Logic Synthesis With Verilog HDL part 4

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Logic Synthesis With Verilog HDL part 4

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[ Team LiB ] 14.5 Verification of Gate-Level Netlist The optimized gate-level netlist produced by the logic synthesis tool must be verified for functionality. Also, the synthesis tool may not always be able to meet both timing and area requirements if they are too stringent.

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Nội dung Text: Logic Synthesis With Verilog HDL part 4

  1. [ Team LiB ] 14.5 Verification of Gate-Level Netlist The optimized gate-level netlist produced by the logic synthesis tool must be verified for functionality. Also, the synthesis tool may not always be able to meet both timing and area requirements if they are too stringent. Thus, a separate timing verification can be done on the gate-level netlist. 14.5.1 Functional Verification Identical stimulus is run with the original RTL and synthesized gate-level descriptions of the design. The output is compared to find any mismatches. For the magnitude comparator, a sample stimulus file is shown below. Example 14-3 Stimulus for Magnitude Comparator module stimulus; reg [3:0] A, B; wire A_GT_B, A_LT_B, A_EQ_B; //Instantiate the magnitude comparator magnitude_comparator MC(A_GT_B, A_LT_B, A_EQ_B, A, B); initial $monitor($time," A = %b, B = %b, A_GT_B = %b, A_LT_B = %b, A_EQ_B = %b", A, B, A_GT_B, A_LT_B, A_EQ_B); //stimulate the magnitude comparator. initial begin A = 4'b1010; B = 4'b1001; # 10 A = 4'b1110; B = 4'b1111; # 10 A = 4'b0000; B = 4'b0000; # 10 A = 4'b1000; B = 4'b1100; # 10 A = 4'b0110; B = 4'b1110; # 10 A = 4'b1110; B = 4'b1110; end endmodule The same stimulus is applied to both the RTL description in Example 14-1 and the
  2. synthesized gate-level description in Example 14-2, and the simulation output is compared for mismatches. However, there is an additional consideration. The gate-level description is in terms of library cells VAND, VNAND, etc. Verilog simulators do not understand the meaning of these cells. Thus, to simulate the gate-level description, a simulation library, abc_100.v, must be provided by ABC Inc. The simulation library must describe cells VAND, VNAND, etc., in terms of Verilog HDL primitives and, nand, etc. For example, the VAND cell will be defined in the simulation library as shown in Example 14-4. Example 14-4 Simulation Library //Simulation Library abc_100.v. Extremely simple. No timing checks. module VAND (out, in0, in1); input in0; input in1; output out; //timing information, rise/fall and min:typ:max specify (in0 => out) = (0.260604:0.513000:0.955206, 0.255524:0.503000:0.936586); (in1 => out) = (0.260604:0.513000:0.955206, 0.255524:0.503000:0.936586); endspecify //instantiate a Verilog HDL primitive and (out, in0, in1); endmodule ... //All library cells will have corresponding module definitions //in terms of Verilog primitives. ... Stimulus is applied to the RTL description and the gate-level description. A typical invocation with a Verilog simulator is shown below. //Apply stimulus to RTL description > verilog stimulus.v mag_compare.v //Apply stimulus to gate-level description. //Include simulation library "abc_100.v" using the -v option > verilog stimulus.v mag_compare.gv -v abc_100.v The simulation output must be identical for the two simulations. In our case, the output is
  3. identical. For the example of the magnitude comparator, the output is shown in Example 14-5. Example 14-5 Output from Simulation of Magnitude Comparator 0 A = 1010, B = 1001, A_GT_B = 1, A_LT_B = 0, A_EQ_B = 0 10 A = 1110, B = 1111, A_GT_B = 0, A_LT_B = 1, A_EQ_B = 0 20 A = 0000, B = 0000, A_GT_B = 0, A_LT_B = 0, A_EQ_B = 1 30 A = 1000, B = 1100, A_GT_B = 0, A_LT_B = 1, A_EQ_B = 0 40 A = 0110, B = 1110, A_GT_B = 0, A_LT_B = 1, A_EQ_B = 0 50 A = 1110, B = 1110, A_GT_B = 0, A_LT_B = 0, A_EQ_B = 1 If the output is not identical, the designer needs to check for any potential bugs and rerun the whole flow until all bugs are eliminated. Comparing simulation output of an RTL and a gate-level netlist is only a part of the functional verification process. Various techniques are used to ensure that the gate-level netlist produced by logic synthesis is functionally correct. One technique is to write a high-level architectural description in C++. The output obtained by executing the high- level architectural description is compared against the simulation output of the RTL or the gate-level description. Another technique called equivalence checking is also frequently used. It is discussed in greater detail in Section 15.3.2, Equivalence