要写一个12小时的时钟。
由题目得知，reset信号的优先级最高，其次是enable，这里很好实现。
我的思路：
写了一个4位的bcd计数器，并实例化了4个，对ss与mm的[7:4]与[3:0]分别考虑，低位到8就可以进行发送进位信号即可，高位加1；
hh有两个临界点，11:59:59->12:00:00与12:59:59->1:00:00，其他就是一个8位bcd计数器了；
pm只有在hh11:59:59->12:00:00时进行反转即可，其他时间保持。
还有就是要注意细节了。
代码
module bcd_count
#(
parameter MOD = 4'd9,
parameter RESET_VAL = 4'd0
)
(
input clk,
input reset,
input ena,
input c_in,
output reg[3:0]q
);
always @(posedge clk) begin
if (reset == 1'b1)
q <= RESET_VAL;
else if (ena == 1'b1 && c_in == 1'b1) begin
if (q == MOD)
q <= RESET_VAL;
else
q <= q + 4'd1;
end
else
q <= q;
end
endmodule
module top_module(
input clk,
input reset,
input ena,
output reg pm,
output reg [7:0] hh,
output [7:0] mm,
output [7:0] ss
);
reg c_in_qss_high;
reg c_in_qmm_low, c_in_qmm_high;
always @(posedge clk) begin
if (reset == 1'b1) begin
pm <= 1'b0;
hh <= 8'b0001_0010;
c_in_qss_high <= 1'b0;
c_in_qmm_low <= 1'b0;
c_in_qmm_high <= 1'b0;
end
else if (ena == 1'b1) begin
if (ss[3:0] == 4'd8)
c_in_qss_high <= 1'b1;
else
c_in_qss_high <= 1'b0;
if (ss[7:4] == 4'd5 && ss[3:0] == 4'd8)
c_in_qmm_low <= 1'b1;
else
c_in_qmm_low <= 1'b0;
if (mm[3:0] == 4'd9 && ss[7:4] == 4'd5 && ss[3:0] == 4'd8)
c_in_qmm_high <= 1'b1;
else
c_in_qmm_high <= 1'b0;
if (mm[7:4] == 4'd5 && mm[3:0] == 4'd9 && ss[7:4] == 4'd5 && ss[3:0] == 4'd9) begin
if (hh == 8'b0001_0001) begin //11:59:59
pm <= ~pm;
hh <= 8'b0001_0010;
end
else if (hh == 8'b0001_0010) begin //12:59:59
pm <= pm;
hh <= 8'b0000_0001;
end
else begin
pm <= pm;
if (hh == 8'b0000_1001) //bcd:9
hh <= 8'b0001_0000; //->bcd:10
else if (hh == 8'b0001_0000) //bcd:10
hh <= 8'b0001_0001; //->bcd11
else if (hh == 8'b0001_0001) //bcd:11
hh <= 8'b0001_0010; //->bcd12
else
hh <= hh + 8'b0000_0001;
end
end
end
else begin
c_in_qss_high <= c_in_qss_high;
c_in_qmm_low <= c_in_qmm_low;
c_in_qmm_high <= c_in_qmm_high;
pm <= pm;
hh <= hh;
end
end
//ss
bcd_count
#(
.MOD(4'd9),
.RESET_VAL(4'd0)
) bcd_count_inst0
(
.clk(clk),
.reset(reset),
.ena(ena),
.c_in(1'b1),
.q(ss[3:0])
);
bcd_count
#(
.MOD(4'd5),
.RESET_VAL(4'd0)
)bcd_count_inst1
(
.clk(clk),
.reset(reset),
.ena(ena),
.c_in(c_in_qss_high),
.q(ss[7:4])
);
//mm
bcd_count
#(
.MOD(4'd9),
.RESET_VAL(4'd0)
)bcd_count_inst2
(
.clk(clk),
.reset(reset),
.ena(ena),
.c_in(c_in_qmm_low),
.q(mm[3:0])
);
bcd_count
#(
.MOD(4'd5),
.RESET_VAL(4'd0)
)bcd_count_inst3
(
.clk(clk),
.reset(reset),
.ena(ena),
.c_in(c_in_qmm_high),
.q(mm[7:4])
);
endmodule