Sequential Verulog Topics part 13

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Example of Sequential Circuit Synthesis In Section 14.4.2, An Example of RTL-to-Gates, we synthesized a combinational circuit. Let us now consider an example of sequential

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Nội dung Text: Sequential Verulog Topics part 13

14.7 Example of Sequential Circuit Synthesis In Section 14.4.2, An Example of RTL-to-Gates, we synthesized a combinational circuit. Let us now consider an example of sequential circuit synthesis. Specifically, we will design finite state machines. 14.7.1 Design Specification A simple digital circuit is to be designed for the coin acceptor of an electronic newspaper vending machine. • Assume that the newspaper cost 15 cents. (Wow! Who gives that kind of a price any more? Well, let us assume that it is a special student edition!!) • The coin acceptor takes only nickels and dimes. • Exact change must be provided. The acceptor does not return extra money. • Valid combinations including order of coins are one nickel and one dime, three nickels, or one dime and one nickel. Two dimes are valid, but the acceptor does not return money. This digital circuit can be designed by using the finite state machine approach. 14.7.2 Circuit Requirements We must set some requirements for the digital circuit. • When each coin is inserted, a 2-bit signal coin[1:0] is sent to the digital circuit. The signal is asserted at the next negative edge of a global clock signal and stays up for exactly 1 clock cycle. • The output of the digital circuit is a single bit. Each time the total amount inserted is 15 cents or more, an output signal newspaper goes high for exactly one clock cycle and the vending machine door is released. • A reset signal can be used to reset the finite state machine. We assume synchronous reset. 14.7.3 Finite State Machine (FSM) We can represent the functionality of the digital circuit with a finite state machine. • input: 2-bit, coin[1:0]—no coin x0= 2'b00, nickel x5 = 2'b01, dime x10 = 2'b10. • output: 1-bit, newspaper—release door when newspaper = 1'b1
• states: 4 states—s0 = 0 cents; s5 = 5 cents; s10 = 10 cents; s15 = 15 cents The bubble diagram for the finite state machine is shown in Figure 14-10. Each arc in the FSM is labeled with a label / where input is 2-bit and output is 1-bit. For example, x5/0 means transition to the state pointed to by the arc, when input is x5 (2'b01), and set the output to 0. Figure 14-10. Finite State Machine for Newspaper Vending Machine 14.7.4 Verilog Description The Verilog RTL description for the finite state machine is shown in Example 14- 6. Example 14-6 RTL Description for Newspaper Vending Machine FSM //Design the newspaper vending machine coin acceptor //using a FSM approach module vend( coin, clock, reset, newspaper); //Input output port declarations input [1:0] coin; input clock; input reset; output newspaper; wire newspaper; //internal FSM state declarations wire [1:0] NEXT_STATE; reg [1:0] PRES_STATE; //state encodings parameter s0 = 2'b00; parameter s5 = 2'b01; parameter s10 = 2'b10; parameter s15 = 2'b11; //Combinational logic function [2:0] fsm;
input [1:0] fsm_coin; input [1:0] fsm_PRES_STATE; reg fsm_newspaper; reg [1:0] fsm_NEXT_STATE; begin case (fsm_PRES_STATE) s0: //state = s0 begin if (fsm_coin == 2'b10) begin fsm_newspaper = 1'b0; fsm_NEXT_STATE = s10; end else if (fsm_coin == 2'b01) begin fsm_newspaper = 1'b0; fsm_NEXT_STATE = s5; end else begin fsm_newspaper = 1'b0; fsm_NEXT_STATE = s0; end end s5: //state = s5 begin if (fsm_coin == 2'b10) begin fsm_newspaper = 1'b0; fsm_NEXT_STATE = s15; end else if (fsm_coin == 2'b01) begin fsm_newspaper = 1'b0; fsm_NEXT_STATE = s10; end else
begin fsm_newspaper = 1'b0; fsm_NEXT_STATE = s5; end s10: //state = s10 begin if (fsm_coin == 2'b10) begin fsm_newspaper = 1'b0; fsm_NEXT_STATE = s15; end else if (fsm_coin == 2'b01) begin fsm_newspaper = 1'b0; fsm_NEXT_STATE = s15; end else begin fsm_newspaper = 1'b0; fsm_NEXT_STATE = s10; end end s15: //state = s15 begin fsm_newspaper = 1'b1; fsm_NEXT_STATE = s0; end endcase fsm = {fsm_newspaper, fsm_NEXT_STATE}; end endfunction //Reevaluate combinational logic each time a coin //is put or the present state changes assign {newspaper, NEXT_STATE} = fsm(coin, PRES_STATE); //clock the state flip-flops. //use synchronous reset always @(posedge clock)
begin if (reset == 1'b1) PRES_STATE
