Read-only memory (ROM) is a class of storage that cannot be erased or modi-fied by the processor. Typical embedded systems may make use of one or more of the following types of ROM: mask ROM PROM, EPROM EEPROM or flash EPROM.
Mask ROM is memory that has been programmed at the time it is manufactured and can never be changed. The data patterns are defined by the photographic masks used to define the circuits on a chip when it is being fabricated. Mask ROMs are used when the programs or data do not need to be changed, when the production quantities are large, and the cost must be as low as possible.
This is the oldest form of ROM and is still used in high volume applications because of its very low manufacturing cost. The program must be permanently defined in advance by including it as part of the master artwork film or “masks”
used to fabricate the chips. It is also the least flexible to change, as a program change necessitates building and packaging new chips, which can take from weeks to months to accomplish.
PROM is user-programmable ROM, which is often used as a generic term for memories that can be programmed one or more times by the user using a special device called a PROM programmer or PROM burner. This was the first
“field programmable” memory, meaning that it can be loaded with data by the
Semiconductor.
Charge on gate leaks off slowly, and must be “refreshed” periodically
Bit Bucket
Figure 4-5: Dynamic RAM bit storage mechanism.
end user using special programming equipment. Bipolar fuse-link PROMs were the first in this category, and were programmed by literally burning out fuses selectively from an array. This is where the term “burning” a PROM came from. (Up to now, you probably thought “burning a PROM” was some reference to the Stephen King novel “Carrie,” didn’t you?) Obviously one time program-mable memory like this was expensive, since it was necessary to discard an obsolete device, and reprogram a new one every time a software revision needs to be tested.
Erasable PROM, or EPROM is used most frequently to store permanent data and programs. It is electrically programmable using an EPROM programmer, and can also be erased by shining a short wavelength ultra violet light through the transparent window in the IC package. The entire memory device is erased since it is not possible to be selective about where the light shines on the chip.
These devices are also referred to as UV EPROMs. A one-time programmable (OTP) EPROM is simply an EPROM enclosed in a low cost package without a transparent lid, meaning it cannot be erased once it is programmed. The storage element in an EPROM is similar to that of a DRAM, as shown in Figure 4-6. However, the EPROM storage transistor gate is a conductor float-ing in an insulatfloat-ing SiO2 (quartz) insulator, which prevents the charge from leaking off. The fact that the
charge is generally guaranteed to remain for at least ten years in the absence of power—as long as the window is covered—
makes this a non-volatile memory.
This would be an ideal storage mechanism except for the way that the charge is stored on the gate. The charge is placed on the floating gate by a method called avalanche induced migration.
This programming method is analogous to routing a river through the room to fill your cup with water. A relatively high voltage, 12 to 25 volts typically, is used to induce avalanche
Semiconductor. lack of charge on gate Metal
SiO2 Metal Gate
Unprogrammed Bit Insulating
Material
Figure 4-6: EPROM storage mechanism.
current flow across the insulating region for up to 50 milliseconds, and some of the charge is stranded on the floating gate. Figure 4-7 illustrates the program and read operations of a typical EPROM.
EPROM erasure is accomplished by shining high-energy photons (UV
light) onto the floating gates for several minutes, as shown in Figure 4-8.
The photons impart enough energy to the trapped electrons to allow them to escape the gate. The EPROM can
be erased and reused many times, which is important when pro-grams are in development, and when a reusable non-volatile memory is required. Some of the larger (less than 1 megabyte) EPROMs are available with a bank switching system to allow access to more locations than can be directly accessed using the address lines. This is accomplished using a write cycle to load the upper address bits into a latch inside the EPROM.
Flash EPROMs are a variation on the standard EPROM, except that
they have been modified so that they do not need to be exposed to UV light to be erased. Like an EPROM, the entire chip is erased at one time, but the erasure is performed electrically using a high reverse polarity voltage to remove the electrons from the gate. They are also easier to program and erase in the application design using relatively simple additional support circuits.
EEPROMs, or E2PROMs, are electrically erasable PROMs. They can be erased and written electrically one byte at a time. The mechanism used is similar to the EPROM except that the insulating region is made very thin, on the order of a few angstroms. The charge is transported using an effect referred to as
Semiconductor.
Figure 4-7: EPROM program and read operation.
Semiconductor.
UV light photons give electrons energy to leave gate UV Transparent Quartz Lid
Ultraviolet Light
Charges are drawn off gate electrically Charge leaks off gate
Metal Gate
Figure 4-8: EPROM/EEPROM erasure.
Fowler-Nordheim tunneling where the insulator is thinned. In an interesting application of quantum physics, the electrons “tunnel” through the insulator.
The operation is similar to an EPROM except that most types can be erased and programmed in circuit, using 5 volt power supplies and a standard micro-processor bus interface. For many of the devices, each byte must be erased by writing ones to a location before it can be programmed. While these devices would seem to be nearly ideal as non-volatile read/write memories, they do have a couple of drawbacks. EEPROM bits have a limited number of write cycles before they get “stuck” in the programmed state. They are typically guaranteed for 10,000 to 100,000 write cycles, which would take only a few seconds if a program gets stuck in a tight loop writing to the EEPROM. This problem is due to the fact that charge can be trapped in defects in the insulator in the gate region resulting in some bits getting “stuck” in the programmed state. The other problem is that they are slow to write, typically taking many microseconds or even milliseconds to erase and write, compared to 100 nano-seconds typical of SRAM.
Small EEPROMs are available with a serial interface so that they will fit into small (8-pin), low cost packages. They are particularly useful in embedded systems for storing configuration data to replace switches and jumpers. They are significantly slower than standard memories due to the serial interface, and are usually accessed using software to manipulate the serial lines directly.