四、Sequential Logic
Counters
1、Four-bit binary counter
Problem Statement：
Build a 4-bit binary counter that counts from 0 through 15, inclusive, with a period of 16. The reset input is synchronous, and should reset the counter to 0.
module top_module (
input clk,
input reset,
output [3:0] q
);
always@(posedge clk)begin
if(reset)
q <= 4'd0;
else if (q == 4'd15)
q <= 4'd0;
else
q <= q + 1'b1;
end
endmodule
2、Decade counter
Problem Statement：
Build a decade counter that counts from 0 through 9, inclusive, with a period of 10. The reset input is synchronous, and should reset the counter to 0.
module top_module (
input clk,
input reset,
output [3:0] q
);
always@(posedge clk)begin
if(reset)
q <= 4'd0;
else if(q == 4'd9)
q <= 4'd0;
else
q <= q + 1'b1;
end
endmodule
3、Decade counter again
Problem Statement：
Make a decade counter that counts 1 through 10, inclusive. The reset input is synchronous, and should reset the counter to 1.
module top_module (
input clk,
input reset,
output [3:0] q
);
always@(posedge clk)begin
if(reset)
q <= 4'd1;
else if(q == 4'd10)
q <= 4'd1;
else
q <= q + 4'd1;
end
endmodule4、Slow decade counter
Problem Statement：
Build a decade counter that counts from 0 through 9, inclusive, with a period of 10. The reset input is synchronous, and should reset the counter to 0. We want to be able to pause the counter rather than always incrementing every clock cycle, so the slowena input indicates when the counter should increment.
module top_module (
input clk,
input slowena,
input reset,
output [3:0] q
);
always@(posedge clk)begin
if(reset)
q <= 4'd0;
else if(slowena)begin
if(q == 4'd9)
q <= 4'd0;
else
q <= q + 4'd1;
end
end
endmodule5、Counter 1-12
Problem Statement：
Design a 1-12 counter with the following inputs and outputs:
- Reset Synchronous active-high reset that forces the counter to 1
- Enable Set high for the counter to run
- Clk Positive edge-triggered clock input
- Q[3:0] The output of the counter
- c_enable, c_load, c_d[3:0] Control signals going to the provided 4-bit counter, so correct operation can be verified.
You have the following components available:
- the 4-bit binary counter (count4) below, which has Enable and synchronous parallel-load inputs (load has higher priority than enable). The count4 module is provided to you. Instantiate it in your circuit.
- logic gates
module count4( input clk, input enable, input load, input [3:0] d, output reg [3:0] Q );The c_enable, c_load, and c_d outputs are the signals that go to the internal counter's enable, load, and d inputs, respectively. Their purpose is to allow these signals to be checked for correctness.
module top_module (
input clk,
input reset,
input enable,
output [3:0] Q,
output c_enable,
output c_load,
output [3:0] c_d
);
assign c_enable = enable;
assign c_load = reset || ((Q == 4'd12) && (enable == 1'b1));
assign c_d = c_load ? 4'd1 : 4'dx;
count4 instance1(.clk(clk), .enable(c_enable), .load(c_load), .d(c_d), .Q(Q));
endmodule6、Counter 1000
Problem Statement：
From a 1000 Hz clock, derive a 1 Hz signal, called OneHertz, that could be used to drive an Enable signal for a set of hour/minute/second counters to create a digital wall clock. Since we want the clock to count once per second, the OneHertz signal must be asserted for exactly one cycle each second. Build the frequency divider using modulo-10 (BCD) counters and as few other gates as possible. Also output the enable signals from each of the BCD counters you use (c_enable[0] for the fastest counter, c_enable[2] for the slowest).
The following BCD counter is provided for you. Enable must be high for the counter to run. Reset is synchronous and set high to force the counter to zero. All counters in your circuit must directly use the same 1000 Hz signal.
module bcdcount ( input clk, input reset, input enable, output reg [3:0] Q );
module top_module (
input clk,
input reset,
output OneHertz,
output [2:0] c_enable
);
wire [3:0]Q0;
wire [3:0]Q1;
wire [3:0]Q2;
bcdcount instance1(.clk(clk), .reset(reset), .enable(c_enable[0]), .Q(Q0));
bcdcount instance2(.clk(clk), .reset(reset), .enable(c_enable[1]), .Q(Q1));
bcdcount instance3(.clk(clk), .reset(reset), .enable(c_enable[2]), .Q(Q2));
assign c_enable[0] = ~reset;
assign c_enable[1] = ~reset && (Q0 == 4'd9);
assign c_enable[2] = ~reset && (Q0 == 4'd9) && (Q1 == 4'd9);
assign OneHertz = ((Q0 == 4'd9) && (Q1 == 4'd9) && (Q2 == 4'd9)) ? 1'b1 : 1'b0;
endmodule7、4-digit decimal counter
Problem Statement：
Build a 4-digit BCD (binary-coded decimal) counter. Each decimal digit is encoded using 4 bits: q[3:0] is the ones digit, q[7:4] is the tens digit, etc. For digits [3:1], also output an enable signal indicating when each of the upper three digits should be incremented.
