FUNCTIONAL DESCRIPTION
Read Mode
CE# should be decoded and used as the primary de-
vice-selecting 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 as-
sures that all deselected memory devices are in their
low-power standby mode and that the output pins are
only active when data is desired from a particular mem-
ory device.
To obtain data at the device outputs, Chip Enable (CE#)
and Output Enable (OE#) must be driven low. CE# con-
trols the power to the device and is typically used to se-
lect the device. OE# enables the device to output data,
independent of device selection. Addresses must be
stable for at least tACC–tOE. Refer to the Switching
Waveforms section for the timing diagram.
System Applications
Standby Mode
During the switch between active and standby condi-
tions, transient current peaks are produced on the ris-
ing and falling edges of Chip Enable. The magnitude of
these transient current peaks is dependent on the out-
put capacitance loading of the device. At a minimum, a
0.1 µF ceramic capacitor (high frequency, low inherent
inductance) should be used on each device between
VCC and VSS to minimize transient effects. In addition,
to overcome the voltage drop caused by the inductive
effects of the printed circuit board traces on Express-
ROM device arrays, a 4.7 µF bulk electrolytic capacitor
should be used between VCC and VSS for each eight
devices. The location of the capacitor should be close
to where the power supply is connected to the array.
The device enters the CMOS standby mode when CE#
is at VCC ± 0.3 V. Maximum VCC current is reduced to
100 µA. The device enters the TTL-standby mode
when CE# is at VIH. Maximum VCC current is reduced
to 1.0 mA. When in either standby mode, the device
places its outputs in a high-impedance state, indepen-
dent of the OE# input.
Output OR-Tieing
To accommodate multiple memory connections, a
two-line control function provides:
■ Low memory power dissipation, and
■ Assurance that output bus contention will not occur.
MODE SELECT TABLE
Mode
CE#
OE#
PGM#
V
Outputs
PP
Read
V
V
X
X
X
X
X
X
X
X
D
OUT
IL
IL
Output Disable
Standby (TTL)
Standby (CMOS)
X
V
High Z
High Z
High Z
IH
V
X
IH
V
± 0.3 V
X
CC
Note:
X = Either V or V .
IH
IL
4
Am27X128
SPANSION [ SPANSION ]
