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What is Programmable Read Only Memory (PROM)?
Programmable Read Only Memory or Programmable ROM or simply PROM refers to that particular computer chip which can be programmed after it is created, unlike a regular ROM.
- A Programmable Read Only Memory is known by different names such as PROM, FPROM or Field Programmable Read Only Memory and OTP or One Time Programmable chip.
- This memory chip usually is manufactured blank. Once it is created, the programmer creates and stores the data in it which cannot be erased or modified.
- The Programmable ROM chips are usually used for cache memory and need a constant power supply to operate.
Understanding Programmable Read Only Memory (PROM)
Programmable Read Only Memory is a permanent memory that was first developed in 1956 by Wen Tsing Chow.
When this chip is programmed, the information stored in it stays there forever and cannot be erased or deleted.
If it is programmed erroneously, it cannot be rectified or updated but needs to be discarded and replaced by a new PROM.
However, an EPROM, which is a variant of the PROM, can be erased for reprogramming without needing to be replaced.
Programmable Read Only Memory is a variant of a Read Only Memory.
It is designed in such a way that it allows the users to put the data or program of their choice on it.
The BIOS or Basic Input Output System in the early computers is an example of a Programmable Read Only Memory.
Ideally, if you want to understand a Programmable ROM fully and comprehensively, you will first need to have some fair idea about Read Only Memory because a PROM is typically based on the concept of a traditional ROM.
Read Only Memory stores data permanently on it. This data is stored on it during the manufacturing process.
The stored data can be read only and cannot be modified or deleted and therefore it is called ‘Read Only Memory.’
Every computer has a ROM and it contains the programs that are essential for the system to operate.
Without the ROM a computer will not work because it will have no one to tell it how to function.
When the system is turned off, the data remains on the memory which makes it a non-volatile memory.
It typically does not need any power to save the data on it.
The only difference between a standard ROM and a Programmable ROM is that the latter is manufactured as a blank chip and the desired program or data is added to it later. Once it is done, the programmable ROM acts just like a standard ROM.
The Programmable ROM chips can be either surface-mount or through-hole chips made from silicon material usually.
However, other materials such as ceramic or glass may also be used to make these chips.
There are leads or wires in the through-hole PROM chips that pass through the holes in the circuit board. These wires are soldered on the other side on the pads on the board.
On the other hand, in the surface-mount PROM chips, the wires are soldered onto the surface of the circuit board itself, directly.
Programming a PROM
In order to program a ROM, a PROM burner or a PROM programmer is used and the process is referred to as PROM burning.
It depends on the design of the PROM as to when it will be programmed.
Typically it can be programmed at different stages such as:
- At the wafer
- During the final test or
- In the system.
The blank PROM chips are plugged into the PROM programmer while programming and it is actually done at the last minute by the companies. This saves them from any large volume commitment.
Ideally, a Programmable ROM contains programs of low level such as a microcode or a firmware.
Features of Programmable Read Only Memory
The Programmable Read Only Memory comes with specific features that make it so useful.
For example, when a Programmable ROM is created it is blank and programmed later on. All the bits stored in it read as 1.
When any of these bits need to be changed during the programming, a gang programmer is used to etch or burn a 0 into the chip.
This is done by the programmer and the chip is then ready for use.
Some of the other notable features of the Programmable ROM are:
- The chip is not reusable
- It is quite inexpensive
- The data stored on it is irreversible
- It has very high storage endurance
- The chip is sheathed completely during a plastic cowl
- It is a written-only type of a ROM
- It is unusable when there is a bug, error, or miscalculation in the program written on it
- It is the predecessor of EPROM or Erasable Programmable Read Only Memory
- It is quite scalable and flexible and
- There are bipolar transistors instead of MOS or Metal Oxide Semiconductor transistors used in it.
Moreover, the specific characteristics that describe a PROM are its ability to run at a clock speed of about 800 MHz.
These chips usually come with a connector that has 240 pins in it and also supports video graphics if the chip is designed specifically to be used in combination with a dedicated Graphics Processing Unit or GPU.
Purpose of Programmable Read Only Memory
The PROM chips are available in a wide selection of shapes and sizes and based on these parameters the applications for which it is to be used are determined.
Typically, the purpose of the Programmable ROMs is to use them in those specific areas where the volume requirement does not make using a factory-programmed Read Only Memory practical or economical.
Since the data stored in the PROM chips cannot be modified, it is therefore used in those areas as well where permanent data storage is essential such as in security systems and medical devices.
There are different uses of a Programmable ROM. Here are some of the most significant applications of PROM chips:
- Microcontrollers – These are small chips that act as single computers and are found in a wide range of electronic devices. PROMs are used in these microcontrollers to store the firmware or programs for it to operate along with other data for other purposes such as constant values and lookup tables.