module top_module (
input clk,
input reset, // Synchronous active-high reset
output [3:1] ena,
output [15:0] q
);
reg[3:0]q0;
reg[3:0]q1;
reg[3:0]q2;
reg[3:0]q3;
always@(posedge clk)begin
if(reset)
q0 <= 4'd0;
else if(q0 == 4'd9)
q0 <= 4'd0;
else
q0 <= q0 + 1'b1;
end
assign ena[1] = (q0 == 4'd9) ? 1'b1 : 1'b0;
always@(posedge clk)begin
if(reset)
q1 <= 4'd0;
else if(q0 == 4'd9)
if(q1 == 4'd9)
q1 <= 4'd0;
else
q1 <= q1 + 1'b1;
else
q1 <= q1;
end
assign ena[2] = ((q0 == 4'd9)&&(q1 == 4'd9)) ? 1'b1 : 1'b0;
always@(posedge clk)begin
if(reset)
q2 <= 4'd0;
else if((q0 == 4'd9)&&(q1 == 4'd9))
if(q2 == 4'd9)
q2 <= 4'd0;
else
q2 <= q2 + 1'b1;
else
q2 <= q2;
end
assign ena[3] = ((q0 == 4'd9)&&(q1 == 4'd9)&&(q2 == 4'd9)) ? 1'b1 : 1'b0;
always@(posedge clk)begin
if(reset)
q3 <= 4'd0;
else if((q0 == 4'd9)&&(q1 == 4'd9)&&(q2 == 4'd9))
if(q3 == 4'd9)
q3 <= 4'd0;
else
q3 <= q3 + 1'b1;
else
q3 <= q3;
end
assign q = {q3,q2,q1,q0};
endmodule8、12-hour counter
Problem Statement：
Create a set of counters suitable for use as a 12-hour clock (with am/pm indicator). Your counters are clocked by a fast-running clk, with a pulse on ena whenever your clock should increment (i.e., once per second).
reset resets the clock to 12:00 AM. pm is 0 for AM and 1 for PM. hh, mm, and ss are two BCD (Binary-Coded Decimal) digits each for hours (01-12), minutes (00-59), and seconds (00-59). Reset has higher priority than enable, and can occur even when not enabled.
The following timing diagram shows the rollover behaviour from 11:59:59 AM to 12:00:00 PM and the synchronous reset and enable behaviour.
module top_module(
input clk,
input reset,
input ena,
output pm,
output [7:0] hh,
output [7:0] mm,
output [7:0] ss
);
reg[3:0]ss1;
reg[3:0]ss2;
reg[3:0]mm1;
reg[3:0]mm2;
reg[3:0]hh1;
reg[3:0]hh2;
always@(posedge clk)begin
if(reset)
pm = 1'b0;
else if((hh1 == 4'd1)&&(hh2 == 4'd1)&&(ss2 == 4'd5)&&(ss1 == 4'd9)&&(mm2 == 4'd5)&&(mm1 == 4'd9))
pm <= ~pm;
end
always@(posedge clk)begin
if(reset)
ss1 <= 4'b0;
else if(ena)begin
if(ss1 == 4'd9)
ss1 <= 4'd0;
else
ss1 <= ss1 + 1'b1;
end
end
always@(posedge clk)begin
if(reset)
ss2 <= 4'b0;
else if(ena)begin
if((ss2 == 4'd5)&&(ss1 == 4'd9))
ss2 <= 4'd0;
else if(ss1 == 4'd9)
ss2 <= ss2 + 1'b1;
end
end
always@(posedge clk)begin
if(reset)
mm1 <= 4'b0;
else if(ena)begin
if((mm1 == 4'd9)&&(ss2 == 4'd5)&&(ss1 == 4'd9))
mm1 <= 4'd0;
else if((ss2 == 4'd5)&&(ss1 == 4'd9))
mm1 <= mm1 + 1'b1;
end
end
always@(posedge clk)begin
if(reset)
mm2 <= 4'b0;
else if(ena)begin
if((mm2 == 4'd5)&&(mm1 == 4'd9)&&(ss2 == 4'd5)&&(ss1 == 4'd9))
mm2 <= 4'd0;
else if((mm1 == 4'd9)&&(ss2 == 4'd5)&&(ss1 == 4'd9))
mm2 <= mm2 + 1'b1;
end
end
always@(posedge clk)begin
if(reset)
hh1 <= 4'd2;
else if(ena)begin
if((ss2 == 4'd5)&&(ss1 == 4'd9)&&(mm2 == 4'd5)&&(mm1 == 4'd9))
if((hh1 == 4'd9)&&(hh2 == 4'd0))
hh1 <= 4'd0;
else if((hh1 == 4'd2)&&(hh2 == 4'd1))
hh1 <= 4'd1;
else
hh1 <= hh1 + 1'b1;
end
end
always@(posedge clk)begin
if(reset)
hh2 <= 4'd1;
else if(ena)begin
if((hh2 == 4'd1)&&(hh1 == 4'd2)&&(ss2 == 4'd5)&&(ss1 == 4'd9)&&(mm2 == 4'd5)&&(mm1 == 4'd9))
hh2 <= 4'd0;
else if((ss2 == 4'd5)&&(ss1 == 4'd9)&&(mm2 == 4'd5)&&(mm1 == 4'd9)&&(hh1 == 4'd9))
hh2 <= hh2 + 1'b1;
end
end
assign ss = {ss2,ss1};
assign mm = {mm2,mm1};
assign hh = {hh2,hh1};
endmodule