The six modes of operation of the EPROM are listed in Table 1. It
should be noted that all inputs for the six modes are at TTL levels.
The power supplies required are VCC and VPP. The VPP power
supply must be at 12.75V during the three programming modes,
and must be at 5V in the other three modes. The VCC power supply
must be at 6.5V during the three programming modes, and at 5V
in the other three modes.
The EPROM has two control functions, both of which must be
logically active in order to obtain data at the outputs. Chip Enable
(CE/PGM) is the power control and should be used for device
selection. Output Enable (OE) is the output control and should be
used to gate data to the output pins, independent of device
selection. Assuming that addresses are stable, address access
time (tACC) is equal to the delay from CE to output (tCE). Data is
available at the outputs tOE after the falling edge of OE, assuming
that CE/PGM has been low and addresses have been stable for
at least tACC –tOE.
The EPROM has a standby mode which reduces the active power
dissipation by over 99%, from 385 mW to 0.55 mW. The EPROM
is placed in the standby mode by applying a CMOS high signal to
the CE/PGM input. When in standby mode, the outputs are in a
high impedance state, independent of the OE input.
The EPROM is placed in output disable by applying a TTL high
signal to the OE input. When in output disable all circuitry is
enabled, except the outputs are in a high impedance state (TRI-
Because the EPROM is usually used in larger memory arrays,
Fairchild has provided a 2-line control function that accommo-
dates this use of multiple memory connections. The 2-line control
function allows for:
1. the lowest possible memory power dissipation, and
2. complete assurance that output bus contention will not
To most efficiently use these two control lines, it is recommended
that CE/PGM be decoded and used as the primary device select-
ing function, while OE be made a common connection to all
devices in the array and connected to the READ line from the
system control bus. This assures that all deselected memory
devices are in their low power standby modes and that the output
pins are active only when data is desired from a particular memory
CAUTION: Exceeding 14V on pin 1 (VPP) will damage the EPROM.
Initially, and after each erasure, all bits of the EPROM are in the
“1’s” state. Data is introduced by selectively programming “0’s”
into the desired bit locations. Although only “0’s” will be pro-
grammed, both “1’s” and “0’s” can be presented in the data word.
The only way to change a “0” to a “1” is by ultraviolet light erasure.
The EPROM is in the programming mode when the VPP power
supply is at 12.75V and OE is at VIH. It is required that at least a
0.1 µF capacitor be placed across VPP, VCC to ground to suppress
spurious voltage transients which may damage the device. The
data to be programmed is applied 8 bits in parallel to the data
output pins. The levels required for the address and data inputs
When the address and data are stable, an active low, TTL program
pulse is applied to the CE/PGM input. A program pulse must be
applied at each address location to be programmed. The EPROM
is programmed with the Turbo Programming Algorithm shown in
Figure 1. Each Address is programmed with a series of 50 µs
pulses until it verifies good, up to a maximum of 10 pulses. Most
memory cells will program with a single 50 µs pulse. (The standard
National Semiconductor Algorithm may also be used but it will
have longer programming time.)
The EPROM must not be programmed with a DC signal applied to
the CE/PGM input.
Programming multiple EPROM in parallel with the same data can
be easily accomplished due to the simplicity of the programming
requirments. Like inputs of the parallel EPROM may be connected
together when they are programmed with the same data. A low
level TTL pulse applied to the CE/PGM input programs the
Programming multiple EPROMs in parallel with different data is
also easily accomplished. Except for CE/PGM, all like inputs
(including OE) of the parallel EPROMs may be common. A TTL
low level program pulse applied to an EPROM’s CE/PGM input
with VPP at 12.75V will program that EPROM. A TTL high level CE/
PGM input inhibits the other EPROMs from being programmed.
A verify should be performed on the programmed bits to determine
whether they were correctly programmed. The verify may be
performed with VPP at 12.75V. VPP must be at VCC, except during
programming and program verify.
Opaque labels should be placed over the EPROM window to
prevent unintentional erasure. Covering the window will also
prevent temporary functional failure due to the generation of photo
MANUFACTURER’S IDENTIFICATION CODE
The EPROM has a manufacturer’s identification code to aid in
programming. When the device is inserted in an EPROM pro-
grammer 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.
The Manufacturer’s Identification code, shown in Table 2, specifi-
cally identifies the manufacturer and device type. The code for
NM27C256 is “8F04”, where “8F” designates that it is made by
Fairchild Semiconductor, and “04” designates a 256K part.
The code is accessed by applying 12V ±0.5V to address pin A9.
Addresses A1–A8, A10–A16, and all control pins are held at VIL.
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 –O7. Proper code access is only guaranteed at 25°C to ±5°C.