Read-only memory: (commonly known by its acronym, Read Only Memory. See Abbreviationfinder). In its strictest sense, it refers only to mask ROM in English MROM (the oldest type of solid state ROM), which is manufactured with permanently stored data, and therefore its content cannot be modified. However, more modern ROMs such as EPROM and Flash EEPROM can be erased and reprogrammed multiple times, still being described as “read-only memory (ROM), because the reprogramming process in general is infrequent, relatively slow, and writing in random places is often not allowed. Despite the simplicity of ROM, reprogrammable devices are more flexible and inexpensive, for this reason ROM masks are rarely found on hardware produced after 2007.
History
The simplest type of solid-state ROM is the same age as the semiconductor technology itself. Combinational logic gates can be used together to index an n-bit memory address into values of m bits in size (a look-up table). With the invention of integrated circuits, the ROM mask was developed. The ROM mask consisted of a grid of lines made up of a word and lines made up of a bit respectively selected from changes in the transistor. In this way they could represent an arbitrary query table and a deductible propagation time.
In ROM masks the data is encoded on the same circuit, so it can only be programmed during manufacture. This has serious disadvantages:
- It is only economical to buy them in large quantities, since the user contracts foundries to produce them according to their needs.
2. The product between completing the mask design and receiving the final result is very long.
3. They are useless for R&D due to the fact that more than one has to be produced during development.
4. If a product has a skin bug, the only way to fix it is by physically changing the ROM.
Later developments took these shortcomings into account, so programmable read-only memory (PROM) was created. Invented in 1956 itallowed users to modify it only once with the application of high voltage pulses. It eliminated the aforementioned problems 1 and 2, since the user could order a large number of empty PROMs and program them with the necessary content chosen by the designers. In 1971 the memory of erasable programmable read only memory (EPROM) allowing restart its contents exposing the device developed strong ultraviolet rays. In this way, point 3 of the previous list was eradicated. Later in 1983 EEPROM was invented, solving conflict number 4 on the list since content could be reprogrammed as long as it provided a mechanism to receive external content (for example, through a serial cable). In the middle of the 1980s Toshiba invented flash memory, a form of EEPROM that allowed content to be removed and reprogrammed in a single operation using electrical pulses thousands of times without suffering any damage.
All these technologies improved the versatility and flexibility of the ROM although the cost per chip increased. This is why ROM skins were the inexpensive solution for quite a few years. Still, keep in mind that newer technologies with more modifiability were designed to remove ROM from the market and replace it.
The latest product is NAND memory, again developed by Toshiba. The designers explicitly broke with the past by saying that it focused “on being a replacement for hard drives and not the old ROM. In 2007, NAND has come a long way toward its goal, offering performance comparable to hard drives, better tolerance to hard drives. physical shocks and extreme miniaturization (such as USB sticks and MicroSD memory cards).
Rom speed
Reading speed
Although the relative speed of RAM vs ROM has varied over time, since 2007 RAM is faster to read than most ROMs, therefore ROM content is normally transferred to RAM when used.
Write speed
For those types of ROMs that can be electrically modified, the speed is much slower than the read speed, and may require exceptionally high voltage,
ROM memory classification
- ROM (programmable by mask)
- PROM (user programmable)
- Fuse PROM
- OTPROM (PROM programmable once, do not support erasure)
- EPROM (Erasable PROMs)
- UVPROM (Ultraviolet Erasable PROM)
- EEPROM (Electrically Erasable PROM)
- EPROM flash (electrically erasable).
Mask ROM
This memory is known simply as ROM and is characterized in that the information
contained within is stored during construction and cannot be altered. They are ideal memories for storing firmware, operating systems, conversion tables, and characters. Generally these memories use MOS transistors to represent the two logic states (1 or 0). The programming is developed through the design of a photographic negative
called mask where internal memory connections are specified. Memory cells are arranged in groups to form records of the same size, and these are physically arranged in an array.
PROM memory
This memory is known as Programmable ROM from the English acronym Programmable Read
Only Memory. This type of memory, unlike ROM, is not programmed during the manufacturing process, instead the programming is done by the user and can be done only once, after which no other information can be erased or stored again..
The programming process is destructive, that is, once recorded, it is as if it were a normal ROM. Two techniques are used to store the information: by destroying the fuse or by destroying the junction. Commonly the information is programmed or burned in the different memory cells applying the address in the address bus, the data in the data input buffers and a pulse of 10 to 30V, in a dedicated terminal to blow the corresponding fuses. When this pulse is applied to a fuse in the cell, a logical 0 is stored, otherwise a logical 1 is stored (default state), thus leaving the information permanently stored.
The process of programming a PROM is usually done with special equipment called a burner. This equipment uses a mechanism of electronic switches controlled by software that allows to load the addresses, the data and generates the pulses to blow the fuses of the internal memory array. In figure 2.2. the function of the programmer is schematically indicated.
