Verilog Modules for Common Digital Functions

Full Adder (Dataflow Description) // // Here is a data flow description of a full_adder module fa(sum, co, a, b, ci) ; output sum, co ; input a, b, ci ; assign sum = a ^ b ^ ci ; assign co = (a & b) | ((a | b) & ci) ; endmodule

Full Adder (Behvioral Description) // 1-bit full-adder (behavioral) module fa(sum, co, a, b, ci) ; input a, b, ci ; output co, sum ; assign {co, sum} = a + b + ci ; endmodule

Full Adder Testbench // testbench for fa cell module fa_tb ; reg a, b, ci ; reg [3:0] i ; wire sum, co ; fa u0(sum, co, a, b, ci) ; initial begin $monitor($time, " a is %b, b is %b, ci is %b, sum is %b, co is %b\n", a, b, ci, sum , co) ; for (i=0; i < 8; i=i+1) #10 {a, b, ci} = i ; end endmodule

Adder (4-bit) – Structural Description // Module to implement a 4-bit adder // Structural description using fa module module adder4(sum, co, a, b, ci) ; output [3:0] sum ; output co ; input [3:0] a ; input [3:0] b ; input ci ; wire [3:1] c ; fa g0(sum[0], c[1], a[0], b[0], ci) ; fa g1(sum[1], c[2], a[1], b[1], c[1]) ; fa g2(sum[2], c[3], a[2], b[2], c[2]) ; fa g3(sum[3], co, a[3], b[3], c[3]) ; endmodule

2-1 Multiplexer // 2-1 MUX // This is a behavioral description. module mux2to1(out, sel, in) ; input sel ; output out ; input [1:0] in ; reg out ; always @(sel or in) begin if (sel == 1) out = in[1] ; else out = in[0] ; end endmodule // This is a dataflow description module mux2to1(out, sel, in) ; input sel ; output out ; input [1:0] in ; assign out = sel ? in[1] : in[0] ; endmodule

2-1 Multiplexer (Case Statement) // 2-1 MUX // This is a behavioral description. module mux2to1(out, sel, in) ; input sel ; output out ; input [1:0] in ; reg out ; always @(sel or in) begin case (sel) 0: out = in[0] ; 1: out = in[1] ; endcase end endmodule

3-8 Decoder // // 3-8 decoder with an enable input // This is a behavioral description. module decoder(out, en, a) ; output [7:0] out ; input [2:0] a ; input en ; reg [7:0] out ; always @(a or en) begin out = 8'd0 ; case(a) 3'd0 : out = 8'd1 ; 3'd1 : out = 8'd2 ; 3'd2 : out = 8'd4 ; 3'd3 : out = 8'd8 ; 3'd4 : out = 8'd16 ; 3'd5 : out = 8'd32 ; 3'd6 : out = 8'd64 ; 3'd7 : out = 8'd128 ; default: $display("The unthinkable has happened.\n") ; endcase if (!en) out = 8'd0 ; end endmodule

3-8 Decoder Testbench // Test bench for 3-8 decoder module decoder_tb ; reg [2:0] a ; reg en ; reg [3:0] i ; wire [7:0] out ; // Instantiate the 3-8 decoder decoder uut(out, en, a); // Exhaustively test it initial begin $monitor($time, " en is %b, a is %b, out is %b\n",en, a, out) ; #10 begin en = 0 ; for (i=0; i < 8; i=i+1) #10 a = i ; end en = 1 ; for (i=0; i < 8; i=i+1) begin #10 a = i ; endmodule

Digital Magnitude Comparator // 4-bit magnitude comparator // This is a behavioral description. module magcomp(AltB, AgtB, AeqB, A, B) ; input [3:0] A, B ; output AltB, AgtB, AeqB ; assign AltB = (A < B) ; assign AgtB = (A > B) ; assign AeqB = (A = B) ; endmodule

