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EE 4271 VLSI Design, Fall 2016 Combinational Circuits Overview • Combinational Circuit • Chip Design styles – Full-custom design – Cell library based design – Programmable Logic Array 2017/5/13 Combinational Logic PJF- 2 Combinational Circuits • A combinational circuit consists of logic gates whose outputs, at any time, are determined by combining the values of the inputs. • For n input variables, there are 2n possible binary input combinations. • For each binary combination of the input variables, there is one possible output. 2017/5/13 Combinational Logic PJF- 3 Combinational Circuits (cont.) • Hence, a combinational circuit can be described by: 1. A truth table that lists the output values for each combination of the input variables, or 2. m Boolean functions, one for each output variable. n-inputs 2017/5/13 •• • Combinational Circuit Combinational Logic •• • m-outputs PJF- 4 Combinational vs. Sequential Circuits Combinational circuits are memory-less. Thus, the output value depends ONLY on the current input values. Sequential circuits consist of combinational logic as well as memory elements (used to store certain circuit states). Outputs depend on BOTH current input values and previous input values (kept in the storage elements). 2017/5/13 Combinational Logic PJF- 5 Combinational vs. Sequential Circuits n-inputs Combinational Circuit m-outputs (Depend only on inputs) Combinational Circuit n-inputs Combinational Circuit m-outputs Next state Storage Elements Present state Sequential Circuit 2017/5/13 Combinational Logic PJF- 6 Important Design Concepts • Modern digital design deals with various methods and tools that are used to design and verify complex circuits and systems. • Concepts: – Design Hierarchy – Computer-Aided-Design (CAD) tools – Hardware Description Languages (HDLs) 2017/5/13 Combinational Logic PJF- 7 Design Hierarchy • “Divide-and-Conquer” approach used to cope with the challenges of designing complex circuits and systems (many times in the order of millions of gates). • Circuit is broken into blocks, repetitively. 2017/5/13 Combinational Logic PJF- 8 Design Hierarchy Example: 9-input odd function (for counting # of 1 in inputs) 2017/5/13 Combinational Logic PJF- 9 Why is Hierarchy useful? • Reduces the complexity required to design and represent the overall schematic of the circuit. • Reuse of blocks is possible. Identical blocks can be used in various places in a design, or in different designs. 2017/5/13 Combinational Logic PJF- 10 Reusable Functions and CAD • Whenever possible, we try to decompose a complex design into common, reusable function blocks • These blocks are – verified and well-documented – placed in libraries for future use 2017/5/13 Combinational Logic PJF- 11 Integrated Circuits • Integrated circuit (a chip) is a semiconductor crystal (most often silicon) containing the electronic components for the digital gates and storage elements which are interconnected on the chip. • Terminology - Levels of chip integration – – – – 2017/5/13 SSI (small-scale integrated) - fewer than 10 gates MSI (medium-scale integrated) - 10 to 100 gates LSI (large-scale integrated) - 100 to thousands of gates VLSI (very large-scale integrated) - thousands to 100s of millions of gates Combinational Logic PJF- 12 Technology Parameters • Specific gate implementation technologies are characterized by the following parameters: – Fan-in – the number of inputs available on a gate – Fan-out – the number of standard loads driven by a gate output – Cost for a gate - a measure of the contribution by the gate to the cost of the integrated circuit – Propagation Delay – The time required for a change in the value of a signal to propagate from an input to an output – Power Dissipation – the amount of power drawn from the power supply and consumed by the gate 2017/5/13 Combinational Logic PJF- 13 Propagation Delay • Propagation delay is the time for a change on an input of a gate to propagate to the output. • Delay is usually measured at the 50% point with respect to the H and L output voltage levels. • High-to-low falling and low-to-high rising delays. 