Example 14-7 Optimized Gate-Level Netlist for Newspaper Vending Machine FSM module vend ( coin, clock, reset, newspaper ); input [1:0] coin; input clock, reset; output newspaper; wire \PRES_STATE[1] , n289, n300, n301, n302, \PRES_STATE243[1] , n303, n304, \PRES_STATE[0] , n290, n291, n292, n293, n294, n295, n296, n297, n298, n299, \PRES_STATE243[0] ; PDFF \PRES_STATE_reg[1] ( .clk(clock), .d(\PRES_STATE243[1] ), .clrbar( 1'b1), .prebar(1'b1), .q(\PRES_STATE[1] ) ); PDFF \PRES_STATE_reg[0] ( .clk(clock), .d(\PRES_STATE243[0] ), .clrbar( 1'b1), .prebar(1'b1), .q(\PRES_STATE[0] ) ); VOR U119 ( .in0(n292), .in1(n295), .out(n302) ); VAND U118 ( .in0(\PRES_STATE[0] ), .in1(\PRES_STATE[1] ), .out(newspaper)); VNAND U117 ( .in0(n300), .in1(n301), .out(n291) ); VNOR U116 ( .in0(n298), .in1(coin[0]), .out(n299) ); VNOR U115 ( .in0(reset), .in1(newspaper), .out(n289) ); VNOT U128 ( .in(\PRES_STATE[1] ), .out(n298) ); VAND U114 ( .in0(n297), .in1(n298), .out(n296) ); VNOT U127 ( .in(\PRES_STATE[0] ), .out(n295) ); VAND U113 ( .in0(n295), .in1(n292), .out(n294) ); VNOT U126 ( .in(coin[1]), .out(n293) ); VNAND U112 ( .in0(coin[0]), .in1(n293), .out(n292) ); VNAND U125 ( .in0(n294), .in1(n303), .out(n300) ); VNOR U111 ( .in0(n291), .in1(reset), .out(\PRES_STATE243[0] ) ); VNAND U124 ( .in0(\PRES_STATE[0] ), .in1(n304), .out(n301) ); VAND U110 ( .in0(n289), .in1(n290), .out(\PRES_STATE243[1] ) ); VNAND U123 ( .in0(n292), .in1(n298), .out(n304) ); VNAND U122 ( .in0(n299), .in1(coin[1]), .out(n303) ); VNAND U121 ( .in0(n296), .in1(n302), .out(n290) ); VOR U120 ( .in0(n293), .in1(coin[0]), .out(n297) ); endmodule The schematic diagram for the gate-level netlist is shown in Figure 14-11. Figure 14-11. Gate-Level Schematic for the Vending Machine
14.7.9 Verification Stimulus is applied to the original RTL description to test all possible combinations of coins. The same stimulus is applied to test the optimized gate- level netlist. Stimulus applied to both the RTL and gate-level netlist is shown in Example 14-8. Example 14-8 Stimulus for Newspaper Vending Machine FSM module stimulus; reg clock; reg [1:0] coin; reg reset; wire newspaper; //instantiate the vending state machine vend vendY (coin, clock, reset, newspaper); //Display the output initial begin $display("\t\tTime Reset Newspaper\n"); $monitor("%d %d %d", $time, reset, newspaper); end //Apply stimulus to the vending machine initial begin clock = 0; coin = 0; reset = 1; #50 reset = 0; @(negedge clock); //wait until negative edge of clock //Put 3 nickels to get newspaper #80 coin = 1; #40 coin = 0; #80 coin = 1; #40 coin = 0; #80 coin = 1; #40 coin = 0;
//Put one nickel and then one dime to get newspaper #180 coin = 1; #40 coin = 0; #80 coin = 2; #40 coin = 0; //Put two dimes; machine does not return a nickel to get newspaper #180 coin = 2; #40 coin = 0; #80 coin = 2; #40 coin = 0; //Put one dime and then one nickel to get newspaper #180 coin = 2; #40 coin = 0; #80 coin = 1; #40 coin = 0; #80 $finish; end //setup clock; cycle time = 40 units always begin #20 clock = ~clock; end endmodule The output from the simulation of RTL and the gate-level netlist is compared. In our case, Example 14-9, the output is identical. Thus, the gate-level netlist is verified. Example 14-9 Output of Newspaper Vending Machine FSM Time Reset Newspaper 0 1 x 20 1 0 50 0 0 420 0 1 460 0 0 780 0 1 820 0 0 1100 0 1
1140 0 0 1460 0 1 1500 0 0 The gate-level netlist is sent to ABC Inc., which does the layout, checks that the layout meets the timing requirements, and then fabricates the IC chip. 14.8 Summary In this chapter, we discussed the following aspects of logic synthesis with Verilog HDL: • Logic synthesis is the process of converting a high-level description of the design into an optimized, gate-level representation, using the cells in the technology library. • Computer-aided logic synthesis tools have greatly reduced the design cycle time and have improved productivity. They allow designers to write technology-independent, high-level descriptions and produce technology- dependent, optimized, gate-level netlists. Both combinational and sequential RTL descriptions can be synthesized. • Logic synthesis tools accept high-level descriptions at the register transfer level (RTL). Thus, not all Verilog constructs are acceptable to a logic synthesis tool. We discussed the acceptable Verilog constructs and operators and their interpretation in terms of digital circuit elements. • A logic synthesis tool accepts an RTL description, design constraints, and technology library and produces an optimized gate-level netlist. Translation, logic optimization, and technology mapping are the internal processes in a logic synthesis tool and are normally invisible to the user. • Functional verification of the optimized gate-level netlist is done by applying the same stimulus to the RTL description and the gate-level netlist and comparing the output. Timing is verified with timing simulation or static timing verification. • Proper Verilog coding techniques must be used to write efficient RTL descriptions, and various design trade-offs must be evaluated. Guidelines for writing efficient RTL descriptions were discussed. • Design partitioning is an important technique used to break the design into smaller blocks. Smaller blocks reduce the complexity of optimization for the logic synthesis tool. • Accurate specification of design constraints is an important part of logic synthesis.
High-level synthesis tools allow the designer to write designs at an algorithmic level. However, high-level synthesis is still an emerging design paradigm, and RTL remains the popular high-level description method for logic synthesis tools.