AM27X512 datasheet(4/10 Pages) AMD

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Am27X512

FUNCTIONAL DESCRIPTION

Read 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-

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.

Standby Mode

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:

s Low memory power dissipation, and

s Assurance that output bus contention will not occur.

CE# should be decoded and used as the primary de-

vice-selecting function, while OE#/VPP be made a com-

mon 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 out-

put pins are only active when data is desired from a

particular memory device.

System Applications

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.

MODE SELECT TABLE

Note:

X = Either VIH or VIL.

Mode

CE#

OE#/VPP

Outputs

Read

VIL

DOUT

Output Disable

VIH

High Z

Standby (TTL)

VIH

High Z

Standby (CMOS)

VCC ± 0.3 V

High Z

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