2017/5/13 Combinational Logic PJF- 14 OUT (volts) IN (volts) Propagation Delay Example 2017/5/13 1.0 ns per division Combinational Logic t (ns) PJF- 15 Chip Design Styles • Full custom - the entire design of the chip down to the smallest detail of the layout is performed – Expensive, its timing and power is hard to analyze – only for dense, fast chips with high sales volume • Standard cell - blocks have been design ahead of time or as part of previous designs – Intermediate cost – Less density and speed compared to full custom • Gate array - regular patterns of gate transistors that can be used in many designs built into chip - only the interconnections between gates are specific to a design – – – – Lowest cost Less density compared to full custom and standard cell Prototype design The base of FPGA 2017/5/13 Combinational Logic PJF- 16 Cell Libraries • Cell - a pre-designed primitive block • Cell library - a collection of cells available for design using a particular implementation technology • Cell characterization - a detailed specification of a cell for use by a designer 2017/5/13 Combinational Logic PJF- 17 Cell Library Based Design Procedure 1. Specification – 2. Write a specification for the circuit if one is not already available Formulation – 3. Derive a truth table or initial Boolean equations that define the required relationships between the inputs and outputs, if not in the specification Optimization – 2017/5/13 Draw a logic diagram or provide a netlist for the resulting circuit using ANDs, ORs, and inverters Combinational Logic PJF- 18 Cell Library Based Design Procedure 4. Technology Mapping – Map the logic diagram to the implementation technology selected – Map to CMOS 5. Evaluation – Evaluate the timing and power 2017/5/13 Combinational Logic PJF- 19 Design Example 1. Specification – BCD to Excess-3 code converter – Transforms BCD code for the decimal digits to Excess-3 code for the decimal digits – BCD code words for digits 0 through 9: 4-bit patterns 0000 to 1001, respectively – Excess-3 code words for digits 0 through 9: 4-bit patterns consisting of 3 (binary 0011) added to each BCD code word 2017/5/13 Combinational Logic PJF- 20 Design Example (continued) 2. Formulation – – – – 2017/5/13 Conversion of 4-bit codes can be most easily formulated by a truth table Variables - BCD: Input BCD Output Excess-3 A,B,C,D ABCD WXYZ Variables 0000 0011 - Excess-3 W,X,Y,Z 0001 0100 0010 0101 Don’t Cares - BCD 1010 0011 0110 to 1111 0100 0111 0101 0110 0111 1000 1001 Combinational Logic 1000 1001 1010 1011 1011 PJF- 21 Design Example (continued) 3. Optimization z 1 1 3 4 5 7 X X 12 13 8 9 1 B 1 4 5 A X CD X 13 8 9 1 X 10 C 1 2 0 4 5 7 6 4 1 1 1 X 13 1 B 14 11 w 3 8 X X 1 A 6 15 1 0 12 7 X 12 10 C X 2 D x 1 3 1 X 14 11 0 D X 2017/5/13 X X 1 1 6 15 1 C 2 1 X A y 1 0 1 a. K-maps W = A + BC + BD X = BC + DB+ B Y = CD + CD Z= D C X 15 X 9 11 Combinational Logic D B X 14 10 1 X 1 1 8 7 X 13 6 X 15 X 9 2 1 5 12 A X 3 11 14 X 10 D PJF- 22 B Design Example (continued) 3. Optimization (continued) Multiple-level using transformations W = A + BC + BD X = BC + DB+ B Y = CD + C D Z= – CD D Perform extraction, finding factor: T1 = C + D W = A + BT1 X = B T1 + B C D Y = CD + CD Z= D 2017/5/13 Combinational Logic PJF- 23 Design Example (continued) 4. Technology Mapping Map with a library containing inverters and 2-input NAND, and then map it to a CMOS based circuit A W B X C D Y 2017/5/13 Z Combinational Logic Z Technology Mapping Example 2017/5/13 Combinational Logic PJF- 25 Timing Analysis • Use static timing analysis which has been covered. 2017/5/13 Combinational Logic PJF- 26 Transmission Gate Based Design Multiplexer • “Selects” binary information from one of many input lines and directs it to a single output line. • Also know as the “selector” circuit, • Selection is controlled by a particular set of inputs lines whose # depends on the # of the data input lines. • For a 2n-to-1 multiplexer, there are 2n data input lines and n selection lines whose bit combination determines which input is selected. 13-May-17 Combinational Logic PJF - 27 Multiplexer (cont.) 13-May-17 Combinational Logic PJF - 28 2-to-1-Line Multiplexer • Since 2 = 21, n = 1 • The single selection variable S has two values: – S = 0 selects input I0 – S = 1 selects input I1 • The equation: Y = S’ I0 + SI1 • The circuit: Enabling Circuits Decoder I0 Y S 13-May-17 Combinational Logic I1 PJF - 29 Example: 4-to-1 MUX - Cell Library Based Design 13-May-17 Combinational Logic PJF - 30 4–to–1-Line Multiplexer using Transmission Gates 13-May-17 Combinational Logic PJF - 31 MUX as a Universal Gate • We can construct AND and NOT gates using 2-to-1 MUXs. Thus, 2-to-1 MUX is a universal gate. z = 0x + 1x’ = x’ 13-May-17 Combinational Logic z = x1x0 + 0x0’ = x1x0 PJF - 32 Programmable Logic Array • The set of functions to be implemented is first transformed to product terms • Since output inversion is available, terms can implement either a function or its complement 13-May-17 Combinational Logic PJF - 33 Programmable Logic Array Example • To implement – F1= A’B’C+A’BC’+AB’C’=(AB+AC+BC+A’B’C’)’ – F2=AB+AC+BC 13-May-17 Combinational Logic PJF - 34 Programmable Logic Array Example A B C X X X 1 X X X 2 X X X Fuse intact Fuse blown X X X 3 X X 4 C C B B A A X X X X 0 X 1 F1 F2 13-May-17 Combinational Logic PJF - 35 Summary • Three design styles – Full custom design – Gate library based design – PLA based design • Transmission gate based design 2017/5/13 Combinational Logic PJF- 36