Ebook Digital electronics: Part 2
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Ebook Digital electronics: Part 2
9Programmable Logic DevicesLogic devices constillite one of the three important classes of devices used to build digital electronics systems, memory d Ebook Digital electronics: Part 2devices and microprocessors being the other two. Memory devices such as ROM and RAM are used to store information such as the software instructions of a program or the contents of a database, and microprocessors execute software instructions to perform a variety of functions, from running a word-pro Ebook Digital electronics: Part 2cessing program to carrying out far more complex tasks. Logic devices implement almost every other function that the system must perform, including deEbook Digital electronics: Part 2
vice-to-device interfacing, data timing, control and display operations and so on. So far. we have discussed those logic devices that perform fixed lo9Programmable Logic DevicesLogic devices constillite one of the three important classes of devices used to build digital electronics systems, memory d Ebook Digital electronics: Part 2ntial logic devices such as flip-flops, counters, registers, etc., to be discussed in the following chapters, also belong to this category of logic devices. In the present chapter, we will discuss a new category of logic devices called programmable logic devices (PLDs). The function to be performed Ebook Digital electronics: Part 2by a programmable logic device is undefined at the time of its manufacture. These devices are programmed by the user to perform a range of functions dEbook Digital electronics: Part 2
epending upon the logic capacity and other features offered by the device. We will begin with a comparison of fixed and programmable logic, and then f9Programmable Logic DevicesLogic devices constillite one of the three important classes of devices used to build digital electronics systems, memory d Ebook Digital electronics: Part 2applications. A brief introduction to the devices offered by some of the major manufacturers of PLDs and PLD programming languages is given towards the end of the chapter.9.1Fixed Logic Versus Programmable LogicAs outlined in the introduction, there are two broad categories of logic devices, namely Ebook Digital electronics: Part 2fixed logic devices and programmable logic devices. Whereas a fixed logic device such as a logic gate or a multiplexer or a flip-flop performs a givenEbook Digital electronics: Part 2
logic function that is known at the time of device manufacture, a programmable logic device can be configured by the user to perform a large variety 9Programmable Logic DevicesLogic devices constillite one of the three important classes of devices used to build digital electronics systems, memory d Ebook Digital electronics: Part 2ic functions. In terms of the internal schematic arrangement of the two types of device, the circuits or building blocks and their interconnections in a fixed logic device are permanent and cannot be altered after the device is manufactured.A programmable logic device offers to the user a wide range Ebook Digital electronics: Part 2 of logic capacity in terms of digital building blocks, which can be configured by the user to perform the intended function or set of functions. ThisEbook Digital electronics: Part 2
configuration can be modified or altered any number of times by the user by reprogramming the device. Figure 9.1 shows a simple logic circuit compris9Programmable Logic DevicesLogic devices constillite one of the three important classes of devices used to build digital electronics systems, memory d Ebook Digital electronics: Part 2e two bits to be added, and c is the carry-in bit. It is a fixed logic device as the circuit is unalterable from outside owing to fixed interconnections between the various building blocks.Figure 9.2 shows the logic diagram of a simple programmable device. The device has an array of four six-input A Ebook Digital electronics: Part 2ND gates at the input and a four-input OR gate at (he output. Each AND gate can handle three variables and thus can produce a product term of three vaEbook Digital electronics: Part 2
riables. The three variables (A. li and c in this case) or (heir complements can be programmed to appear at the inputs of any of (he four AND gates th9Programmable Logic DevicesLogic devices constillite one of the three important classes of devices used to build digital electronics systems, memory d Ebook Digital electronics: Part 2 an antifuse performs a function that is opposite to that performed by a conventional electrical fuse. A fuse has a low initial resistance and permanently breaks an electrically conducting path when current through it exceeds a certain limiting value. In the case of an antifuse, the initial resistan Ebook Digital electronics: Part 2ce is very high and it is designed to create a low-resistance electrically conducting path when voltage across it exceeds a certain level. As a resultEbook Digital electronics: Part 2
