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Introduction to Programmable ROMs
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Today, weβre going to discuss Programmable ROMs or PROMs. Can anyone tell me what they think a PROM does?
Isn't it a type of memory?
Exactly! PROMs store binary information, and you can program them to perform various logic functions. They help implement complex combinational functions.
How do they do that?
Great question! PROMs have an arrangement of hard-wired AND gates for every possible input combination and programmable OR gates to output selected functions.
Working Mechanism of PROMs
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Letβs dive deeper into how PROMs work. Who can explain the role of the fusible links in a PROM?
Are those links what connect the gates together?
Correct! These links can be broken or connected depending on what we want to program. In a conventional fuse, breaking the connection means programming it, while an antifuse creates a connection.
Can we visualize how this looks when programmed?
Absolutely! After programming, we will see certain paths activated, allowing the specific Boolean functions to output the intended results as shown in the circuit diagram.
Applications of PROMs
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Now that we understand PROM architecture, why do you think they are used for complex logic functions?
Maybe because they can create many different combinations?
And they can hold their functions even when the power is off!
Exactly! That persistence allows flexibility in design. However, they aren't always the best option for simpler functions due to their layout constraints.
Introduction & Overview
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Quick Overview
Standard
This section discusses Programmable ROMs (PROMs and EPROMs) as essential components in implementing arbitrary combinational logic functions. These devices use fusible links to program data and retain the logic configuration even without power, although they may be inefficient for simple functions.
Detailed
Programmable ROMs
Programmable Read Only Memories (PROMs) and Erasable Programmable Read Only Memories (EPROMs) are types of programmable ROMs that serve dual purposes as memory devices and programmable logic devices.
A PROM typically has 'n' input lines and 'm' output lines, designated as a 2βΏ Γ m configuration. When used in logic implementation, these devices can realize any conceivable n-variable Boolean function by programming the connections between their wired AND gates and programmable OR gates. The internal architecture involves a combination of hard-wired AND gates that generate all possible minterms and programmable OR gates that select desired outputs from these minterms.
For example, a PROM with four input lines can produce up to 16 product terms, while offering flexibility in defining up to four different output lines according to user specifications.
Moreover, while PROMs excel at implementing complex Boolean functions, they may be inefficient for simpler cases involving numerous 'don't care' conditions due to the static nature of their architecture. Serve as foundational programmable logic devices, PROMs pave the way for understanding what is now categorized under PLDs.
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Introduction to Programmable ROMs
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A read only memory (ROM) is essentially a memory device that can be used to store a certain fixed set of binary information. As outlined earlier, these devices have certain inherent links that can be made or broken depending upon the type of fusible link to store any user-specified binary information in the device.
Detailed Explanation
A ROM is a type of memory that retains data even when the power is turned off. It stores a fixed set of binary information, which can include essential program instructions. The ROM can be 'programmed' to store specific information by altering the internal connections using fusible links. There are two types of fusible links: conventional fuses, which break connections to program the device, and antifuses, which establish connections to store new data.
Examples & Analogies
Think of a programmable ROM like a pre-filled book where you can customize the content. The book retains its content even when closed (like data stored in ROM). You can either 'delete' chapters (like breaking a connection) or 'add' chapters (like making a connection), depending on whether you are using a fuse or antifuse.
Operation of PROMs
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While, in the case of a conventional fusible link, relevant interconnections are broken to program the device, in the case of an antifuse the relevant interconnections are made to do the same job. This is illustrated in Fig. 9.8.
Detailed Explanation
Programming a PROM involves modifying its internal connections to produce outputs corresponding to specific inputs. In a conventional system using fuses, connections are permanently broken to program new data. Conversely, in a system with antifuses, connections are made where necessary, effectively creating paths for the data to flow. This modification is what allows the PROM to perform specific functions based on the user's needs.
Examples & Analogies
Imagine a telephone switchboard. In a traditional system (like a conventional fuse), you'd disconnect wires to reroute calls. In an antifuse-like system, you would connect wires to create new routes. Both methods adjust connections to direct information, but they do so in opposite ways.
Programmability and Applications of PROMs
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Once a given pattern is formed, it remains as such even if power is turned off and on. In the case of PROMs, the user can erase the data already stored on the ROM chip and load it with fresh data.
