A CHEAP AND CHEERFUL EPROM ERASER
P. D. COKER
There are a number of ways in which redundant EPROMS can be erased before
reprogramming – such as exposing the uncovered window to bright sunlight for a
few days or for a rather shorter time to short-wave ultra-violet radiation.
It should even be possible to use a series of flashes from a xenon
photographic flash tube although I haven’t tried this method. All commonly
avaliable EPROM erasers tend to use a quartz UV tube and the design which
follows uses this method, allowing up to 6 devices to be erased at one time.
The time taken to erase an existing program from an EPROM is dependent upon
the wavelength and intensity of the radiation which is employed, the type of
EPROM and, to a lesser extent, upon the age and type of device; generally, an
exposure time of 15 – 20 minutes with a standard 8 watt tube 1″ from the EPROM
is sufficient although double this time can be given without undue damage
being caused. A shorter time will not fully erase the contents to FF (hex).
Some EPROMS may take much longer than 20 minutes, due to their method of
manufacture, but these are the exception rather than the rule and can I
sometimes apply to some Texas types. Sadists who erase their EPROMS with the
aid of a combined UV/heat lamp are asking for trouble since the devices will
be at risk of death from heatstroke – and unless they take precautions, their
eyesight may be affected permanently and adversely by the high level of UV
radiation (up to 300 watts).
A programmed EPROM has a pattern of charges placed in its memory locations
(which are effectively twin gate MOS FETs), where, owing to the excellent
insulating power of the silicon dioxide layer between the two gate electrodes,
it can be retained for very long periods of time (in excess of 10 years
according to most manufacturers). The effect of UV light is to allow this
charge to leak away – which is why a programmed EPROM must have the ‘window’
covered; it is made out of a high-silica glass which allows UV light to pass
through – most ordinary types of glass block the short wavelength (253.7 nm)
ultra-violet. Additionally, EPROMS don’t like high temperatures (above 70
deg. C) which is another good reason for keeping them cool while erasing them
and for fitting your computing engine with a decent fan if it acts as a part-time
A typical device in the 27XX range requires a radiation dose of about 15
Joules/sq. cm (1 Joule is 1 Watt/second) at an intensity of about 0.012
Watts/sq.cm and for a new UV tube, this would be given by about 20 minutes
exposure at 1″ from the light source. Tubes lose emission intensity as they
age but this is not likely to be much of a problem in this application since
the exposure time can be increased if needed. The only drawback is that in
order to check the state of erasure, the devices must be taken out of the
eraser and checked in the programmer, and replaced if necessary for another
dose of sun tan! A device which will not erase properly is likely to have one
or more of the following attributes:
- Made by Texas – try a longer erasure time (up to 2 hours).
- a dirty window – clean off the muck with a solvent and try again.
- open or short-circuited – had it been plugged in the wrong way
round when-it was last used?
If the last is true, then throw it away. Incidentally, new, unused EPROMS
should be blank, with FF in all locations. If not, your dealer should change