Types of ROM |PROM, EPROM and EEPROM
ROM stands(Read Only Memory) is the memory from which we can only read but cannot write on it. This is a volatile memory because the information is stored permanently in such memories during the time of manufacture.
A ROM, stores such instructions that are required to start a computer. This operation is referred to as bootstrap. ROM chips are not only used by the computer but also in other electronic items like a washing machine and microwave oven. ROM is of three types which are as follows:
PROM (Programmable Read-Only Memory):
↦ PROM is a type of ROM that can be programmed or written to only once. It is initially blank, and users or manufacturers can burn data onto it using a PROM programmer.
↦ Once the data is written, it becomes permanent, and the information cannot be altered. PROM is cost-effective for mass production of devices with fixed firmware or software. However, its limitation lies in the inability to reprogram the memory once programmed.
↦ PROM chips are essentially blank slates that can be programmed with specific data using a PROM programmer. The programming process involves blowing fuses or setting certain bits in the memory cells to store the desired information permanently.
↦ Once programmed, the data remains fixed, and the PROM cannot be reprogrammed. This type of ROM is suitable for applications where the data is not expected to change frequently, and the main focus is on cost-effectiveness in mass production.
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Application: Fixed Firmware in Consumer Electronics Example: Inexpensive household appliances or consumer electronic devices, like microwave ovens or basic digital calculators, may use PROM to store fixed firmware or software that doesn’t require frequent updates. Once the firmware is programmed during manufacturing, it remains unchanged throughout the device’s lifespan. |
EPROM (Erasable and Programmable Read-Only Memory)
↦ EPROM addresses the inflexibility of PROM by allowing the erasure of the programmed data using ultraviolet (UV) light. The EPROM chip is covered with a quartz crystal window, and the data can be erased by exposing the chip to UV light for a specified duration.
↦ This exposure removes the charge that holds the data, making the EPROM chip reusable. EPROMs are commonly used during the development and testing phases of electronic systems, where frequent changes to the code may be necessary.
↦ However, the process of erasing and reprogramming EPROMs is somewhat cumbersome compared to modern alternatives.
↦ EPROM introduces the capability to erase the programmed data and reprogram the memory. The erasure process is achieved by exposing the EPROM chip to ultraviolet (UV) light.
↦ The EPROM chip has a quartz crystal window on top that allows the UV light to reach the memory cells. This exposure removes the charge from the floating gate transistors, resetting the memory cells to their default state.
↦ After erasure, new data can be programmed into the EPROM using a programmer. EPROMs find application in scenarios where occasional updates or modifications to the stored data are necessary, and the convenience of erasability outweighs the complexity of the erasure process.
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Application: Development and Testing of Embedded Systems Example: During the development phase of a microcontroller-based system, an EPROM might be used to store and test different versions of firmware. Engineers can make changes to the code, erase the EPROM, and reprogram it with the updated code to test new features or troubleshoot issues. Once the development is complete, the final firmware can be transferred to a more cost-effective ROM type for mass production. |
EEPROM (Electrical Erasable and Programmable Read-Only Memory)
↦ EEPROM takes the concept of reprogrammability further by allowing the erasure and rewriting of data electrically, without the need for UV light.
↦ This makes EEPROM more convenient and user-friendly than EPROM. EEPROM can be selectively erased and reprogrammed at the byte level, providing granular control over the data stored in the memory.
↦ As a result, EEPROM is commonly used in applications where data may need to be updated or modified periodically, such as in firmware updates or storing user-specific information in electronic devices.
↦ EEPROM improves upon EPROM by eliminating the need for UV light for erasure. Instead, EEPROM allows selective erasure and rewriting of data using electrical signals.
↦ This makes EEPROM more user-friendly and versatile, as it can be updated without removing the chip from the circuit.
↦ EEPROM memory cells are based on floating gate transistors, similar to EPROM, but the erasure is achieved through a process known as tunneling, where electrons tunnel through a thin oxide layer.
↦ The electrical nature of erasure makes EEPROM suitable for applications where frequent updates or modifications to the data are required, such as in firmware storage, configuration settings, or data logging in electronic devices.
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Application: Firmware Updates in Electronic Devices Example: Modern smartphones often use EEPROM to store firmware and configuration data. This allows for convenient firmware updates, enabling users to receive new features, security patches, or bug fixes without replacing the entire device. EEPROM’s ability to selectively erase and rewrite data at the byte level is crucial for accommodating incremental updates. |
Types of ROM
| PROM(Programmable Read-Only Memory) | EPROM(Erasable and Programmable Read-Only Memory) | EEPROM(Electrical Erasable and Programmable Read-Only Memory) |
| PROM is is a type of ROM that can be modified only once by a user. | EPROM is a type of ROM that can be erased by exposing it to ultra-violet light and can be reprogrammed. | EEPROM is a type of ROM that is programmed and erased electrically. |
| PROM cannot be changed once you program it. | EPROM can be reprogrammed by erasing the content on it exposing it to ultra-violet light. | EPROM can be reprogrammed by erasing the content on it using electricity. |
| PROM was developed by Weng Tsing Chow in 1959. | EPROM was developed by Dov Frohman at Intel in 1971. | EEPROM was developed by George Perlegos at Intel in 1978. |
References
- Read Only Memory (ROM): Definition, types, features, differences. (2021, May 20). Toppr-guides. https://www.toppr.com/guides/computer-science/computer-fundamentals/primary-memory/rom-read-only-memory/
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Worku. (2020, May 31). Types of ROM | PROM, EPROM, EEPROM, Flash EPROM & Mask ROM. StudyElectrical.Com. https://studyelectrical.com/2017/06/different-types-of-rom-prom-eprom.html
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