Detailed Explanation
Once programmed, a PROM retains its data, meaning that the settings are preserved even when the device loses power. Users can update and erase data as needed, allowing for flexible programming of various logic functions. The PROMβs architecture, which combines a fixed AND array for input and programmable OR gates for outputs, enables a user to define a multitude of different logical operations based on binary inputs.
Examples & Analogies
This is similar to how you can change the content of a dry-erase board. You can write down (program) whatever you need, and even when the board is wiped clean (powered off), you can easily write new information on it. It allows for easy updates while retaining this flexible approach.
Internal Architecture of PROMs
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APROM ingeneralhasn input lines and m output lines and is designated as a 2n Γ m PROM. Looking at the internal architecture of a PROM device, it is a combinational circuit with the AND gates wired as a decoder and having OR gates equal to the number of outputs.
Detailed Explanation
In technical terms, a PROM is defined by its inputs and outputs - specifically it can be described as a 2^n x m configuration, where 'n' represents the number of inputs and 'm' the number of outputs. The internal layout typically features an AND gate array set up as a decoder, used to generate all possible combinations of input signals. The results from the AND gates are fed into OR gates, which produce the final outputs according to the programmed connections.
Examples & Analogies
Think of this like a vending machine. The buttons (inputs) represent different choices, and the machine entails internal mechanisms (AND gates) to determine which item to dispense based on the combination of button presses. The output is the selected item dispensed by the machine (OR gates), showing how input combinations lead to specific outputs.
Complexity and Limitations of PROMs
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It may be mentioned here that in practice a PROM would not be used to implement as simple a Boolean function as that illustrated above. The purpose here is to indicate to readers how a PROM implements a Boolean function.
Detailed Explanation
Even though a PROM can implement complex logical operations, it is not always the most efficient choice, especially for straightforward Boolean functions. A PROM's design can lead to inefficient use of logic capacity, particularly in cases where only a few specific conditions need to be met. Other programmable devices, like PLAs or PALs, might be better suited for these situations due to their more flexible architectures.
Examples & Analogies
Using a PROM is akin to using a bulldozer to clear a small garden patch. While it can do the job (implement a function), itβs not the most efficient solution. Sometimes simpler tools (like a shovel or rake) would be better for less complex tasks, just as PLAs or PALs can manage simpler Boolean functions more effectively than a PROM.
Definitions & Key Concepts
Learn essential terms and foundational ideas that form the basis of the topic.
Key Concepts
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Programmable ROMs: Enable manipulation of stored binary data for specific applications.
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Fusible and antifuse links: Two methods of programming PROMs.
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Combinational logic functions: Important applications of PROMs in digital design.
Examples & Real-Life Applications
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Examples
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Example of a PROM designed to implement a specific Boolean function.
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Using a PROM to store configuration settings in a microcontroller after programming.
Memory Aids
Use mnemonics, acronyms, or visual cues to help remember key information more easily.
π΅ Rhymes Time
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In a PROM for each input, you build, connections that flow, but mostly are filled, with fuses and antifuses, a mix of delight, programming logic to behave just right.
π Fascinating Stories
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Imagine a smart library (PROM) where each book (input) creates a story (output) upon request. The librarian (programmable OR gate) only tells stories that have been selected (programmed) by linking specific paths.
π§ Other Memory Gems
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To remember PROM purposes: P-R-O-M = Persistent Read Only Memory.
π― Super Acronyms
P-L-C = Programmable Logic Capability of PROMs.
Flash Cards
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Glossary of Terms
Review the Definitions for terms.
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Term: PROM
Definition:
Programmable Read Only Memory, a type of memory that can be programmed to store binary information.
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Term: EPROM
Definition:
Erasable Programmable Read Only Memory, a type of PROM that can be erased and reprogrammed.
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Term: Fusible Link
Definition:
A connection in a PROM that can be broken to program the device.
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Term: Antifuse
Definition:
A type of fuse in a PROM that, instead of breaking a connection, creates a new one to store information.
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Term: Combinational Logic
Definition:
A type of logic circuit where the output is a function of the present input only.