STK14CA8
Document Number: 001-51592 Rev. **
Page 11 of 16
nvSRAM Operation
nvSRAM
The STK14CA8 nvSRAM has two functional components paired
in the same physical cell. These are the SRAM memory cell and
a nonvolatile QuantumTrap cell. The SRAM memory cell
operates similar to 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 operations, SRAM
READ and WRITE operations are inhibited. The STK14CA8
supports unlimited read and writes similar to a typical SRAM. In
addition, it provides unlimited RECALL operations from the
nonvolatile cells and up to 200K STORE operations.
SRAM READ
The STK14CA8 performs a READ cycle whenever E and G are
low while W and HSB are high. The address specified on pins
A
0-16
determine which of the 131,072 data bytes are accessed.
When the READ is initiated by an address transition, the outputs
are valid after a delay of t
AVQV
(READ cycle #1). If the READ is
initiated by E and G, the outputs are valid at t
ELQV
or at t
GLQV
,
whichever is later (READ cycle #2). The data outputs repeatedly
responds to address changes within the t
AVQV
access time
without the need for transitions on any control input pins, and
remains valid until another address change or until E
or G is
brought high, or W and 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 are written into memory if it is valid t
DVWH
before the end
of a W controlled WRITE or t
DVEH
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 turns off the output buffers t
WLQZ
after
W goes low.
AutoStore Operation
The STK14CA8 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 STK14CA8.
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 15
shows the proper connection of the storage capacitor
(V
CAP
) for automatic store operation. Refer to
DC Characteristics
on page 3 for the size of the capacitor. The voltage on the V
CAP
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 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. The HSB signal can be monitored by the system to detect
an AutoStore cycle is in progress.
Figure 15. AutoStore Mode
Hardware STORE (HSB) Operation
The STK14CA8 provides the HSB pin for controlling and
acknowledging the STORE operations. The HSB pin is used to
request a hardware STORE cycle. When the HSB pin is driven
low, the STK14CA8 conditionally initiates a STORE operation
after t
DELAY
. An actual STORE cycle only begins 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
STK14CA8 continues to allow SRAM operations for t
DELAY
.
During t
DELAY
, multiple SRAM READ operations may take place.
If a WRITE is in progress when HSB is pulled low, it is allowed a
time t
DELAY
to complete. However, any SRAM WRITE cycles
requested after HSB goes low are inhibited until HSB returns
high.
If HSB is not used, it should be left unconnected.
Hardware RECALL (Power Up)
During power up or after any low power condition
(V
CC
<V
SWITCH
), an internal RECALL request is latched. When
V
CC
again exceeds the sense voltage of V
SWITCH
, a RECALL
cycle is automatically initiated and takes t
HRECALL
to complete.
V
CC
V
CAP
10
k
O
h
m
0.
1
µ
F
V
CC
V
CAP
W
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