120 lines
5.4 KiB
Plaintext
120 lines
5.4 KiB
Plaintext
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Kingpin [RDT Syn.] presents:
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---==== A CLOSER LOOK AT EPROMS ====---
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A brief introduction to EPROMS, what they are used for, and how they work..
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(Stolen from an undisclosed source..)
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Whassup to Brian Oblivion and the rest of the RDT posse..
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---=======================---
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How an EPROM works:
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With the advent of Large Scale Integration (LSI) and the decreasing
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cost of semiconductors, the use of semiconductors as memory elements became
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possible. Over the last few years, non-volatile semiconductor memory such as
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the EPROM (Erasable Programmable Read-Only Memory) has become an increasingly
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important part of microcomputer systems. Memory chips in general are carrying
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enough bits to even challenge magnetic storage in many applications.
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The EPROM is an important part of microcomputer memory and offers
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many advantages over available memory devices. The EPROM is not a permanently
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programmed device; rather it can be erased and then re-programmed when
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changes are required. Also, the device configuration and fabrication
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technique has allowed EPROMS to be cost effective for a variety of
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applications. The heart of the EPROM'S flexibility is in the FAMOS
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(floating-gate avalanche-injection metal oxide semiconductor) type transistor.
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__________________________
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/ Polysilicon Select Gate |\
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__________/ ______________________ V \__________
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| / ____________________ \ |
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| SiO2 ___/ / __________________ \ \___ |
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| |____/ |__________________| \____| |
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| Polysilicon Floating Gate ^ |
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|__________________________________________________|
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Figure 1. FAMOS-Type EPROM
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Programming the EPROM consist of storing instructions in the form of
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electrical charges. By application of a sufficiently latge voltage to the
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cells, electrons are injected into the floating silicon gate. Storage of the
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charge is possible due to the high impedance of the floating gate structure.
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The stored charge causes a shift in the cell threshold and switches the
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transistor to a non-active state.
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------ +25 Volts
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____________|_____________
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/ | \
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__________/ __________|___________ \__________
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| / ____________________ \ |
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| SiO2 ___/ / __________________ \ \___ |
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| |____/ |______e-__e-______| \____| |
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| e- e- |
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|__________________________________________________|
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^^^^^^^^ Electrons stored
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Figure 2. Programming an EPROM Cell
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Because there are no electrical connections to the floating gate,
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erasure must be achieved by a non-electrical process. The EPROM has a fused
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quartz lid which permits the ultraviolet radiation to penetrate the cell. By
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exposure of the polysilicon gate to UV radiation, the stored charge can be
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drained as a result of photo excitation of the stored electrons. The excited
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electrons then dissipate into the oxide layer reducing the floating gate
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capacitance to its original, un-programmed state.
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The EPROM is then ready to be reprogrammed.
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UV Light __________________________ UV Light
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\ / \ /
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___\_______/ ______________________ \____/_____
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| \ / ____________________ \ / |
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| SiO2 ___/ / __________________ \ \___ / |
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| \ |____/ |______e-__e-______| \____|/ |
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| \____________e-_______e-__________/ |
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|__________________________________________________|
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| | | | Electrons exit
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V V V V
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Figure 3. Erasing an EPROM Cell
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Calculating EPROM Erasure Times:
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EPROM erasure is accomplished by a minimum dosage which is the total
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amount of UV energy incident on the chip surface during a period of time.
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EPROM manufacturers normally assign a nominal dose specification to their
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devices to assist the user in determining proper erase times.
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The minimum time required to erase an EPROM with a given dosage
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specification can be calculated from the following equation:
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Erase Time =
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EPROM Dosage (W-sec/cm2) x 1000
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-------------------------------
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Intensity (uW/cm2)
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Note that this equation is only valid for an EPROM that has been
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calibrated with a source which has a spectrum equivalent to that of the light
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source being used to erase the EPROM. Virtually all EPROM manufacturers use a
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254nm source to calculate the dosage requirements for their EPROMS.
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While most EPROM manufacturers' performance specifications ensure a
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high quality, non-volatile memory device, the UV dose specified is typically
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a dose sufficient to erase approximately 95% of all such devices. Due to
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process variations in manufacuring, some 15 W-sec/cm2 EPROMS may require as
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much as 40 W-sec/cm2. These variations do not normally reflect other aspects
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of EPROM performance. Most EPROMS will erase well below the given dose
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requirements.
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---=======================---
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