HT27LC020
Read Mode
It is recommended that CE be decoded and used as the
primary device-selection function, while OE be made a
common connection 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.
The HT27LC020 has two control functions, both of
which must be logically satisfied in order to obtain data
at outputs. Chip Enable (CE) 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. As-
suming 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 the CE has been LOW and addresses
System Considerations
During the switch between active and standby condi-
tions, transient current peaks are produced on the rising
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
0.1mF ceramic capacitor (high frequency, low inherent
inductance) should be used on each device between
VCC and VPP to minimize transient effects. In addition,
to overcome the voltage drop caused by the inductive
effects of the printed circuit board traces on EPROM ar-
rays, a 4.7mF bulk electrolytic capacitor should be used
between VCC and VPP for each eight devices. The lo-
cation of the capacitor should be close to where the
power supply is connected to the array.
have been stable for at least tACC-tOE
.
Standby Mode
The HT27LC020 has CMOS standby mode which re-
duces the maximum VCC current to 10mA. It is placed in
CMOS standby when CE is at VCC±0.3V. The
HT27LC020 also has a TTL-standby mode which re-
duces the maximum VCC current to 0.6mA. It is placed
in TTL-standby when CE is at VIH. When in standby
mode, the outputs are in a high-impedance state, inde-
pendent of the OE input.
Two-line Output Control Function
To accommodate multiple memory connections, a
two-line control function is provided to allow for:
·
·
Low memory power dissipation
Assurance that output bus contention will not occur
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Figure 1. A.C. Waveforms for Read Operation
Rev. 1.50
6
December 8, 2003
HOLTEK [ HOLTEK SEMICONDUCTOR INC ]