- Automotive electronics – The PROM chips are often found in the ECUs or Electronic Control Units of the automobiles that store the specific programs that help in controlling transmission, the engine, and other systems in the vehicle.
- Industrial electronics – You will also see the PROM chips to be used in different types of industrial electronic devices such as Human Machine Interfaces or HMIs and Programmable Logic Controllers or PLCs.
- Consumer electronics – The PROM chips are also used in several different kinds of consumer electronics devices such DVD players, digital cameras, and more.
- Medical devices – A lot of medical devices and appliances use the PROM chips in them such as defibrillators, pacemakers, and other implantable medical equipment to store the instructions necessary for their instructions.
- Security systems – An extensive use of the PROM chips are also seen in the variety of security systems available in the market today to store their programs for operation and other data like access codes and fingerprint templates such as in door locks, in alarm systems and more.
- Computers – Probably, one of the most extensive uses of the PROM chips is in the computers to store the data for BIOS on motherboards that tell the computer how to start up and the specific functions to perform.
In addition to the above, you will also find a programmable ROM in several electronic devices which include and are not limited to:
- Video game consoles
- Cell phones
- High Definition Multimedia Interfaces or HDMI
- RFID or Radio Frequency Identification tags and others.
It is also used while building a system that may be converted eventually into a ROM especially for the purpose of mass production.
Types of Programmable Read Only Memory
Apart from the basic Programmable ROM, there are a few other types of PROM chips available in the market and these are explained as under.
Erasable Programmable Read Only Memory or EPROM:
These chips use UltraViolet or UV light to expunge and rewrite the data on it. It was developed in 1971 by Dov Frohman.
These chips are particularly necessary while developing a prototype of any project based on a microprocessor.
You can program or erase an EPROM a thousand times however, erasing the contents from these chips takes a considerable amount of time, often 20 minutes or more.
Another drawback of it is that the entire content in it is removed while erasing the chip.
Since there is a window to shine UV radiation into it to erase its contents, these chips are often referred to as UV-Erasable EPROM or just as UV-EPROM.
Electrically Erasable Programmable Read Only Memory or EEPROM:
These chips are programmed as well as erased by using electricity. It was developed in 1978 by George Perlegos.
The good thing about the EEPROMs is that data stored in them can be erased instantly and it can select the particular byte to be erased, much unlike an EPROM.
Another good thing about this chip which offers a higher cost per bit than EPROM is that you do not have to remove the chip externally from the socket while programming or erasing.
It can be done while it is still on the system board without the need of any external programming or erasure device.
Flash Memory Erasable Programmable Read Only Memory:
The Flash EPROM has become one of the most popular memory chips to be used since the early 1990s that are user programmable, and there are several good reasons for that.
- First, it takes less than a second to erase the contents from a Flash EPROM and hence it gets its name and
- Second, electricity is used to erase the contents from a Flash EPROM.
However, like EPROM, the Flash memory, which is also referred to as a Flash memory, does not allow selecting the contents to be erased.
In fact, the entire device is wiped clean when you erase it.
Though there are a few specific types of modern Flash memories that come with a feature that allows dividing the memory into blocks and erasing them block by block, it certainly does not offer a selected byte erasure option like the EEPROM.
Flash memory can also be programmed without removing it from the socket in the system board and therefore it is widely used for upgrading the BIOS ROM of a computer.
This is another ROM that is programmable by the IC manufacturer only and hence it is not a user-programmable memory.
Typically, mask is a term that is used extensively in IC fabrication.
This is a costly process and is therefore used typically when the volume requirement is high, in fact hundreds of thousands.
One of the most significant advantages of a Mask ROM is its cost as it is relatively cheaper in comparison to other types of ROMs.
However, programming should be done with extreme caution because if there is a mistake in the data or program code, the whole batch is to be thrown away.
Other than the above major types, there are a few other types of Programmable ROMs available in the market as well.
- Field Programmable Read Only Memory or FPROM – These chips are very much the same as a regular Programmable ROM and also carry out the same functions and
- Addressable Programmable Read Only Memory or APROM – These chips are programmed without any special programming device such as the PROM programmer and the data is stored in a selection of cells that can be addressed individually.
So now it is all up to you as to which particular PROM you would use for your computing needs. However, when you choose make sure that you choose the right one based on the following selection criteria:
- The amount of data required to store
- The type of device being used
- The cost involved
- The level and type of security required and
- The ease in use and programming.
When all these criteria are fulfilled, you will get a high return from your investment.
So, as you can see from this article the Programmable Read Only Memory is one of the best ways to store data permanently on a chip.
These chips are non-volatile and depending on the type of PROM you use, you can even erase the old data and program new data on it to serve your specific purpose.