, this circuit can be programmed to generate any three-variable sum-of-products Boolean function having four minienns by activating the desired fusibl9Programmable Logic DevicesLogic devices constillite one of the three important classes of devices used to build digital electronics systems, memory d Ebook Digital electronics: Part 2of a full adder) or to produce difference outputs resulting from subtraction of two bits with a borrow-in (the difference output in the case of a full subtractor).We can visualize that the logic circuit of Fig. 9.2 has a programmable AND array at the input and a fixed OR gate at the output. Incident Ebook Digital electronics: Part 2ally, this is the architecture of programmable logic devices called programmable array logic (PAL). Practical PAL devices have a much larger number ofEbook Digital electronics: Part 2
programmable AND gates and fixed OR gates to have enhanced logic capacity and performance capability. PAL devices are discussed in detail in (he latt9Programmable Logic DevicesLogic devices constillite one of the three important classes of devices used to build digital electronics systems, memory d Ebook Digital electronics: Part 2isadvantages1If we want to build a fixed logic device to perform a certain specific function, the time required from design to the final stage when the manufactured device is actually available for use could easily be several months to a year or so. PLD-based design requires much less time from desi Ebook Digital electronics: Part 2gn cycle to production run.2In the case of fixed logic devices, the process of design validation followed by incorporation of changes, if any. involveEbook Digital electronics: Part 2
s substantial nonrecurring engineering (NRE) costs, which leads to an enhanced cost of the initial prototype device. In the case of PLDs. inexpensive 9Programmable Logic DevicesLogic devices constillite one of the three important classes of devices used to build digital electronics systems, memory d Ebook Digital electronics: Part 2 quick testing of the device in an actual application environment. In this case, the device used for prototyping is the same as the one that would qualify for use in the end equipment.302Digital Electronics3In the case of programmable logic devices, users can change the circuit as often as they want Ebook Digital electronics: Part 2 to until the design operates to their satisfaction. PLDs offer to the users much more flexibility during the design cycle. Design iterations are nothEbook Digital electronics: Part 2
ing but changes to the programming file.4Fixed logic devices have an edge for large-volume applications as they can be mass produced more economically9Programmable Logic DevicesLogic devices constillite one of the three important classes of devices used to build digital electronics systems, memory d Ebook Digital electronics: Part 2 types of programmable logic device, distinguishable from one another in terms of architecture, logic capacity, programmability and certain other specific features. In this section, we will briefly discuss commonly used PLDs and their salient features. A detailed description of each of them will fol Ebook Digital electronics: Part 2low in subsequent sections.9.2./ Programmable ROMsPROM (Programmable Read Only Memory) and EPROM (Erasable Programmable Read Only Memory) can be consiEbook Digital electronics: Part 2
dered to be predecessors to PLDs. The architecture of a programmable ROM allows the user to hardware-implement an arbitrary combinational function of 9Programmable Logic DevicesLogic devices constillite one of the three important classes of devices used to build digital electronics systems, memory d Ebook Digital electronics: Part 2 be used to store 2"m-bit words. When used as a PLD. it can be used to implement III different combinational functions, with each function being a chosen function of II variables. Any conceivable n-variable Boolean function can be made to appear at any of the m output lines. A generalized ROM device Ebook Digital electronics: Part 2 with n inputs and in outputs has 2" hard-wired AND gales at the input and III programmable OR gates at the output. Each AND gate has II inputs, and eEbook Digital electronics: Part 2
ach OR gate has 2" inputs. Thus, each OR gate can be used to generate any conceivable Boolean function of It variables, and this generalized ROM can b9Programmable Logic DevicesLogic devices constillite one of the three important classes of devices used to build digital electronics systems, memory d Ebook Digital electronics: Part 2the programmable OR array allows only (he desired mintenns to appear at their inputs. Figure 9.3 shows the internal architecture of a PROM having four input lines, a hard-wired array of 16 AND gates and a programmable array of four OR gates. A cross (x) indicates an intact (or unprogrammed) fusible Ebook Digital electronics: Part 2link or interconnection, and a dot (•) indicates a hard-wired interconnection. PROMs. EPROMs and EEPROMs (Electrically Erasable Programmable Read OnlyEbook Digital electronics: Part 2
Memory) can be programmed using standard PROM programmers. One of the major disadvantages of PROMs is their inefficient use of logic capacity. It isGọi ngay
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