By Bert Huneault, CET
Reading and writing with PROMs
This is the second part of a .4-part series on non-volatile memories. Part one covered the basic read-only memory (ROM), which is mask-programmed at the factory and cannot be erased.
Although the basic read-only memory is useful for purposes where you do not want any chance of the contents being erased, there are uses for which a programmable ROM is more convenient. For one thing, the ordinary mask-programmed ROM is very expensive, and is therefore not cost-effective in small production batches. In low-volume applications, it is more economical to use fusible link ROMs that are user-programmable. These programmable read-only memories (PROMs) are not programmed during the IC manufacturing process, but are custom programmed by the user. Once programmed, however, the PROM is like an ordinary ROM-it can't be erased or reprogrammed. Thus, if the program in a PROM is faulty or the user decides to alter instructions or data, the PROM must be thrown away.
PROM ICs generally feature blown-junction transistor (BJT) technology, with transistor leads comprising fusible links. These links come intact from the manufacturer. By applying a suitably high voltage to specific addresses, the user can selectively blow any of these fuse links to produce the desired data storage. Connections left intact can represent a binary 1, while those burned open store a binary 0. PROMs are field-programmable devices, and the equipment used to program them is called a PROM programmer (or PROM burner), a machine that permits keyboard entry of the desired program or data. During the programming process, the chip's address pins and data pins determine which memory cells will get their fuse links burned out.
As already mentioned, ROMs and PROMs are neither erasable nor reprogrammable. A more versatile category of read-only memories are both erasable and reprogrammable.
Erasable PROMs The erasable programmable read-only memory, or EPROM, is a device that can not only be programmed by the user, but can also be erased and reprogrammed as often as desired. Once programmed, the EPROM is a nonvolatile memory that can hold its stored data practically indefinitely.
The memory cells in an EPROM are field effect transistors (FETs) that feature a floating gate, an insulated gate with no electrical connections. By applying a relatively high-voltage programming pulse to the device (25V to 50V between source and drain for about 50ms per address location), electrons are injected into the insulated floating gate and remain trapped there once the pulse is removed because there is no discharge path. Thus, programming places a charge on the MOS transistor, and the MOSFET is such a good "capacitor" that is retains the charge for years.
Of course, there can be thousands of MOS transistors on the chip, each one representing a binary digit--bit 1 or bit 0, depending on whether it stores a charge or not. Naturally, during programming operations the IC's address and data pins are used to determine which memory cells get a charge and which don't.
The charge is erasable by means of ultraviolet (UV) light. A transparent quartz window is built into the top of the EPROM case (DIP package) for this purpose. High-intensity UV light is concentrated through this window onto the memory chip inside, causing a photocurrent to flow from the floating gate back to the silicon substrate; this neutralizes the trapped electrical charge. All the stored bit patterns are thus erased simultaneously.
Figure 1. The EPROM can not only be custom programmed by the user, but can
be erased and reprogrammed as often as desired. Like the ROM, the EPROM has
eight data lines, but its eleven address lines (compared to 10 in the ROM)
gives it a storage capacity of 2,048 eight-bit words, or 2Kbytes.
Although both have a CE (chip enable) pin, the EPROM also has an OE (output enable) pin that allows it to write as well as read.
A main disadvantage of the EPROM is the long time (15 to 30 minutes) it takes to erase. Once the erasing operation is completed, an opaque label is placed over the clear window to prevent stray UV (from fluorescent lights or sunlight) from gradually erasing the new contents of the EPROM.
The EPROM can then be reprogrammed by means of a PROM programmer. Note that for both erasing and programming operations, the EPROM must be removed from the printed circuit board in which it is used.
EPROMs are very useful in design and development of computer systems. They allow the designer to try out a new program and debug it by erasing and reprogramming as often as necessary until it works perfectly. When the design is finalized, the data can be burned into PROMs for small production runs, or it can be sent to an IC manufacturer who produces a ROM mask for mass production.
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