Functional Description (Continued)
are held at V
IL. Address pin A0 is held at VIL for the manufacturer’s
code, and held at VIH for the device code. The code is read on the
eight data pins, O0 –O 7 . Proper code access is only guaranteed
The erasure characteristics of the device are such that erasure
begins to occur when exposed to light with wavelengths shorter
than approximately 4000 Angstroms (Å). It should be noted that
sunlight and certain types of fluorescent lamps have wavelengths
in the 3000Å–4000Å range.
The recommended erasure procedure for the EPROM is expo-
sure to short wave ultraviolet light which has a wavelength of
2537Å. The integrated dose (i.e., UV intensity x exposure time) for
erasure should be minimum of 15W-sec/cm2.
The EPROM should be placed within 1 inch of the lamp tubes
during erasure. Some lamps have a filter on their tubes which
should be removed before erasure
An erasure system should be calibrated periodically. The distance
from lamp to device should be maintained at one inch. The erasure
time increases as the square of the distance from the lamp (if
distance is doubled the erasure time increases by factor of 4).
Lamps lose intensity as they age. When a lamp is changed, the
distance has changed, or the lamp has aged, the system should
be checked to make certain full erasure is occurring. Incomplete
erasure will cause symptoms that can be misleading. Program-
mers, components, and even system designs have been errone-
ously suspected when incomplete erasure was the problem.
The power switching characteristics of EPROMs require careful
decoupling of the devices. The supply current, ICC, has three
segments that are of interest to the system designer: the standby
current level, the active current level, and the transient current
peaks that are produced by voltage transitions on input pins. The
magnitude of these transient current peaks is dependent on the
output capacitance loading of the device. The associated VCC
transient voltage peaks can be suppressed by properly selected
decoupling capacitors. It is recommended that at least a 0.1
ceramic capacitor be used on every device between VCC and
GND. This should be a high frequency capacitor of low inherent
inductance. In addition, at least a 4.7
µF bulk electrolytic capacitor
should be used between VCC and GND for each eight devices. The
bulk capacitor should be located near where the power supply is
connected to the array. The purpose of the bulk capacitor is to
overcome the voltage drop caused by the inductive effects of the
PC board traces.
The modes of operation of the NM27C512 are listed in Table 1. A single 5V power supply is required in the read mode. All inputs are
TTL levels excepts for V
PP and A9 for device signature.
TABLE 1. Mode Selection
X (Note 13)
Note 13: X can be V
IL or VIH.
TABLE 2. Manufacturer’s Identification Code