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ROMs are another form of programmable logic, implementing a collection of functions by placing their truth tables in an array of memory elements. PLAs are attractive when there is a high degree of sharing among the product terms of the multiple output functions. However, a two-level minimization method is critical for using PLAs
We examined switch logic in more detail. Building logic from individual transistors makes it is possible to implement some functions in a more transistor-efficient manner than with discrete gates. Steering logic devices, such as multiplexers and demultiplexers, are examples of logic of this form. Even complex functions, like the Tally circuit, can be implemented efficiently by casting them in terms of data signals routed through a network of switches.
We also looked at discrete gate implementations of multiplexers/selectors and demultiplexers/decoders and described methods for building general-purpose logic from such building blocks.
Some logic gates are not restricted to outputs of 1 and 0. A tri-state gate has a third state that is called high impedance and has the effect of disconnecting itself from its output circuit node. A gate with open-collector outputs can only drive its output to a 0; in conditions where its output would be a 1, the output actually "floats." A circuit node connected to an open-collector output must also be connected to a pull-up resistor. This guarantees that the node will be treated as a logic 1 when none of the connected gates are driving the node to a logic 0.
We closed this chapter by examining combinational word problems. The procedure is to understand the word statement of the problem and then to formulate it in terms of a suitable design representation, such as a truth table. Once an implementation strategy has been determined-for example, choosing whether to implement the circuit in terms of a ROM, two-level logic
The standard handbook on PLD components is Monolithic Memories' Programmable Logic Handbook, 4th edition, Monolithic Memories, Inc., Santa Clara, CA, 1985. The book contains a wealth of information on PLD parts from the company that invented the concept.
The section on switch logic in this chapter is based on the pioneering text by C. Mead and L. Conway, Introduction to VLSI Systems Design, Addison-Wesley, Reading, MA, 1979, especially Chapters 1 and 3. A more modern treatment of CMOS VLSI and its application to switching circuits and conventional gate logic can be found in E. D. Fabricius's textbook Introduction to VLSI Design, McGraw-Hill, New York, 1990. The basic concept of MOS switches is presented in Chapter 1, with a more detailed description of MOS implementations of Tally circuits, NAND, NOR, AOI, and multiplexers in Chapter 8.
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This file last updated on 07/13/96 at 21:42:49.
Chapter Review
The theme of this chapter has been how to construct digital systems with more complex logic building blocks than the discrete gates of Chapters 2 and 3. We began with PALs and PLAs, particularly dense implementations of AND-OR logic defined over multiple inputs and outputs. PALs and PLAs have the important capability of being programmable: the connections among the gates can be wired "on demand" to implement a particular function.ROMs are another form of programmable logic, implementing a collection of functions by placing their truth tables in an array of memory elements. PLAs are attractive when there is a high degree of sharing among the product terms of the multiple output functions. However, a two-level minimization method is critical for using PLAs
(or PALs) effectively. ROM-based designs require nothing more than a truth table, but a ROM does not exploit don't-care conditions or shared product terms among output functions.We examined switch logic in more detail. Building logic from individual transistors makes it is possible to implement some functions in a more transistor-efficient manner than with discrete gates. Steering logic devices, such as multiplexers and demultiplexers, are examples of logic of this form. Even complex functions, like the Tally circuit, can be implemented efficiently by casting them in terms of data signals routed through a network of switches.
We also looked at discrete gate implementations of multiplexers/selectors and demultiplexers/decoders and described methods for building general-purpose logic from such building blocks.
Some logic gates are not restricted to outputs of 1 and 0. A tri-state gate has a third state that is called high impedance and has the effect of disconnecting itself from its output circuit node. A gate with open-collector outputs can only drive its output to a 0; in conditions where its output would be a 1, the output actually "floats." A circuit node connected to an open-collector output must also be connected to a pull-up resistor. This guarantees that the node will be treated as a logic 1 when none of the connected gates are driving the node to a logic 0.
We closed this chapter by examining combinational word problems. The procedure is to understand the word statement of the problem and then to formulate it in terms of a suitable design representation, such as a truth table. Once an implementation strategy has been determined-for example, choosing whether to implement the circuit in terms of a ROM, two-level logic
(PAL or PLA), or multilevel logic-the appropriate combinational logic design method is applied (ROM programming, K-maps, CAD tools for simplification) to obtain the circuit realization. Further Reading
Books on digital design with programmable devices have appeared only in the past several years. R. Alford, Programmable Logic Designer's Guide, Sams and Co., Indianapolis, IN, 1989, provides a good discussion of programmable technology and design methods. Chapter 3 describes the wide range of programmable devices currently available, including more general logic structures than PALs and PLAs. Another good book on programmable logic is G. Bostock, Programmable Logic Devices: Technology and Applications, McGraw-Hill, New York, 1988. Chapter 2 describes the processing and electronic circuit technology of programmable devices. Chapter 4 covers logic design with PROMs, PLAs, and PALs.The standard handbook on PLD components is Monolithic Memories' Programmable Logic Handbook, 4th edition, Monolithic Memories, Inc., Santa Clara, CA, 1985. The book contains a wealth of information on PLD parts from the company that invented the concept.
The section on switch logic in this chapter is based on the pioneering text by C. Mead and L. Conway, Introduction to VLSI Systems Design, Addison-Wesley, Reading, MA, 1979, especially Chapters 1 and 3. A more modern treatment of CMOS VLSI and its application to switching circuits and conventional gate logic can be found in E. D. Fabricius's textbook Introduction to VLSI Design, McGraw-Hill, New York, 1990. The basic concept of MOS switches is presented in Chapter 1, with a more detailed description of MOS implementations of Tally circuits, NAND, NOR, AOI, and multiplexers in Chapter 8.
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