D-Latch /* Verilog description of a negative-level senstive D latch with preset (active low) */ module dlatch(q, clock, preset, d) ; output q ; input clock, d, preset ; reg q ; always @(clock or d or preset) begin if (!preset) q = 1 ; else if (!clock) q = d ; end endmodule

D Flip Flop // Negative edge-triggered D FF with sync clear module D_FF(q, d, clr, clk) ; output q ; input d, clr, clk ; reg q ; always @ (negedge clk) begin if (clr) q <= 1'b0 ; else q <= d ; end endmodule

D Flip Flop Testbench // Testbench for D flip flop module test_dff ; wire q ; reg d, clr, clk ; D_FF u0(q, d, clr, clk) ; initial begin clk = 1'b1 ; forever #5 clk = ~clk ; end initial fork #0 begin clr = 1'b1 ; d = 1'b0 ; #20 begin clr=1'b0 ; #30 d = 1'b1 ; #50 d = 1'b0 ; join $monitor($time, "clr=%b, d=%b,q=%b", clr, d, q) ; endmodule

4-bit D Register with Parallel Load // // 4-bit D register with parallel load module dreg_pld(Dout, Din, ld, clk) ; input [3:0] Din ; input ld, clk ; output [3:0] Dout ; reg [3:0] Dout ; always @(posedge clk) begin if (ld) Dout <= Din ; else Dout <= Dout ; end endmodule

1-Bit Counter Cell // 1 bit counter module module cnt1bit(q, tout, tin, clr, clk) ; output q ; output tout ; input tin ; input clr ; input clk ; reg q ; assign tout = tin & q ; always @ (posedge clk) begin if (clr == 1'b1) q <= 0 ; else if (tin == 1'b1) q <= ~q ; end endmodule

2-Bit Counter // 2-bit counter module cnt2bit(cnt, tout, encnt, clr, clk) ; output [1:0] cnt ; output tout ; input encnt ; input clr ; input clk ; wire ttemp ; cnt1bit u0(cnt[0], ttemp, encnt, clr, clk) ; cnt1bit u1(cnt[1], tout, ttemp, clr, clk) ; endmodule

74161 Operation

4-bit Synchronous Counter (74LS161) // 4-bit counter module counter_4bit(clk, nclr, nload, ent, enp, rco, count, parallel_in) ; input clk ; input nclr ; input nload ; input ent ; input enp ; output rco ; output [3:0] count ; input [3:0] parallel_in ; reg [3:0] count ; reg rco ; always @ (count or ent) if ((count == 15) && (ent == 1'b1)) rco = 1'b1 ; else rco = 1'b0 ; always @ (posedge clk or negedge nclr) begin if (nclr == 1'b0) count <= 4'd0 ; else if (nload == 1'b0) count <= parallel_in ; else if ((ent == 1'b1) && (enp == 1'b1)) count <= (count + 1) % 16 ; end endmodule

T Flip-flops // T type flip-flop built from D flip-flop and inverter module t_ff(q, clk, reset) ; output q ; input clk, reset ; wire d ; d_ff dff0(q, d, clk, reset) ; not not0(d, q) ; endmodule

Ripple Counter /* This is an asynchronout ripple carry counter. It is 4-bits wide. */ module ripple_carry_counter(q, clk, reset) ; output [3:0] q ; input clk, reset ; t_ff tff0(q[0], clk, reset) ; t_ff tff1(q[1], q[0], reset) ; t_ff tff2(q[2], q[1], reset) ; t_ff tff3(q[3], q[2], reset) ; endmodule

Up/Down Counter // 4-bit up/down counter module up_dwn(cnt, up, clk, nclr) ; output [3:0] cnt ; input up, clk, nclr ; reg [3:0] cnt ; always @(posedge clk) begin if (~nclr) cnt <= 0 ; else if (up) cnt <= cnt + 1 ; else cnt <= cnt - 1 ; end endmodule

Shift Register // 4 bit shift register `define WID 3 module serial_shift_reg(sout, pout, sin, clk) ; output sout ; output [`WID:0] pout ; input sin, clk ; reg [`WID:0] pout ; always @(posedge clk) begin pout <= {sin, pout[`WID:1]} ; end assign sout = pout[0] ; endmodule

