counting;
output		ctr_expired;

reg 	[7:0]	count255,count255_nxt;
reg		[7:0]	cnt_perset_stored;
wire	[7:0]	cnt_perset_stored_nxt;
reg				ctr_expired;
wire			ctr_expired_nxt;
reg				enable_cnt_dn_d1,enable_cnt_up_d1;
wire			enable_cnt_up_risedge,enable_cnt_dn_risedge;
//**********************************************************

assign	enable_cnt_up_risedge = enable_cnt_up&!enable_cnt_up_d1;
assign	enable_cnt_dn_risedge = enable_cnt_dn&!enable_cnt_dn_d1;

assign	cnt_perset_stored_nxt = new_cntr_perset?new_cntr_perset_value:cnt_perset_stored_nxt;

assign ctr_expired_nxt = enable_cnt_up?(count255_nxt == cnt_perset_stored_nxt):
									   (enable_cnt_dn?(count255_nxt = 'd0):1'b0);
									 
	always@(*)
		begin
			count255_nxt = count255;
			
			if(enable_cnt_dn_risedge)
				count255_nxt = cnt_perset_stored	//初始化最大值
			else if(enable_cnt_up_risedge)
				count255_nxt = 'd0;
			else if(pause_counting)
				count255_nxt = count255;
			else if(enable_cnt_dn && ctr_expired)
				count255_nxt = cnt_perset_stored;
			else if(enable_cnt_dn)
				count255_nxt = count255 - 1'b1;
			else if(enable_cnt_up && ctr_expired)
				count255_nxt = 'd0;
			else if(enable_cnt_up)
				count255_nxt = count255 + 1'b1;
		end
	always@(posedge clk or negedge restb)
		begin
			if(!restb)begin
				count255  				<= 'd0;
				cnt_perset_stored 		<= 'd0;
				ctr_expired				<= 1'b1;
				enable_cnt_up_d1		<= 1'b0;
				enable_cnt_dn_d1		<= 1'b0;
			end
			else begin
				count255				<= count255_nxt;
				cnt_perset_stored		<= cnt_perset_stored_nxt;
				ctr_expired				<= ctr_expired_nxt;
				enable_cnt_dn_d1		<= enable_cnt_dn;
				enable_cnt_up_d1		<= enable_cnt_up;
			end
		end
endmodule