Nonvolatile ROM

ROM chips come in a few different types. Like all memory chips, ROM prices are based mostly on capacity, with a small price premium for speed. Some ROMs can retrieve data faster than others, and this might be worth a little extra money to someone building a fast system.

Masked ROM

The simplest and cheapest ROMs are called masked ROMs (MROMs). These chips are mass-produced with particular data bits already stored in them. Once the MROMs are made, the data can never be changed. This is usually the point: MROMs are good for mass-produced products like cars, in which the software for the antilock brake system should never be altered.

Because their data patterns are permanently set when the chips are built, MROMs are only made in large volumes for big customers. It isn't cost-effective for a semiconductor maker to produce just a few thousand ROMs once in awhile. The cost of stopping the production line and setting up the tooling is prohibitive. So although MROMs are the cheapest form of ROMs, they're reserved for only the largest customers or the most popular products.

Programmable ROM

A more cost-effective alternative for the average consumer is the programmable ROM (PROM). These are like ROM chips but they are manufactured blank, with no data bits stored in them. (Viewed another way, all the bits are zeros.) The customer then loads the contents of the PROM using a machine called a PROM burner. There's only one chance to program a PROM; after the data has been loaded, the PROM's contents are permanent.

Because PROMs can only be “burned” once, they're called one-time programmable ROMs (OTP ROMs). These chips are a good way for medium-sized companies to enjoy the low cost of mass-produced ROMs without the risk of large volume commitments. Companies can keep a supply of blank PROMs in stock for as long as necessary, burning their contents at the last minute. Sometimes this is literally true: Fast-selling electronics goods are sometimes stored in the warehouse with a wire hanging from the shipping carton. The product receives its final programming just before it goes on the truck.

Erasable Programmable ROM

The next step in the cost-versus-flexibility scale is the erasable programmable ROM (EPROM). Unlike ROMs and PROMs, EPROMs can be erased if you have the right equipment. EPROMs are more expensive than either ROMs or PROMs, and they can't be used in some safety-critical equipment because of the possibility of erasure. Still, EPROMs are extremely popular in computers and other high-volume products where standards and features need to change from time to time.

EPROMs are manufactured and shipped blank, just like PROMs. Customers load data bits into them just the same way, too, using a PROM burner. The only difference between a PROM and an EPROM is that you can erase an EPROM if you expose it to ultraviolet light for long enough. Usually 30 minutes under a special lamp is enough to erase an EPROM and return it back to its original blank condition.

This violates our earlier rule about ROMs being permanent and not erasable, but EPROMs still qualify as ROMs. Because it takes half an hour or more to erase and reprogram an EPROM, these chips do not compete with RAMs, which can be erased and reprogrammed in a fraction of a second. Even though EPROMs can be erased, they're still nonvolatile; any data stored in an EPROM will last for years unless it's deliberately erased.

EPROMs have a small round window on the top of their package, shown in Figure 7.2, to let ultraviolet light through. The window allows light to shine directly on the silicon chip inside. The EPROM chip, unlike most chips, is designed to be sensitive to ultraviolet light, gradually losing its memory.

Electrically Erasable Programmable ROM

Electrically erasable programmable ROMs (EEPROMs) are an advancement over EPROMs because they can be erased electronically, instead of through exposure to ultraviolet light. This makes EEPROM chips easier to use in many systems. For example, an EEPROM can be permanently soldered onto a printed circuit board and still be erased without removing it, whereas a normal EPROM would have to be removed from the system and placed under an ultraviolet lamp. This makes EEPROMs more user friendly in PCs and other consumer items where the tools and the skills to remove and erase an EPROM just aren't there.

Typically for engineers, EEPROMs are often called E²PROMs, or “e-squareds.” EEPROMs have many useful advantages over traditional EPROMs, but they never captured very much market share because they were quickly overtaken by flash ROMs, which are covered next.

Flash ROM

Flash ROMs are now the most popular type of erasable ROM (and no, erasable ROM is not really an oxymoron). Like EEPROMs, flash ROMs can be erased and reprogrammed completely electronically, without removing the chip from the system. This makes flash ROMs very useful in products for which a relatively naïve customer might be asked to erase and reprogram the chips without any specialized knowledge. MP3 music players and occasionally PCs fall into this category.

Where flash ROMs differ from EEPROMs is speed. EEPROMs are slow to erase but flash ROMs are pretty quick, as their name suggests. You can erase and reprogram an entire flash ROM in a few seconds, versus several minutes for a conventional EEPROM. For most consumer products, that difference has enormous psychological value. Customers are often nervous enough about reprogramming or updating the software on their products; getting it over with quickly alleviates a lot of tension.

Flash ROMs were originally much more expensive than EEPROMs, which were, in turn, more expensive than EPROMs, and so on. The price of flash chips has come down considerably in the 10 years or so since they were first developed because their volume has increased. Flash ROMs have now almost entirely replaced EEPROMs and are even encroaching on EPROM and PROM territory. The advantages of flash ROM's erasability, relative speed, and high volume make it nearly ideal as an all-purpose permanent memory chip.