Introduction to Digital Design

Text Book Digital Design by - John F. Wakerly www.ddpp.com - you will find some solutions at this site. www.xilinx.com - Xlinix Web site

Digital Design Digital Design is also know as “Logical Design”. The purpose is to build Digital Systems.

Analog versus Digital Analog systems process time-varying signals that can take on any value across a continuous range of voltages (in electrical/electronics systems). Digital systems process time-varying signals that can take on only one of two discrete values of voltages (in electrical/electronics systems). Discrete values are called 1 and 0 (ON and OFF, HIGH and LOW, TRUE and FALSE, etc.)

Digital Revolution Digital systems started back in 1940s. Digital systems cover all areas of life: still pictures digital video digital audio telephone traffic lights Animation

Benefits of Digital over Analog Reproducibility Not effected by noise means quality Ease of design Data protection Programmable Speed Economy

Digital Devices Gates Flip-Flops PLDs FPGAs

Gates The most basic digital devices are called gates. Gates got their name from their function of allowing or blocking (gating) the flow of digital information. A gate has one or more inputs and produces an output depending on the input(s). A gate is called a combinational circuit. Three most important gates are: AND, OR, NOT

Digital Logic Binary system -- 0 & 1, LOW & HIGH, negated and asserted. Basic building blocks -- AND, OR, NOT

AND, OR, NOT Gates

Electronic Aspects of Digital Design How we represent digital information in electronic devices? By discrete voltages.

What is the Basic Digital Element in Electronics ? a Switch

Using Switch to represent digital information

Digital Abstraction It is difficult to make ideal switches means a switch is completely ON or completely OFF. So, we impose some rules that allow analog behavior to be ignored in most cases, so circuits can be modeled as if they really did process 0s and 1s, known as digital abstraction. Digital abstraction allows us to associate a noise margin with each logic values (0 and 1).

Real Switches to represent digital information 5v 5v 1k Output Output 5v 4.5v 10k

Logic levels Undefined region is inherent digital, not analog amplification, weak => strong Switching threshold varies with voltage, temp, process, phase of the moon need “noise margin” The more you push the technology, the more “analog” it becomes. Logic voltage levels decreasing with new processors. 5 -> 3.3 -> 2.5 -> 1.8 V

MOS Transistors Voltage-controlled resistance PMOS NMOS

CMOS Inverter

Switch model

Alternate transistor symbols

CMOS Gate Characteristics No DC current flow into MOS gate terminal However gate has capacitance ==> current required for switching (CV2f power) No current in output structure, except during switching Both transistors partially on Power consumption related to frequency Slow input-signal rise times ==> more power Symmetric output structure ==> equally strong drive in LOW and HIGH states

Flip-flops A device that stores either a 0 or 1. Stored value can be changed only at certain times determined by a clock input. New value depend on the current state and it’s control inputs A digital circuit that contains filp-flops is called a sequential circuit

Flip-flops S-R latch symbols D flip-flop J-K flip-flops

Integrated Circuits A collection of one or more gates fabricated on a single silicon chip is called an integrated circuit (IC). ICs were classified by size: SSI - small scale integration - 1~20 gates MSI - medium scale integration - 20~200 gates LSI - large scale integration - 200~200,000 gates VLSI - very large scale integration - over 1M transistors Pentium-III - 40 million transistors

DIP Packages

Gates in ICs

Programmable Logic Devices PLDs allow the function to be programmed into them after they are manufactured. Complex PLDs (CPLD) are a collection of PLDs on the same chip. Another programmable logic chip is FPGA - field-programmable gate arrays.

CPLDs and FPGAs CPLD FPGA

Application Specific Ics (ASICs) Chips designed for a particular application are called semicustom ICs or application-specific ICs (ASICs). ASICs generally reduce the total component and manufacturing cost of a product by reducing chip count, physical size, and power consumption, and they often provide higher performance. But costly if not produced in bulk.

Printed-Circuit Boards An IC is normally mounted on a printed-circuit board (PCB) that connects it to other ICs in a system. Individual wire connection or traces can be as narrow as 4 mils with 4 mils spacing (one-thousandth of an inch) Now a days, most of the components use surface mount technology. They are normally layered.

Software Aspects of Digital Design Today software tools are an essential part of digital design. Software tools improve productivity, correctness and quality of designs Software tools are: Schematic entry HDL (Hardware Description Language) Editors Simulators - to verify the behaviour of the design Synthesis tools - circuit design Timing analyzers and verifiers

Digital Design Levels the lowest level of design is device physics and IC manufacturing processes. design at the transistor level level of functional building blocks level of logic design using HDLs computer design and overall system design.

Different Design Levels Consider a simple design example: Build a multiplexer with two data inputs A and B, a control input S, and an output Z. Switch model for the example multiplexer

Designing at the transistor level Transistor-level circuit diagrams Gate symbols (for simple elements)

Logic design using Truth tables

Logic design using boolean algebra Equations: Z = S¢ × A + S × B Logic diagrams

Prepackaged building blocks, e.g. multiplexer

Various hardware description languages ABEL VHDL We’ll start with gates and work our way up

Name of the program module the type of PLD pin numbers ABEL statement to achieve the multiplexer Standard library and a set of definitions Inputs and outputs functions behaviour

Structural VHDL program for the multiplexer

Summery Design to minimize cost. Rule of thumb is to minimize the number of ICs. Though PLDs costs more but uses less PCB area. Unless mass production avoid ASIC design. Design to solve real life problems.