In the previous tutorials, Boolean functions, Boolean expressions, minimization of Boolean expressions, and implementation of a Boolean function in the logic gate diagram were discussed. It is possible to minimize a Boolean function with less number of Boolean variables and implement a logic gate diagram for it manually. But as the number of variables in a Boolean function increases, not only its minimization becomes complex, but designing a logic gate implementation for it also becomes complicated. In this case, computer-based design tools are the ultimate resource.
These digital circuit design tools use hardware description language to describe, prototype, and test a digital circuit. A hardware description language is a programming language that describes the hardware architecture and functioning of a digital circuit in the form of text. A hardware description language basically describes the relationship between the input signals of a digital circuit and its output signals. In this way, it can textually represent any truth table, Boolean function or Boolean expression. HDL generally has a syntax that can be read by both humans and software tools (design applications).
Fig. 1: Representational Image for Hardware Description Languages
The process of designing a digital integrated circuit using HDL involves the following steps –
1) Design Input : This is the first step in designing a digital integrated circuit. In this step, HDL is used to describe the functionality of the digital circuit. The description can be in the form of a truth table, Boolean equation, netlist of interconnected logic gates or behavioral models. A large and complex digital circuit can be divided into small functional units or models that are then interconnected, with each unit having its own functionality and behavior.
2) Logic Simulation : In this step, the digital circuit based on HDL instructions is simulated as a time-ordered sequence of input and output signals or the waveform of input and output signals. Digital circuit simulation is useful to detect any error in the logical design of the circuit even before it is manufactured on a chip. The logic values of the input signals used to simulate the digital circuit are called test bench. The test bench is also written in HDL. By simulating the digital design, errors in the circuit logic are detected and the respective HDL statements are then corrected to derive the desired output signals.
3) Design Synthesis : In this step, the digital circuit is programmatically synthesized as a database of required physical components and their interconnections. The interconnection of the physical components of the digital circuit is called the netlist. This database can be used to manufacture the circuit on a silicon chip or as a layout on a printed circuit board. The digital database generated by synthesis tools is useful for automating digital integrated circuit manufacturing as the actual procedures involved in implementing the digital circuit are incorporated into the database.
4) Timing Check : Digital circuits must output signals within a desired time limit. There is some propagation delay in the signal when it transitions from one logic gate to another. Practically, this propagation delay must be minimized (such as by implementing two-level Boolean functions in the circuit). By checking the time, the speed of the digital circuit is confirmed. The process involves checking each signal path and confirming that it is not compromised by propagation delays. This is the final step before manufacturing the digital circuit.
5) Fault Simulation : Before the circuit is produced, it is compared with the ideal circuit initially described in HDL. The production circuit may not present the same behavior as the ideal circuit if it could induce a failure in the circuit. The production circuit must be checked for faults before shipping. Failure simulation is always done before production to test the internal logic of the integrated circuit.
6) Physical Design : In the process, the digital circuit is manufactured on a silicon chip or PCB inside a clean room. This is the production stage of the circuit. The circuit must be manufactured in a dust-free environment. Even a single speck of dust can cause circuit malfunction.
There are two standard hardware description languages – VHDL and Verilog. VHDL is an abbreviation for VHSIC Hardware Description Language, where VHSIC stands for Very High Scale Integrated Circuit. VHDL is an IEEE standard HDL language required by the Department of Defense. Verilog is a proprietary HDL language from Cadence Design Systems. Cadence transferred control of Verilog to a consortium of companies and universities known by the name – Open Verilog International. OVI Verilog was developed as an adoption of the IEEE standard. Verilog is simple to use and learn compared to VHDL.
In the next tutorial, learn about arithmetic circuits . Arithmetic circuits are the basic digital circuits that form part of an Arithmetic Logic Unit (ALU) in any Central Processing Unit (CPU).
