Mastering Read-Only Memory: ROM, EPROM, EEPROM, and Flash EPROM Explained
📑 Table of Contents
- Introduction to Memories
- Types of RAM
- Introduction to Read-Only Memory (ROM)
- Mask Programmable ROM
- Erasable Programmable ROM (EPROM)
- Electrically Erasable Programmable ROM (EEPROM)
- Flash EEPROM
- Ideal Memory
- Conclusion
Introduction to Memories
Memories play a crucial role in the storage and retrieval of data in electronic devices. They are essential components of computer systems, smartphones, and other digital devices. In this article, we will explore different types of memories, with a focus on Read-Only Memory (ROM). We will discuss various categories of ROM, their structures, working principles, programming, erasing, and the advantages and disadvantages of each type.
Types of RAM
Static RAM (SRAM)
SRAM, or Static Random Access Memory, is a type of volatile memory that uses flip-flop circuits to store each bit of data. It provides fast access and retrieval of data, as there is no need for refreshing like in dynamic RAM. However, SRAM is more expensive and consumes more power compared to other types of RAM.
Dynamic RAM (DRAM)
DRAM, or Dynamic Random Access Memory, is another type of volatile memory commonly used in electronic devices. Unlike SRAM, DRAM stores each bit of data as a charge in a capacitor within a memory cell. This charge needs to be refreshed periodically to prevent data loss. DRAM is less expensive and offers higher storage density but has slower access times compared to SRAM.
Introduction to Read-Only Memory (ROM)
Read-Only Memory (ROM) is a non-volatile memory that retains its stored data even when the power is turned off. As the name suggests, ROM is primarily used for storing data that cannot be modified by normal computer operations. However, certain types of ROM allow limited writing operations. Let's delve further into the types of ROM and how they function.
Mask Programmable ROM
Structure and Working Principle
Mask Programmable ROM (PROM) is a type of ROM that can be programmed once and retains the stored data permanently. The basic structure of a PROM involves rows and columns of memory cells. Each memory cell consists of a MOS transistor located at the intersection of a row and a column. The transistor's gate may or may not be connected to the WORD line.
During the reading process, only one word line can be activated at a time. When the word line goes high, the selected transistors in that row are turned on, causing the corresponding bit line voltage to decrease and the output to go high. The absence of a transistor results in a high bit line voltage and a low output, indicating a stored zero. By fabricating transistors selectively, ones and zeros can be stored at specific memory cell locations.
Advantages and Disadvantages
The main advantage of PROM is its non-volatile nature, ensuring data retention without a power supply. It is primarily used when the stored program or data needs to remain unchanged permanently. However, PROM has a significant drawback: once programmed, the data cannot be modified or erased. This lack of flexibility limits its usage in applications where frequent modifications are required.
Erasable Programmable ROM (EPROM)
FAMOS Device
Erasable Programmable ROM (EPROM) is a type of ROM that allows data to be programmed and erased. The EPROM's basic active device is a Floating Gate Avalanche Injection MOS (FAMOS) transistor. It is similar to a regular MOS transistor, but with an additional floating gate. This floating gate is not electrically accessible, giving rise to the EPROM's non-volatile property.
Programming and Erasing
EPROM programming involves applying a high gate voltage and a low drain voltage to implant electrons into the floating gate. This increases the threshold voltage of the transistor, preventing it from turning on at normal gate voltages. Once programmed, the data remains intact for many years.
To erase the EPROM, it needs to be exposed to ultraviolet (UV) light. UV erasable PROMs have Glass windows on top to facilitate the exposure to UV light. The high-energy UV light excites the electrons in the floating gate, allowing them to overcome the silicon dioxide barrier and return to the substrate or control gate. This reduces the threshold voltage and erases the stored data.
While programming and erasing EPROMs can be accomplished, the process is time-consuming and cumbersome. Each chip needs to be taken out of the circuit, exposed to UV light for about half an hour, and then reprogrammed. Partial erasing is not possible, requiring the entire memory to be erased before reprogramming.
Electrically Erasable Programmable ROM (EEPROM)
FLOTOX Device
Electrically Erasable Programmable ROM (EEPROM) is another type of non-volatile memory that allows electrical programming and erasing. EEPROMs use a FLOTOX (Floating Gate Tunneling Oxide) device as the active element. The FLOTOX structure consists of a floating gate, control gate, source and drain contacts, and a thin tunneling oxide layer.
Structure and Operation
An EEPROM cell typically consists of a select transistor and a floating gate transistor. The select transistor allows the selection of a specific memory cell for programming or erasing. When the select transistor is enabled, the drain voltage of the selected cell is modified, allowing programming or erasing of that particular bit of memory.
EEPROMs provide bit erasability, allowing selective erasing of individual bits. This eliminates the need to erase the entire memory before rewriting. However, the writing and erasing times for EEPROMs are significantly longer compared to the reading time. The writing operation typically takes microseconds, whereas reading is performed in nanoseconds.
Advantages and Disadvantages
EEPROMs offer the advantage of bit erasability, allowing for flexible modification of data. They can be programmed and erased internally, eliminating the need to remove the chip from the circuit. However, EEPROMs have slower writing and erasing times compared to reading, making them less suitable for applications requiring frequent modifications.
Flash EEPROM
Structure and Operation
Flash EEPROM, a variant of EEPROM, is a one-transistor cell memory that also allows electric programming and erasing. It utilizes a floating gate MOS structure similar to EEPROM, but with the absence of the select transistor. Flash EEPROM requires a global erase operation, erasing the entire memory before rewriting.
Bit Erasability
Unlike traditional EEPROMs, flash EEPROMs do not offer bit erasability. Erasing is done in a bulk manner, which means that the entire memory needs to be erased before rewriting. This limitation contributes to higher writing times compared to reading operations.
Ideal Memory
An ideal memory would combine the desirable features of different memory types. It would be non-volatile, have low power consumption, high density, and be fully bit-erasable. Researchers are constantly working towards realizing this ideal memory, which would revolutionize the field of data storage.
Conclusion
In this article, we explored the different types of memories, focusing on Read-Only Memory (ROM) and its subsets. We discussed the structure, function, and advantages and disadvantages of Mask Programmable ROM, Erasable Programmable ROM (EPROM), Electrically Erasable Programmable ROM (EEPROM), and Flash EEPROM. While each type offers unique characteristics and purposes, current research aims to develop an ideal memory that combines the best attributes of various memory technologies.
Resources
No difficulty
WHY YOU SHOULD CHOOSE TOOLIFY
