14.3 Verilog HDL Synthesis For the purpose of logic synthesis, designs are currently written in an HDL at a register transfer level (RTL). The term RTL is used for an HDL description style that utilizes a combination of data flow and behavioral constructs. Logic synthesis tools take the register transfer-level HDL description and convert it to an optimized gate-level netlist. Verilog and VHDL are the two most popular HDLs used to describe the functionality at the RTL level. In this chapter, we discuss RTL-based logic synthesis with Verilog HDL. Behavioral synthesis tools that convert a behavioral description into an RTL description are slowly evolving, but RTL- b ased synthesis is currently the most popular design method. Thus, we will address only RTL-based synthesis in this chapter. 14.3.1 Verilog Constructs N ot all constructs can be used when writing a description for a logic synthesis tool. In general, any construct that is used to define a cycle-by-cycle RTL description is acceptable to the logic synthesis tool. A list of constructs that are typically accepted by logic synthesis tools is given in Table 14-1 . The capabilities of individual logic synthesis tools may vary. The constructs that are typically acceptable to logic synthesis tools are also shown. Table 14-1. Verilog HDL Constructs for Logic Synthesis Construct Type Keyword or Description Notes ports input, inout, output parameters parameter module definition module signals and variables wire, reg, tri Vectors are allowed instantiation module instances, primitive gate instances E.g., mymux m1(out, i0, i1, s); E.g., nand (out, a, b); functions and tasks function, task Timing constructs ignored procedural always, if, then, else, initial is not supported case, casex, casez procedural blocks begin, end, named blocks, disable Disabling of named blocks allowed data flow assign Delay information is ignored loops for, while, forever, while and forever loops must contain @(posedge clk) or @(negedge clk) Remember that we are providing a cycle-by-cycle RTL description of the circuit. Hence, there are restrictions on the way these constructs are used for the logic synthesis tool. For example, the while and forever loops must be broken by a @ (posedge clock) or @ (negedge clock) statement to enforce cycle-by-cycle behavior and to prevent combinational feedback. Another restriction is that logic synthesis ignores all timing delays specified by #<delay> construct. Therefore, pre- and post-synthesis Verilog simulation results may not match. The designer must use a description style that eliminates these mismatches. Also, the initial construct is not supported by logic synthesis tools. Instead, the designer must use a reset mechanism to initialize the signals in the circuit. It is recommended that all signal widths and variable widths be explicitly specified. Defining unsized variables can result in large, gate-level netlists because synthesis tools can infer unnecessary logic based on the variable definition. 14.3.2 Verilog Operators Almost all operators in Verilog are allowed for logic synthesis. Table 14-2 is a list of the operators allowed. Only operators such as === and !== that are related to x and z are not allowed, because equality with x and z does not have much meaning in logic synthesis. While writing expressions, it is recommended that you use p arentheses to group logic the way you want it to appear. If you rely on operator p recedence, logic synthesis tools might produce an undesirable logic structure. Table 14-2. Verilog HDL Operators for Logic Synthesis Operator Type Operator Symbol Operation Performed Arithmetic * / multiply divide + - % + - add subtract modulus unary plus unary minus Logical ! && || logical negation logical and logical or Relational > < >= <= greater than less than greater than or equal less than or equal Equality == != equality inequality Bit-wise ~ & | ^ ^~ or ~^ bitwise negation bitwise and bitwise or bitwise ex-or bitwise ex-nor Reduction & ~& | reduction and reduction nand reduction or ~| ^ ^~ or ~^ reduction nor reduction ex-or reduction ex-nor Shift >> << >>> <<< right shift left shift arithmetic right shift arithmetic left shift Concatenation { } concatenation Conditional ?: conditional 14.3.3 Interpretation of a Few Verilog Constructs Having described the basic Verilog constructs, let us try to understand how logic synthesis tools frequently interpret these constructs and translate them to logic gates. The assign statement The assign construct is the most fundamental construct used to describe combinational logic at an RTL level. Given below is a logic expression that uses the assign statement. assign out = (a & b) | c; This will frequently translate to the following gate-level representation: If a, b, c, and out are 2-bit vectors [1:0], then the above assign statement will frequently translate to two identical circuits for each bit. If arithmetic operators are used, each arithmetic operator is implemented in terms of arithmetic hardware blocks available to the logic synthesis tool. A 1-bit full adder is implemented below. assign {c_out, sum} = a + b + c_in; Assuming that the 1-bit full adder is available internally in the logic synthesis tool, the above assign statement is often interpreted by logic synthesis tools as follows: If a multiple-bit adder is synthesized, the synthesis tool will perform optimization and the designer might get a result that looks different from the above figure. If a conditional operator ? is used, a multiplexer circuit is inferred. assign out = (s) ? i1 : i0; It frequently translates to the gate-level representation shown in Figure 14-3 . Figure 14-3. Multiplexer Description The if-else statement Single if-else statements translate to multiplexers where the control signal is the signal or variable in the if clause. if(s) out = i1; else out = i0; The above statement will frequently translate to the gate-level description shown in Figure 14-3 . In general, multiple if-else-if statements do not synthesize to large multiplexers. The case statement The case statement also can used to infer multiplexers. The above multiplexer would have been inferred from the following description that uses case statements: case (s) 1'b0 : out = i0; 1'b1 : out = i1; endcase Large case statements may be used to infer large multiplexers. for loops The for loops can be used to build cascaded combinational logic. For example, the following for loop builds an 8-bit full adder: c = c_in; for(i=0; i <=7; i = i + 1) {c, sum[i]} = a[i] + b[i] + c; // builds an 8-bit ripple adder c_out = c; The always statement The always statement can be used to infer sequential and combinational logic. For sequential logic, the always statement must be controlled by the change in the value of a clock signal clk. always @(posedge clk) q <= d; This is inferred as a positive edge-triggered D-flipflop with d as input, q as output, and clk as the clocking signal. Similarly, the following Verilog description creates a level-sensitive latch: always @(clk or d) if (clk) q <= d; For combinational logic, the always statement must be triggered by a signal other than the clk, reset, or preset. For example, the following block will be interpreted as a 1-bit full adder: always @(a or b or c_in) {c_out, sum} = a + b + c_in; The function statement Functions synthesize to combinational blocks with one output variable. The output might be scalar or vector. A 4-bit full adder is implemented as a function in the Verilog description below. The most significant bit of the function is used for the carry bit. function [4:0] fulladd; input [3:0] a, b; input c_in; begin fulladd = a + b + c_in; //bit 4 of fulladd for carry, bits[3:0] for sum. end endfunction . c_out = c; The always statement The always statement can be used to infer sequential and combinational logic. For sequential logic, the always statement must be controlled by the change in