Checking, in this book. Timing verification The gate-level netlist is typically checked for timing by use of timing simulation or by a static timing verifier. If any timing constraints are violated, the designer must either redesign part of the RTL or make trade-offs in design constraints for logic synthesis. The entire flow is iterated until timing requirements are met. Details of static timing verifiers are beyond the scope of this book. Timing simulation is discussed in Chapter 10, Timing and Delays. [ Team LiB ] [ Team LiB ] 14.6 Modeling Tips for Logic Synthesis The Verilog RTL design style used by the designer affects the final gate-level netlist produced by logic synthesis. Logic synthesis can produce efficient or inefficient gate- level netlists, based on the style of RTL descriptions. Hence, the designer must be aware of techniques used to write efficient circuit descriptions. In this section, we provide tips
  4. about modeling trade-offs, for the designer to write efficient, synthesizable Verilog descriptions. 14.6.1 Verilog Coding Style[2] [2] Verilog coding style suggestions may vary slightly based on your logic synthesis tool. However, the suggestions included in this chapter are applicable to most cases. The IEEE Standard Verilog Hardware Description Language document also adds a new language construct called attribute. Attributes such as full_case, parallel_case, state_variable, and optimize can be included in the Verilog HDL specification of the design. These attributes are used by synthesis tools to guide the synthesis process. The style of the Verilog description greatly affects the final design. For logic synthesis, it is important to consider actual hardware implementation issues. The RTL specification should be as close to the desired structure as possible without sacrificing the benefits of a high level of abstraction. There is a trade-off between level of design abstraction and control over the structure of the logic synthesis output. Designing at a very high level of abstraction can cause logic with undesirable structure to be generated by the synthesis tool. Designing at a very low level (e.g., hand instantiation of each cell) causes the designer to lose the benefits of high-level design and technology independence. Also, a "good" style will vary among logic synthesis tools. However, many principles are common across logic synthesis tools. Listed below are some guidelines that the designer should consider while designing at the RTL level. Use meaningful names for signals and variables Names of signals and variables should be meaningful so that the code becomes self- commented and readable. Avoid mixing positive and negative edge-triggered flipflops Mixing positive and negative edge-triggered flipflops may introduce inverters and buffers into the clock tree. This is often undesirable because clock skews are introduced in the circuit. Use basic building blocks vs. use continuous assign statements Trade-offs exist between using basic building blocks versus using continuous assign statements in the RTL description. Continuous assign statements are a very concise way of representing the functionality and they generally do a good job of generating random logic. However, the final logic structure is not necessarily symmetrical. Instantiation of basic building blocks creates symmetric designs, and the logic synthesis tool is able to optimize smaller modules more effectively. However, instantiation of building blocks is
  5. not a concise way to describe the design; it inhibits retargeting to alternate technologies, and generally there is a degradation in simulator performance. Assume that a 2-to-1, 8-bit multiplexer is defined as a module mux2_1L8 in the design. If a 32-bit multiplexer is needed, it can be built by instantiating 8-bit multiplexers rather than by using the assign statement. //Style 1: 32-bit mux using assign statement module mux2_1L32(out, a, b, select); output [31:0] out; input [31:0] a, b; wire select; assign out = select ? a : b; endmodule //Style 2: 32-bit multiplexer using basic building blocks //If 8-bit muxes are defined earlier in the design, instantiating //these muxes is more efficient for //synthesis. Fewer gates, faster design. //Less efficient for simulation module mux2_1L32(out, a, b, select); output [31:0] out; input [31:0] a, b; wire select; mux2_1L8 m0(out[7:0], a[7:0], b[7:0], select); //bits 7 through 0 mux2_1L8 m1(out[15:7], a[15:7], b[ 15:7], select); //bits 15 through 7 mux2_1L8 m2(out[23:16], a[23:16], b[23:16], select); //bits 23 through 16 mux2_1L8 m3(out[31:24], a[31:24], b[31:24], select); //bits 31 through 24 endmodule Instantiate multiplexers vs. Use if-else or case statements We discussed in Section 14.3.3, Interpretation of a Few Verilog Constructs, that if-else and case statements are frequently synthesized to multiplexers in hardware. If a structured implementation is needed, it is better to implement a block directly by using multiplexers, because if-else or case statements can cause undesired random logic to be generated by the synthesis tool. Instantiating a multiplexer gives better control and faster synthesis, but it has the disadvantage of technology dependence and a longer RTL description. On the other hand, if-else and case statements can represent multiplexers
