Mask ROM
Encyclopedia
Mask ROM is a type of read-only memory
(ROM) whose contents are programmed by the integrated circuit
manufacturer (rather than by the user). The terminology "mask" comes from integrated circuit fabrication, where regions of the chip are masked off
during the process of photolithography
.
It is common practice to use rewritable non-volatile memory
– such as UV-EPROM
or EEPROM
– for the development phase of a project, and to switch to mask ROM when the code has been finalized. For example, Atmel
microcontrollers come in both EEPROM and mask ROM formats.
The main advantage of mask ROM is its cost.
Per bit, mask ROM is more compact than any other kind of semiconductor memory
. Since the cost of an integrated circuit
strongly depends on its size, mask ROM is significantly cheaper than any other kind of semiconductor memory.
However, the one-time masking cost is high and there is a long turn-around time from design to product phase. Design errors are costly: if an error in the data or code is found, the mask ROM is useless and must be replaced in order to change the code or data.
Some integrated circuits contain only mask ROM.
As of 2003, four companies produce most such mask ROM chips: Samsung Electronics
, NEC Corporation, Oki Electric Industry
, and Macronix.
Other integrated circuits contain mask ROM as well as a variety of other devices. In particular, many microprocessor
s have mask ROM to store their microcode
. Some microcontroller
s have mask ROM to store the bootloader or all of their firmware
.
Read-only memory
Read-only memory is a class of storage medium used in computers and other electronic devices. Data stored in ROM cannot be modified, or can be modified only slowly or with difficulty, so it is mainly used to distribute firmware .In its strictest sense, ROM refers only...
(ROM) whose contents are programmed by the integrated circuit
Integrated circuit
An integrated circuit or monolithic integrated circuit is an electronic circuit manufactured by the patterned diffusion of trace elements into the surface of a thin substrate of semiconductor material...
manufacturer (rather than by the user). The terminology "mask" comes from integrated circuit fabrication, where regions of the chip are masked off
Photomask
A photomask is an opaque plate with holes or transparencies that allow light to shine through in a defined pattern. They are commonly used in photolithography.-Overview:...
during the process of photolithography
Photolithography
Photolithography is a process used in microfabrication to selectively remove parts of a thin film or the bulk of a substrate. It uses light to transfer a geometric pattern from a photomask to a light-sensitive chemical "photoresist", or simply "resist," on the substrate...
.
It is common practice to use rewritable non-volatile memory
Non-volatile memory
Non-volatile memory, nonvolatile memory, NVM or non-volatile storage, in the most basic sense, is computer memory that can retain the stored information even when not powered. Examples of non-volatile memory include read-only memory, flash memory, ferroelectric RAM, most types of magnetic computer...
– such as UV-EPROM
EPROM
An EPROM , or erasable programmable read only memory, is a type of memory chip that retains its data when its power supply is switched off. In other words, it is non-volatile. It is an array of floating-gate transistors individually programmed by an electronic device that supplies higher voltages...
or EEPROM
EEPROM
EEPROM stands for Electrically Erasable Programmable Read-Only Memory and is a type of non-volatile memory used in computers and other electronic devices to store small amounts of data that must be saved when power is removed, e.g., calibration...
– for the development phase of a project, and to switch to mask ROM when the code has been finalized. For example, Atmel
Atmel
Atmel Corporation is a manufacturer of semiconductors, founded in 1984. Its focus is on system-level solutions built around flash microcontrollers...
microcontrollers come in both EEPROM and mask ROM formats.
The main advantage of mask ROM is its cost.
Per bit, mask ROM is more compact than any other kind of semiconductor memory
Semiconductor memory
Semiconductor memory is an electronic data storage device, often used as computer memory, implemented on a semiconductor-based integrated circuit. Examples of semiconductor memory include non-volatile memory such as Read-only memory , magnetoresistive random access memory , and flash memory...
. Since the cost of an integrated circuit
Integrated circuit
An integrated circuit or monolithic integrated circuit is an electronic circuit manufactured by the patterned diffusion of trace elements into the surface of a thin substrate of semiconductor material...
strongly depends on its size, mask ROM is significantly cheaper than any other kind of semiconductor memory.
However, the one-time masking cost is high and there is a long turn-around time from design to product phase. Design errors are costly: if an error in the data or code is found, the mask ROM is useless and must be replaced in order to change the code or data.
Some integrated circuits contain only mask ROM.
As of 2003, four companies produce most such mask ROM chips: Samsung Electronics
Samsung Electronics
Samsung Electronics is a South Korean multinational electronics and information technology company headquartered in Samsung Town, Seoul...
, NEC Corporation, Oki Electric Industry
Oki Electric Industry
, commonly referred to as OKI, OKI Electric or the OKI Group, is a Japanese company manufacturing and selling info-telecom and printer products. Headquartered in Tokyo, Japan, OKI operates in over 120 countries around the world....
, and Macronix.
Other integrated circuits contain mask ROM as well as a variety of other devices. In particular, many microprocessor
Microprocessor
A microprocessor incorporates the functions of a computer's central processing unit on a single integrated circuit, or at most a few integrated circuits. It is a multipurpose, programmable device that accepts digital data as input, processes it according to instructions stored in its memory, and...
s have mask ROM to store their microcode
Microcode
Microcode is a layer of hardware-level instructions and/or data structures involved in the implementation of higher level machine code instructions in many computers and other processors; it resides in special high-speed memory and translates machine instructions into sequences of detailed...
. Some microcontroller
Microcontroller
A microcontroller is a small computer on a single integrated circuit containing a processor core, memory, and programmable input/output peripherals. Program memory in the form of NOR flash or OTP ROM is also often included on chip, as well as a typically small amount of RAM...
s have mask ROM to store the bootloader or all of their firmware
Firmware
In electronic systems and computing, firmware is a term often used to denote the fixed, usually rather small, programs and/or data structures that internally control various electronic devices...
.