9

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NMC27C64 Rev. C

Program Inhibit

Programming multiple NMC27C64s in parallel with different data

is also easily accomplished. Except for CE all like inputs (including

OE and PGM) of the parallel NMC27C64 may be common. A TTL

low level program pulse applied to an NMC27C64

’s PGM input

with CE at V

TTL high level CE input inhibits the other NMC27C64s from being

programmed.

Program Verify

A verify should be performed on the programmed bits to determine

whether they were correctly programmed. The verify may be

performed with V

programming and program verify.

MANUFACTURER

’S IDENTIFICATION CODE

The NMC27C64 has a manufacturer

’s identification code to aid in

programming. The code, shown in Table 2, is two bytes wide and

is stored in a ROM configuration on the chip. It identifies the

manufacturer and the device type. The code for the NMC27C64

is

“8FC2”, where “8F” designates that it is made by Fairchild

Semiconductor, and

“C2” designates a 64k part.

The code is accessed by applying 12V

0.5V to address pin A9.

Addresses A1

–A8, A10–A12, CE, and OE are held at V

A0 is held at V

device code. The code is read out on the 8 data pins. Proper code

access is only guaranteed at 25 C

5 C.

The primary purpose of the manufacturer

’s identification code is

automatic programming control. When the device is inserted in a

EPROM programmer socket, the programmer reads the code and

then automatically calls up the specific programming algorithm for

the part. This automatic programming control is only possible with

programmers which have the capability of reading the code.

ERASURE CHARACTERISTICS

The erasure characteristics of the NMC27C64 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 3000

Å – 4000Å range.

After programming, opaque labels should be placed over the

NMC27C64

’s window to prevent unintentional erasure. Covering

the window will also prevent temporary functional failure due to the

generation of photo currents.

The recommended erasure procedure for the NMC27C64 is

exposure to short wave ultraviolet light which has a wavelength of

2537 Angstroms (

Å). The integrated dose (i.e., UV intensity x

The NMC27C64 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 unit should be maintained at one inch. The erasure

time increases as the square of the distance. (If distance is

doubled the erasure time increases by a 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. Programmers, compo-

nents, and even system designs have been erroneously sus-

pected when incomplete erasure was the problem.

SYSTEM CONSIDERATION

The power switching characteristics of EPROMs require careful

decoupling of the devices. The supply current, I

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 V

CC

transient voltage peaks can be suppressed by properly selected

decoupling capacitors. It is recommended that at least a 0.1 F

ceramic capacitor be used on every device between V

GND. This should be a high frequency capacitor of low inherent

inductance. In addition, at least a 4.7 F bulk electrolytic capacitor

should be used between V

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.

TABLE 2. Manufacturer

’s Identification Code

Pins

A0

O7

O6

O5

O4

O3

O2

O1

O0

Hex

(10)

(19)

(18)

(17)

(16)

(15)

(13)

(12)

(11)

Data

Manufacturer Code

V

IL

1000111

1

8F

1100001

0

C2