  6. very concisely and are used to create technology-independent RTL descriptions. Use parentheses to optimize logic structure The designer can control the final structure of logic by using parentheses to group logic. Using parentheses also improves readability of the Verilog description. //translates to 3 adders in series out = a + b + c + d; //translates to 2 adders in parallel with one final adder to sum results out = (a + b) + (c + d) ; Use arithmetic operators *, /, and % vs. Design building blocks Multiply, divide, and modulo operators are very expensive to implement in terms of logic and area. However, these arithmetic operators can be used to implement the desired functionality concisely and in a technology-independent manner. On the other hand, designing custom blocks to do multiplication, division, or modulo operation can take a longer time, and the RTL description becomes more technology-dependent. Be careful with multiple assignments to the same variable Multiple assignments to the same variable can cause undesired logic to be generated. The previous assignment might be ignored, and only the last assignment would be used. //two assignments to the same variable always @(posedge clk) if(load1) q
  7. //whenever control = 1, out = a which implies a latch behavior. //no branch for control = 0 always @(control or a) if (control) out
  8. The downside of horizontal partitioning is that global minima can often be different local minima. Thus, by use of bit slices, each block is optimized individually, but there may be some global redundancies that the synthesis tool may not be able to eliminate. Vertical Partitioning Vertical partitioning implies that the functionality of a block is divided into smaller submodules. This is different from horizontal partitioning. In horizontal partitioning, all blocks do the same function. In vertical partitioning, each block does a different function. Assume that the 4-bit ALU described earlier is a four-function ALU with functions add, subtract, shift right, and shift left. Each block is distinct in function. This is vertical partitioning. Vertical partitioning of the 4-bit ALU is shown in Figure 14-8. Figure 14-8. Vertical Partitioning of 4-bit ALU
  9. Figure 14-8 shows vertical partitioning of the 4-bit ALU. For logic synthesis, it is important to create a hierarchy by partitioning a large block into separate functional sub- blocks. A design is best synthesized if levels of hierarchy are created and smaller blocks are synthesized individually. Creating modules that contain a lot of functionality can cause logic synthesis to produce suboptimal designs. Instead, divide the functionality into smaller modules and instantiate those modules. Parallelizing design structure In this technique, we use more resources to produce faster designs. We convert sequential operations into parallel operations by using more logic. A good example is the carry lookahead full adder. Contrast the carry lookahead adder with a ripple carry adder. A ripple carry adder is serial in nature. A 4-bit ripple carry adder requires 9 gate delays to generate all sum and carry bits. On the other hand, assuming that up to 5-input and and or gates are available, a carry lookahead adder generates the sum and carry bits in 4 gate delays. Thus, we use more logic gates to build a carry lookahead unit, which is faster compared to an n-bit ripple carry adder. Figure 14-9. Parallelizing the Operation of an Adder
  10. 14.6.3 Design Constraint Specification Design constraints are as important as efficient HDL descriptions in producing optimal designs. Accurate specification of timing, area, power, and environmental parameters such as input drive strengths, output loads, input arrival times, etc., are crucial to produce a gate-level netlist that is optimal. A deviation from the correct constraints or omission of a constraint can lead to nonoptimal designs. Careful attention must be given to specifying design constraints. [ Team LiB ]
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