STK14D88
Document Number: 001-52037 Rev. **
Page 11 of 17
nvSRAM Operation
nvSRAM
The STK14D88 nvSRAM is made up of two functional compo-
nents paired in the same physical cell. These are the SRAM
memory cell and a nonvolatile QuantumTrap™ cell. The SRAM
memory cell operates like a standard fast static RAM. Data in the
SRAM can be transferred to the nonvolatile cell (the STORE
operation), or from the nonvolatile cell to SRAM (the RECALL
operation). This unique architecture allows all cells to be stored
and recalled in parallel. During the STORE and RECALL opera-
tions SRAM READ and WRITE operations are inhibited. The
STK14D88 supports unlimited read and writes like a typical
SRAM. In addition, it provides unlimited RECALL operations
from the nonvolatile cells and up to 200K STORE operations.
SRAM READ
The STK14D88 performs a READ cycle whenever E and G are
low while W and HSB are high. The address specified on pins
A0-16 determine which of the 32,768 data bytes will be accessed.
When the READ is initiated by an address transition, the outputs
will be valid after a delay of tAVQV (READ cycle #1). If the READ
is initiated by E and G, the outputs will be valid at tELQV or at
tGLQV, whichever is later (READ cycle #2). The data outputs will
repeatedly respond to address changes within the tAVQV access
time without the need for transitions on any control input pins,
and will remain valid until another address change or until either
E or G is brought high, or W or HSB is brought low.
SRAM WRITE
A WRITE cycle is performed whenever E and W are low and HSB
is high. 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 the common I/O pins
DQ0-7 will be written into memory if it is valid tDVWH before the
end of a W controlled WRITE or tDVEH before the end of an E
controlled WRITE.
It is recommended that G be kept high during the entire WRITE
cycle to avoid data bus contention on common I/O lines. If G is
left low, internal circuitry will turn off the output buffers tWLQZ after
W goes low.
AutoStore Operation
The STK14D88 stores data to nvSRAM using one of three
storage operations. These three operations are Hardware Store
(activated by HSB), Software Store (activated by an address
sequence), and AutoStore (on power down).
AutoStore operation is a unique feature of Cypress Quantum
Trap technology is enabled by default on the STK14D88.
During normal operation, the device will draw current from VCC
to charge a capacitor connected to the VCAP pin. This stored
charge will be used by the chip to perform a single STORE
operation. If the voltage on the VCC pin drops below VSWITCH,
the part will automatically disconnect the VCAP pin from VCC. A
STORE operation will be initiated with power provided by the
VCAP capacitor.
Figure 12 shows the proper connection of the storage capacitor
(VCAP) for automatic store operation. Refer to the DC CHARAC-
TERISTICS table for the size of the capacitor. The voltage on the
VCAP pin is driven to 5V by a charge pump internal to the chip. A
pull up should be placed on W to hold it inactive during power up.
To
reduce
unneeded
nonvolatile
stores,
AutoStore
and
Hardware Store operations will be 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. The HSB signal can be monitored by the system to detect
an AutoStore cycle is in progress.
Figure 12. AutoStore Mode
Hardware STORE (HSB) Operation
The STK14D88 provides the HSB pin for controlling and
acknowledging the STORE operations. The HSB pin can be
used to request a hardware STORE cycle. When the HSB pin is
driven low, the STK14D88 will conditionally initiate a STORE
operation after tDELAY. An actual STORE cycle will only begin if
a WRITE to the SRAM took place since the last STORE or
RECALL cycle. The HSB pin has a very resistive pull up and is
internally driven low to indicate a busy condition while the
STORE (initiated by any means) is in progress. This pin should
be externally pulled up if it is used to drive other inputs.
SRAM READ and WRITE operations that are in progress when
HSB is driven low by any means are given time to complete
before the STORE operation is initiated. After HSB goes low, the
STK14D88 will continue SRAM operations for tDELAY. During
tDELAY, multiple SRAM READ operations may take place. If a
WRITE is in progress when HSB is pulled low, it will be allowed
a time, tDELAY, to complete. However, any SRAM WRITE cycles
requested after HSB goes low will be inhibited until HSB returns
high.
If HSB is not used, it should be left unconnected.
Software STORE
Data can be transferred from the SRAM to the nonvolatile
memory by a software address sequence. The STK14D88
software STORE cycle is initiated by executing sequential E
controlled READ cycles from six specific address locations in
exact order. During the STORE cycle, previous data is erased
and then the new data is programmed into the nonvolatile
elements. Once a STORE cycle is initiated, further memory
inputs and outputs are disabled until the cycle is completed.
VCC
VCC
VCAP
W
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