The small quartz window admits UV light for erasure. Development of the EPROM memory cell started with investigation of faulty integrated circuits where the gate connections of transistors had broken. Stored charge on these isolated gates changes their threshold voltage. In , he noted the movement of charge through oxide onto a gate.
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So, here is my humble attempt to provide a dose of information about them. If you think this information contains any errors, I would be glad to hear about it; email inform xtronics.
This article is written with the philosophy that there is no such thing as a stupid question; only stupid mistakes from not asking stupid questions. Things only seem complicated until you figure out how they work, so lets dig in. If you are familiar with binary, bits, bytes, hexadecimal, and ASCII you might want to skip the first section.
Binary refers to base two or a two-state digit called a bit. A byte or multiples of bytes set the register size for microcomputers. Hexadecimal hex is a base 16 way of representing one byte. A byte requires just 2 hex digits. Thus, FFh is a two hex digit representation of a byte. We are fast approaching the time when 64 bit words or doublewide words will be common.
ASCII, is a code that represents the letters and numbers you can type on a computer keyboard. Each letter you press is represented by a unique set of 8 bits or one byte. But when we talk about memory size, we speak in terms of k-bytes. To change bits to bytes, simply divide the number of bits by 8 to get bytes. Now, 16, bits divided by 8 gives us 2, bytes or 2 k-bytes.
What is a Rom? They are programmed at the factory at the time of manufacture with a special mask, thus called a masked ROM. This is the cheapest way to manufacture ROMs once you need more than 10, at a time. The drawback is, if there is even one little bug in the software, that pile of 10, ROMs becomes worthless.
Programming is accomplished with a current instead of a voltage as are EPROMs and requires a different type of programmer. To reduce the cost, these EPROMs come in a windowless plastic carrier, which is cheaper than the costly ceramic package required for the erasing window.
They can be programmed one time only, so these are used after the code is bug free. This eliminates the need of a window. In- circuit writing and erasing is possible because no special voltages are required. To accomplish in-circuit operation, you have to write special application software routines. EPROM memory cells use floating gate technology. A floating gate is a gate with a special capacitor for its only electrical connection.
This special capacitor takes on an electrical charge in a quantum physics effect called tunneling. The presence of a charge determines the value 1 or 0 of the memory cell.
In our example below, a room with a very narrow door represents the memory cell. People in the room represent electrons with their associated charge. These people can only enter or exit through a much too narrow door with much pushing or shoving to represent the tunneling effect. Think of a room with about 30 people acting as the electron charge.
When we program an EPROM bit, we shove these people until they pop back out by applying a pulse of high voltage to the memory cell. When programming a bit we can only change a 1 to a 0 because changing a 0 to a 1 requires erasing.
If it were digital, we would only have an empty room or a room with one person in it. Instead, a cell sensor circuitry compares how many people are in the room to a reference to determine if the cell represents a 1 or 0. With more than 15 people in the room, the cell is considered a 1. When programming an EPROM you have to apply the specified programming voltage for the specified time.
Too short a time or too low a voltage and not all the people get shoved out of the room. On the other hand, if you apply too high of a program voltage or over erase the EPROM, it is equivalent to blowing the doorframe right out of the wall! Our imaginary people now enter and exit the room, milling around on their own whims and we no longer have information storage!
A little off the topic here, you may have seen little solid state recording devices, sometimes used in answering machines, that record voice grade audio. The recorded cells have intermediate values that are not compared when read but out put as a voltage. When the memory cells are played back in consecutive order, the continuously changing values form an audio signal!
Yet a fourth voltage is required to program them! You no longer see three supply EPROMs in new products, but they are still out there in older equipment and are somewhat available for replacements.
A good example is the three supply made by Texas Instruments, TI never made a single supply. The CE pin enables and disables the data output. When disabled most of the chip is in a low power sleep mode. The access time of a chip is given from the time CE becomes active until data appears. The access time using the OE pin is a lot shorter than CE because the OE pin disables only the data output pins, but not the rest of the chip. This allows data to be accessed at a much higher speed at the cost of increased power consumption.
Enabling both CE and OE causes all the bits to appear on the data lines; you can not enable individual bits. The address lines form an address bus made of inputs that together select which location you want to read or program.
These have two pins, OE and A16, that are switched around. Once the code is completely debugged and you are ready to switch to masked ROMs, there is no reason to change the board layout. Remember, they are not pin compatible with or If you program a byte to A5h , only the 0s locations change state. We can go back and change any of the 1s to 0s in a second programming session called patching.
For example, the A5h byte can be changed to 21h but it can not be changed to F5h Parts that use a Vpp of Then using one of the programming algorithms, we apply a programming pulse.
The best deal in programmers is the pocket programmer. Programming algorithms Standard This simplest of the algorithms, uses a set time of a 50mS programming pulse on each byte. After all bytes are programmed each byte is then verified or compared to the EPROM image in the buffer. Fast or intelligent This starts with a 1mS programming pulse to program a byte.
Then the byte is checked against the programming data for verification. If it does not verify, the program pulse is doubled keeping track of the accumulated pulse time and the byte is programmed again. This process is kept up until the byte is programmed.
The byte is then programmed again with a pulse time equal to the accumulated pulse multiplied by 3 to be sure it sticks. Extreme caution must be used with this protocol. Next a verification of the whole EPROM is done, if a byte does not verify it is programmed again using a Us pulse until the byte is successfully programmed.
On the down side it also requires a very high accuracy Vpp supply and is not found on many programmers. Then a programming pulse is applied which programs all 4 bytes at once. After all location are programmed a verification pass is done and any bytes that do not verify are programmed again using the 8 bit Fast algorithm. The 32 bit algorithm programs large EPROMS 4 times faster, but the programming current is also 4 times as much and thus requires a special Vpp supply.
Ah, yes you can, but it can take about 3 weeks of sunny weather!. Well then, how about a fluorescent light? Well I have a black light? The answer is still no. In fact the high frequency UV light used will not pass through plastic or most glass. The intensity of the light source combined with the distance from the light source determines the intensity of the exposure.
Light intensity varies inversely with the distances from the source. The longer the distance the lower the intensity of the light. Besides bulb to chip distance, bulb age also effects exposure time. Older 1. With all these variables, the best way to determine exposure time is to run an empirical exposure test.
If they are not erased, expose them again for another minute. Keeping track of the total exposure time, repeat this process until they are erased. Now take the total time and multiply by 1. For example, if it takes 3 minutes we should use a 4.
Remember if you leave an EPROM in the eraser too long, it can remain erased forever and will no longer program. Erasing cycles will slow down access times, but this is usually of no consequence unless you erase them an extreme number of cycles or for an excessive exposure time. A worn out EPROM may program correctly yet fail to work in the equipment that requires a fast access time. Making Your Own Eraser? Or why not to Germicidal light bulbs are used in hospitals for sterilizing equipment by killing germs.
Germs are living cells.
How EPROMs Work or:
27C010 - 27C010 1024K EPROM Datasheet