STK12C68
Document Number: 001-51027 Rev. **
Page 3 of 20
Device Operation
The STK12C68 nvSRAM is made up of two functional compo-
nents paired in the same physical cell. These are an SRAM
memory cell and a nonvolatile QuantumTrap cell. The SRAM
memory cell operates as a standard fast static RAM. Data in the
SRAM is transferred to the nonvolatile cell (the STORE
operation) or from the nonvolatile cell to SRAM (the RECALL
operation). This unique architecture enables the storage and
recall of all cells in parallel. During the STORE and RECALL
operations, SRAM Read and Write operations are inhibited. The
STK12C68 supports unlimited reads and writes similar to a
typical SRAM. In addition, it provides unlimited RECALL opera-
tions from the nonvolatile cells and up to one million STORE
operations.
SRAM Read
The STK12C68 performs a Read cycle whenever CE and OE are
LOW while WE and HSB are HIGH. The address specified on
pins A
0–12
determines the 8,192 data bytes accessed. When the
Read is initiated by an address transition, the outputs are valid
after a delay of t
AA
(Read cycle 1). If the Read is initiated by CE
or OE, the outputs are valid at t
ACE
or at t
DOE
, whichever is later
(Read cycle 2). The data outputs repeatedly respond to address
changes within the t
AA
access time without the need for transi-
tions on any control input pins, and remains valid until another
address change or until CE or OE is brought HIGH, or WE or
HSB is brought LOW.
SRAM Write
A Write cycle is performed whenever CE and WE are LOW and
HSB is HIGH. The address inputs must be stable prior to entering
the Write cycle and must remain stable until either CE or WE
goes HIGH at the end of the cycle. The data on the common IO
pins DQ
0–7
are written into the memory if it has valid t
SD
, before
the end of a WE controlled Write or before the end of an CE
controlled Write. Keep OE HIGH during the entire Write cycle to
avoid data bus contention on common IO lines. If OE is left LOW,
internal circuitry turns off the output buffers t
HZWE
after WE goes
LOW.
AutoStore Operation
The STK12C68 stores data to nvSRAM using one of three
storage operations:
1. Hardware store activated by HSB
2. Software store activated by an address sequence
3. AutoStore on device power down
AutoStore operation is a unique feature of QuantumTrap
technology and is enabled by default on the STK12C68.
During normal operation, the device draws current from V
CC
to
charge a capacitor connected to the V
CAP
pin. This stored
charge is used by the chip to perform a single STORE operation.
If the voltage on the V
CC
pin drops below V
SWITCH
, the part
automatically disconnects the V
CAP
pin from V
CC
. A STORE
operation is initiated with power provided by the V
CAP
capacitor.
Figure 2
shows the proper connection of the storage capacitor
(V
CAP
) for automatic store operation. A charge storage capacitor
between 68 µF and 220 µF (+20%) rated at 6V should be
provided. The voltage on the V
CAP
pin is driven to 5V by a charge
pump internal to the chip. A pull up is placed on WE to hold it
inactive during power up.
In system power mode, both V
CC
and V
CAP
are connected to the
+5V power supply without the 68
μ
F capacitor. In this mode, the
AutoStore function of the STK12C68 operates on the stored
system charge as power goes down. The user must, however,
guarantee that V
CC
does not drop below 3.6V during the 10 ms
STORE
cycle.
To reduce unnecessary nonvolatile stores, AutoStore, and
Hardware Store operations are ignored, unless at least one Write
operation has taken place since the most recent STORE or
RECALL cycle. Software initiated STORE cycles are performed
regardless of whether a Write operation has taken place. An
optional pull up resistor is shown connected to HSB
.
The HSB
signal is monitored by the system to detect if an AutoStore cycle
is in progress.
Figure 2. AutoStore Mode
9FF
9
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P
K
2
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5
Y
P
K
2
N
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)
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