Moreover, when the circuit has a pattern similar to that of Figure 7, we can use the “for-generate” statement to dramatically simplify the code. The use of VHDL components is a helpful technique, particularly when we need to implement the same functionality many times or when a subcircuit is complicated and has a lengthy VHDL description. The following code uses the “for-generate” statement to implement a four-bit adder:ġ8 uut: FA port map (a => a(i), b => b(i), c_in => c(i), s => s(i), c_out => c(i 1)) Since i takes the values 0 to 3, lines 17-20 of Listing 3 will be created. This means that the statement of Line 2 will be executed four times with i=0, 1, 2, and 3. The range of i is from 0 to 3 in this example. Here, gen is an arbitrary label for the “for-generate” statement, and i, which comes between the keywords for and in, is an identifier that specifies the number of repetitions of the statements inside the “for-generate” structure. The “for-generate” statement that describes Figure 7 would be: 1 gen: for i in 0 to 3 generateĢ uut: FA port map (a => a(i), b => b(i), c_in => c(i), s => s(i), c_out => c(i 1)) To describe such repeated structures, VHDL allows us to use the “for-generate” statement. Where i=0 gives the rightmost FA instantiation (line 17 of Listing 3), i=3 gives the leftmost FA instantiation (line 20 of Listing 3), and so on. Similarly, we can use the following code to describe the leftmost FA in Figure 1. The notation a => A0 means that the a port of the FA component is connected to A0 from the main circuit. Here, u1 is an arbitrary label for this component instantiation and FA specifies the component name. ![]() Adding these three bits gives a sum, $$S_i$$, and a carry $$C_=\big ( A_i \oplus B_i \big ) C_i A_iB_i$$ĩ s A0, b => B0, c_in => C0, s => S0, c_out => C1) Here, $$A_i$$ and $$B_i$$ are the digits of the ith column and $$C_i$$ is the carry produced by the previous column. This circuit, called a full adder (FA), has the following truth table: This means that binary addition requires a circuit that can add three bits. The carry produced in each column must be added to the digits of the next bit position. This gives us a sum and a carry for each bit position. To add two n-bit numbers, we add the digits of each bit position together from right to left. To discuss the advantages of using VHDL components, let’s consider writing the VHDL code for a four-bit adder.īefore we move on, if you'd like a refresher on VHDL and the purpose of hardware description languages, please check out my article Getting Started with VHDL for Digital Circuit Design. Also, we’ll look at the “for-generate” statement, which can sometimes dramatically simplify the code. ![]() Moreover, if we use this capability the code will be more readable. Components allow us to break a large design into smaller and more manageable parts. ![]() This article will discuss use of VHDL components. It also touches on the "for-generate" statement and its uses. This article defines VHDL components, describes component declaration, and gives examples of how to use VHDL components in your code.
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