4Am27X020
FUNCTIONAL DESCRIPTIO N
Rea d Mode
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-
le ct th e de v i c e. OE # en ab l es th e de v i c e to ou tp u t da t a,
independent of device selection. Addresses must be
stable for at least tACC–tOE. Refer to the Switching
Waveforms section for the timing diagram.
Stand by Mode
The device enters the CMOS standby mode when CE#
is at VCC ± 0.3 V. Ma ximu m VCC curr ent is re duc ed t o
100 µA. The device enters the TTL-standby mode
when CE# is at VIH. Maxi mum V CC current is r educe d
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-Ti e ing
To accommodate multiple memory connections, a
two-line control function provides:
■Low memory power dissipation, and
■Assurance th at output b us contention will not occur .
CE# should be decoded and u sed as the prim ary 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 memor y devices are in their
low-p ower st an dby mo de a nd th at th e o utput pins ar e
onl y a ct iv e w hen d ata i s de sire d fr om a particu lar mem-
ory device.
System Applications
During the switch between active and standby condi-
tions, transient current peaks are produced on the r is-
in g a nd f al l in g ed ge s of C hip E n ab l e . The ma gn i t ud e 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 m ini m ize tran si e nt e ffects. In a dd iti on ,
to ove rcome the vol tage dro p cau sed by th e i nduct ive
effects of the printed cir cuit board traces on Express-
ROM device arra ys, a 4.7 µF bulk electrolytic capacitor
should be used between VCC and VSS for each eight
devices. The loca tion 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 V
IL
.
Mode CE# OE# PGM# VPP Outputs
Read VIL VIL XXD
OUT
Output Disable X VIH X X High Z
Standby (TTL) VIH X X X High Z
Standby (CMOS) VCC ± 0.3 V X X X High Z