EEPROMs are changed 1 byte at a time, which makes them versatile but slow. This erases the targeted cells of the EEPROM, which can then be rewritten. Instead of using UV light, you can return the electrons in the cells of an EEPROM to normal with the localized application of an electric field to each cell. Changing the contents does not require additional dedicated equipment.The entire chip does not have to be completely erased to change a specific portion of it.The chip does not have to removed to be rewritten.In such a case, the EPROM's floating gates are charged to the point that they are unable to hold the electrons at all. An EPROM that is left under too long can become over-erased. The EPROM must be removed from the device it is in and placed under the UV light of the EPROM eraser for several minutes. The EPROM must be very close to the eraser's light source, within an inch or two, to work properly.Īn EPROM eraser is not selective, it will erase the entire EPROM. Because this particular frequency will not penetrate most plastics or glasses, each EPROM chip has a quartz window on top of it. In a standard EPROM, this is best accomplished with UV light at a frequency of 253.7. To erase it, you must supply a level of energy strong enough to break through the negative electrons blocking the floating gate. To rewrite an EPROM, you must erase it first. A blank EPROM has all of the gates fully open, giving each cell a value of 1. When the charge passing through drops below the 50-percent threshold, the value changes to 0. If the flow through the gate is greater than 50 percent of the charge, it has a value of 1. A device called a cell sensor monitors the level of the charge passing through the floating gate. These negatively charged electrons act as a barrier between the control gate and the floating gate. The excited electrons are pushed through and trapped on the other side of the thin oxide layer, giving it a negative charge. This charge causes the floating-gate transistor to act like an electron gun. The charge comes from the column ( bitline), enters the floating gate and drains to a ground. An electrical charge, usually 10 to 13 volts, is applied to the floating gate.
Tunneling is used to alter the placement of electrons in the floating gate. To change the value to 0 requires a curious process called Fowler-Nordheim tunneling. As long as this link is in place, the cell has a value of 1. The floating gate's only link to the row ( wordline) is through the control gate. One of the transistors is known as the floating gate and the other as the control gate. The two transistors are separated from each other by a thin oxide layer. In an EPROM, the cell at each intersection has two transistors. Once again we have a grid of columns and rows. EPROMs are configured using an EPROM programmer that provides voltage at specified levels depending on the type of EPROM used. Erasing an EPROM requires a special tool that emits a certain frequency of ultraviolet (UV) light.
Erasable programmable read-only memory (EPROM) addresses this issue. Even though they are inexpensive per chip, the cost can add up over time. Working with ROMs and PROMs can be a wasteful business. So the charge on the column does not get transferred to the row. The neat part of ROM is that if the cell's value is 0, there is no diode at that intersection to connect the column and row. If a diode is present at that cell, the charge will be conducted through to the ground, and, under the binary system, the cell will be read as being "on" (a value of 1). By taking advantage of the unique properties of a diode, a ROM chip can send a charge that is above the forward breakover down the appropriate column with the selected row grounded to connect at a specific cell. In silicon-based items such as processors and memory chips, the forward breakover voltage is approximately 0.6 volts. If the value is 0, then the lines are not connected at all.Ī diode normally allows current to flow in only one direction and has a certain threshold, known as the forward breakover, that determines how much current is required before the diode will pass it on. While RAM uses transistors to turn on or off access to a capacitor at each intersection, ROM uses a diode to connect the lines if the value is 1. But where the columns and rows intersect, ROM chips are fundamentally different from RAM chips. Similar to RAM, ROM chips (Figure 1) contain a grid of columns and rows.
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