4 Am27X1024
FUNCTIONAL DESCRIPTION
Read Mode
T o obtain dat a at the device outputs, Chip Enable ( CE#)
and Output Enable (OE#) must be driv en low . CE# controls the power to the de vice and is typically used t o select the device . OE# en ables th e de v ice to out put dat a,
independent of device selection. Addresses must be
stable for at least t
ACC–tOE.
Refer to the Switching
Waveforms section for the timing diagr am.
Standby Mode
The device enters the CMOS standby mode when CE#
is at V
CC
± 0.3 V. Maximum V
CC
current is reduced to
100 µA. The device enters the TTL-standby mode
when CE# is at V
IH
. Maximum V
CC
current is reduced
to 1.0 mA. When in either standby mode, the device
places its outputs in a high-impedance state, independent 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.
CE# should be decoded and used as the primary device-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 assures 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 memory device.
System Applications
During the switch between active and standby conditions, transient current peaks are produced on the rising and falling edges of Chip Enab le . The magnitude of
these transient current peaks is dependent on the output capacitance loading of the de vi ce. At a minim um, a
0.1 µF ceramic capacitor (high frequency, low inherent
inductance) sho uld be used on each device between
V
CC
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 ExpressROM device arrays, a 4.7 µF bulk electrolytic capacitor
should be used between V
CC
and VSS for each eight
devices. The location of the capacitor should be close
to where the power supply is connected to the array.
MODE SELECT TABLE
Note:
X = Either V
IH
or VIL.
Mode CE# OE# PGM# V
PP
Outputs
Read V
IL
V
IL
XXD
OUT
Output Disable X V
IH
X X High Z
Standby (TTL) V
IH
X X X High Z
Standby (CMOS) V
CC
± 0.3 V X X X High Z