Universal Shift Register // 4 bit universal shift register module universal(pout, pin, sinr, sinl, s1, s0, clk) ; output [3:0] pout ; input sinl, sinr, clk, s1, s0 ; input [3:0] pin ; reg [3:0] pout ; always @(posedge clk) begin case ({s1,s0}) 0: pout <= pout ; 1: pout <= {pout[2:0], sinl} ; 2: pout <= {sinr, pout[3:1]} ; 3: pout <= pin ; endcase end endmodule

Mealy FSM

DRAM Controller // Example of a Mealy machine for a DRAM controller module mealy(clk, cs, refresh, ras, cas, ready) ; input clk, cs, refresh ; output ras, cas, ready ; parameter s0 = 0, s1 = 1, s2 = 2, s3 = 3, s4 = 4 ; reg [2:0] present_state, next_state ; reg ras, cas, ready ; // state register process always @ (posedge clk) begin present_state <= next_state ; end

DRAM Controller (part 2) // state transition process always @ (present_state or refresh or cs) begin case (present_state) next_state = s0 ; ras = 1'bX ; cas = 1'bX ; ready = 1'bX ; s0 : begin if (refresh) next_state = s3 ; ras = 1'b1 ; cas = 1'b0 ; ready = 1'b0 ; end else if (cs) next_state = s1 ; ras = 1'b0 ; cas = 1'b1 ; else ready = 1'b1 ;

DRAM Controller (part 3) s1 : begin next_state = s2 ; ras = 1'b0 ; cas = 1'b0 ; ready = 1'b0 ; end s2 : begin if (~cs) begin next_state = s0 ; ras = 1'b1 ; cas = 1'b1 ; ready = 1'b1 ; else s3 : begin next_state = s4 ; s4 : begin endcase endmodule

1-bit ALU Module // 1-bit alu like the one in Mano text (ECE580) module alui(aci, clk, control, acip1, acim1, dri) ; output aci ; input clk ; input [2:0] control ; input acip1, acim1, dri ; reg aci ; parameter nop = 3'b000, com = 3'b001, shr = 3'b010, shl = 3'b011, dr = 3'b100, clr = 3'b101, and_it = 3'b110 ; always @ (posedge clk) begin case (control) nop: aci <= aci ; com: aci <= ~aci ; shr: aci <= acip1 ; shl: aci <= acim1 ; dr: aci <= dri ; clr: aci <= 0 ; and_it: aci <= aci & dri ; default: aci <= aci ; endcase end endmodule

Modeling Memory // memory model, bidir data bus module memory(data, addr, ce, rw) ; inout [7:0] data ; input ce, rw ; input [5:0] addr ; reg [7:0] mem[0:63] ; reg [7:0] data_out ; tri [7:0] data ; wire tri_en ; always @(ce or addr or rw or data) begin if (~ce) if (rw) data_out = mem[addr] ; else mem[addr] = data ; end assign tri_en = ~ce & rw ; assign data = tri_en ? data_out : 8'bz ; endmodule

Binary to BCD // // We need to convert a 5-bit binary number // to 2 BCD digits module bin2bcd(bcd1, bcd0, bin) ; output [3:0] bcd1 ; output [3:0] bcd0 ; input [4:0] bin ; reg [3:0] bcd1 ; reg [3:0] bcd0 ; reg [7:0] bcd ; integer i ; always @ (bin) begin bcd1 = 4'd0 ; bcd0 = 4'd0 ; for (i=0; i < 5; i= i + 1) begin if (bcd0 >= 4'd5) bcd0 = bcd0 + 4'd3 ; if (bcd1 >= 4'd5) bcd1 = bcd1 + 4'd3 ; bcd = {bcd1, bcd0} ; bcd = bcd << 1 ; bcd[0] = bin[4-i] ; bcd1 = bcd[7:4] ; bcd0 = bcd[3:0] ; end endmodule