Notes on erasing EPROM

what is and what is not allowed

In order for programmable and UV-light erasable circuits to bring maximum benefit, it is necessary to ensure the correct algorithm during programming on a correctly set and correct type of programmer. That is self-evident and clear.

But what do you need to know about UV erasers? A mistake or the wrong use of an eraser can cause the same amount of problems as the wrong use of a programmer. To clarify how such errors can occur, briefly from the theory.

An erased EPROM has all cells in the HIGH state. A change occurs during programming. The EPROM cells, which are to be programmed to the LOW state, are charged with an electric charge of defined parameters. The erasing process involves using UV radiation to increase the conductivity of the otherwise non-conductive gate insulation enough that charge from the cell can migrate to the surrounding silicon. This puts the cell in the HIGH state.

Now let me describe how EPROM erasing errors can occur and dispel some of the myths that circulate around this process.

Fiction:

Erasing the circuit for 5 minutes and then checking the erasure is sufficient and finding out if the circuit is erased correctly.

FACT:

EPROMs need a precisely specified dose of UV radiation to be properly erased. This dose is specified by the EPROM manufacturer in the form of a total dose (integrated dose) of radiation per unit area. This dose is typically 15Ws/cm2. The given value is the minimum for correct erasure and may vary depending on the manufacturer and type of circuit (for example, heroic Soviet EPROMs). In order to calculate the erasion time, it is therefore necessary to know this total dose (for now, let's consider 15Ws/cm2) and the UV power of the used EPROM eraser (EREPROM-01, approx. 19mW/cm2). Of this, the erasion time is 15/0.019 = 790 seconds, i.e. with a slight reserve of 15 minutes.
For further use, let's introduce the term erased EPROM (or cell in EPROM). It is an EPROM (or a cell in the EPROM) that did not receive the full prescribed dose of UV radiation.

Fiction:

A greased EPROM will show up as unerased during the BLANK CHECK or pre-program erasure test.

FACT:

About 90% of erased EPROM will pass erasure tests. For most programmers, this test is performed under standard conditions, i.e. VCC=5V, or VCC=5V+10%. Thus, even with the cells still partially charged, a greased EPROM is read as erased. However, during programming, the circuit is exposed to VCC from 6V and higher and VPP from 12V and higher (according to the type of algorithm) in addition with internal heating. Erased cells that we do not intend to program can be read as programmed under these conditions of increased power and rising temperature. This will at best cause an error during programming, but if such an EPROM goes through the programming process and its unprogrammed cells show up as faulty only later, then you are wasting valuable time identifying the fault in the device.

You have to remember:

• A circuit that is read as cleared may not actually be empty.
• Always ensure that the circuits are wiped for a long enough time and therefore receive the minimum prescribed dose of UV radiation.
• Makes sure the circuit window is clear when erasing. Glue residues after removing the protective sticker can reduce UV radiation by up to 90%.
• If you are not sure about the quality of the UV tube in the eraser, replace it sooner, e.g. every year or after 2000 hours of use.
• If you have a production programmer (LabProg-48LV or PREPROM-02aLV) with the possibility of a marginal test (verification at the minimum and maximum permissible supply voltage) after programming, it will set this option.
• Remove the protective sticker from the perimeter window only during the erasing stage.

Finally, a word of advice: It's a good idea to stop looking at the eraser lamp before you stop seeing it.

Yours sincerely

Ing. Ján Puobiš, Elnec s.r.o. PREŠOV