U631H256
10
March 31, 2006
STK Control #ML0043
Rev 1.0
Device Operation
The U631H256 has two separate modes of operation:
SRAM mode and nonvolatile mode. The memory ope-
rates in SRAM mode as a standard fast static RAM.
Data is transferred in nonvolatile mode from SRAM to
EEPROM shadow (the STORE operation) or from
EEPROM to SRAM (the RECALL operation). In this
mode SRAM functions are disabled.
SRAM READ
The U631H256 performs a READ cycle whenever E
and G are LOW while W is HIGH. The address speci-
fied on pins A0 - A14 determines which of the 32768
data bytes will be accessed. When the READ is initia-
ted by an address transition, the outputs will be valid
after a delay of tcR. If the READ is initiated by E or G,
the outputs will be valid at ta(E) or at ta(G), whichever is
later. The data outputs will repeatedly respond to
address changes within the tcR access time without the
need for transition on any control input pins, and will
remain valid until another address change or until E or
G is brought HIGH or W is brought LOW.
SRAM WRITE
A WRITE cycle is performed whenever E and W are
LOW. The address inputs must be stable prior to
entering the WRITE cycle and must remain stable until
either E or W goes HIGH at the end of the cycle. The
data on pins DQ0 - 7 will be written into the memory if it
is valid tsu(D) before the end of a W controlled WRITE
or tsu(D) before the end of an E controlled WRITE.
It is recommended that G is kept HIGH during the
entire WRITE cycle to avoid data bus contention on the
common I/O lines. If G is left LOW, internal circuitry will
turn off the output buffers tdis(W) after W goes LOW.
Noise Consideration
The U631H256 is a high speed memory and therefore
must have a high frequency bypass capacitor of appro-
ximately 0.1
μF connected between VCC and VSS using
leads and traces that are as short as possible. As with
all high speed CMOS ICs, normal carefull routing of
power, ground and signals will help prevent noise pro-
blems.
Software Nonvolatile STORE
The U631H256 software controlled STORE cycle is
initiated by executing sequential READ cycles from six
specific address locations. By relying on READ cycles
only, the U631H256 implements nonvolatile operation
while remaining compatible with standard 32K x 8
SRAMs. During the STORE cycle, an erase of the pre-
vious nonvolatile data is first performed, followed by a
program of the nonvolatile elements. Once a STORE
cycle is initiated, further inputs and outputs are disabled
until the cycle is completed .Because a sequence of
addresses is used for STORE initiation, it is important
that no other READ or WRITE accesses intervene in
the sequence or the sequence
will be aborted and no STORE or RECALL will take
place.
To initiate the STORE cycle the following READ
sequence must be performed:
1.
Read addresses 0E38
(hex) Valid READ
2.
Read addresses 31C7
(hex) Valid READ
3.
Read addresses 03E0
(hex) Valid READ
4.
Read addresses 3C1F (hex) Valid READ
5.
Read addresses 303F
(hex) Valid READ
6.
Read addresses 0FC0 (hex) Initiate STORE
Cycle
Once the sixth address in the sequence has been
entered, the STORE cycle will commence and the chip
will be disabled. It is important that READ cycles and
not WRITE cycles be used in the sequence, although it
is not necessary that G be LOW for the sequence to be
valid. After the tSTORE cycle time has been fulfilled, the
SRAM will again be activated for READ and WRITE
operation.
Software Nonvolatile RECALL
A RECALL cycle of the EEPROM data into the SRAM
is initiated with a sequence of READ operations in a
manner similar to the STORE initiation. To initiate the
RECALL cycle the following sequence of READ opera-
tions must be performed:
1.
Read addresses 0E38
(hex) Valid READ
2.
Read addresses 31C7
(hex) Valid READ
3.
Read addresses 03E0
(hex) Valid READ
4.
Read addresses 3C1F (hex) Valid READ
5.
Read addresses 303F
(hex) Valid READ
6.
Read addresses 0C63
(hex) Initiate RECALL
Cycle
Internally, RECALL is a two step procedure. First, the
SRAM data is cleared and second, the nonvolatile
information is transferred into the SRAM cells. The
RECALL operation in no way alters the data in the
EEPROM cells. The nonvolatile data can be recalled an
unlimited number of times.
Automatic Power Up RECALL
On power up, once VCC exceeds the sense voltage of
VSWITCH, a RECALL cycle is automatically initiated.
The voltage on the VCC pin must not drop below
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