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LHF32KZR
33
5 DESIGN CONSIDERATIONS
5.1 Three-Line Output Control
The device will often be used in large memory arrays.
SHARP provides three control inputs to
accommodate multiple memory connections. Three-
Line control provides for:
a. Lowest possible memory power dissipation.
b. Complete assurance that data bus contention will
not occur.
To use these control inputs efficiently, an address
decoder should enable BE# while OE# should be
connected to all memory devices and the system’s
READ# control line. This assures that only selected
memory devices have active outputs while
deselected memory devices are in standby mode.
RP# should be connected to the system
POWERGOOD signal to prevent unintended writes
during system power transitions. POWERGOOD
should also toggle during system reset.
5.2 STS and Block Erase, Bank Erase,
(Multi) Word/Byte Write and Block
Lock-Bit Configuration Polling
STS is an open drain output that should be
connected to V CC by a pullup resistor to provide a
hardware method of detecting block erase, bank
erase, (multi) word/byte write and block lock-bit
configuration completion. In default mode, it
transitions low after block erase, bank erase, (multi)
word/byte write or block lock-bit configuration
commands and returns to V OH when the WSM has
finished executing the internal algorithm. For
alternate STS pin configurations, see the
Configuration command.
STS can be connected to an interrupt input of the
system CPU or controller. It is active at all times.
STS, in default mode, is also High Z when the device
is in block erase suspend (with (multi) word/byte write
inactive), (multi) word/byte write suspend or deep
power-down modes.
5.3 Power Supply Decoupling
Flash memory power switching characteristics require
careful device decoupling. System designers are
interested in three supply current issues; standby
current levels, active current levels and transient
peaks produced by falling and rising edges of BE#
and OE#. Transient current magnitudes depend on
the device outputs’ capacitive and inductive loading.
Two-line control and proper decoupling capacitor
selection will suppress transient voltage peaks. Each
device should have a 0.1μF ceramic capacitor
connected between its V CC and GND and between its
V PP and GND. These high-frequency, low inductance
capacitors should be placed as close as possible to
package leads. Additionally, for every eight devices,
a 4.7μF electrolytic capacitor should be placed at the
array’s power supply connection between V CC and
GND. The bulk capacitor will overcome voltage
slumps caused by PC board trace inductance.
5.4 V PP Trace on Printed Circuit Boards
Updating flash memories that reside in the target
system requires that the printed circuit board
designer pay attention to the V PP Power supply trace.
The V PP pin supplies the memory cell current for
block erase, bank erase, (multi) word/byte write and
block lock-bit configuration. Use similar trace widths
and layout considerations given to the V CC power
bus. Adequate V PP supply traces and decoupling will
decrease V PP voltage spikes and overshoots.
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