Applying the STK11C88, STK15C88
and STK16C88 32K x 8 nvSRAM
Abstract
Simtek Corporation’s family of high speed 256 kilobit
nonvolatile Static Random Access Memories
includes the STK15C88
AutoStore™
and
STK11C88 Software Store and STK16C88
AutoStorePlus™
devices. All three memories have
industry-standard 32K x 8 architectures and pin-outs
(Figure 1), and are drop-in replacements for many
standard SRAM and BatRAM products. As with all
Simtek nvSRAM products, the STK11C88,
STK15C88 and STK16C88 do not require batteries
or other power sources to maintain nonvolatility for a
guaranteed minimum of 100 years, even at high
temperatures.
The STK15C88
AutoStore™
and STK16C88
AutoStorePlus™
memories both automatically
STORE
the SRAM data into EEPROM upon power
loss, and require no external power storage compo-
nents such as batteries or capacitors. They are
available in speeds of 20, 25, 35, and 45 nanosec-
onds.
The STK11C88 Software Store memory is designed
for safe storage of data under processor control.
The STK11C88 virtually eliminates the danger of
inadvertent data loss due to human error or elec-
tronics failure. It is ideally suited for applications that
would normally require the combination of high
speed SRAM and EEPROM, and performs both
functions in one package. The resultant savings in
board space, glue logic (parts count), and power
consumption helps to reduce costs and increase
packaging density. The STK11C88 is available in
20, 25, 35, and 45 nanosecond versions, and can
be interfaced to most standard microprocessors
without interface logic or memory wait states.
Both memories automatically
RECALL
nonvolatile
data into the SRAM portion of the chip at power-up
without processor intervention or external control.
Simtek's family of nonvolatile memories give the
product development engineer the ability to design
modern embedded and state machines without the
worry of data loss or corruption due to power failure.
SRAM Operations
The STK11C88, STK15C88 and STK16C88 nvS-
RAM memories are identical in the operation of their
RAM front ends, and look to the design engineer like
industry standard 32K x 8 fast SRAMs.
A
14
A
12
A
7
A
6
A
5
A
4
A
3
A
2
A
1
A
0
DQ
0
DQ
1
DQ
2
V
SS
1
2
3
4
5
6
7
8
9
10
11
12
13
14
28
27
26
25
24
23
22
21
20
19
18
17
16
15
V
CC
W
A
13
A
8
A
9
A
11
G
A
10
E
DQ
7
DQ
6
DQ
5
DQ
4
DQ
3
Figure 1
STK11C88/STK15C88/STK16C88 Pin-Out
The three control lines, Write Enable (W), Chip
Enable (E), and Output Enable (G) are utilized in the
same manner as a standard SRAM. This front end
functional equivalency allows improvements to be
made to old designs without PWB (Printed Wiring
Board) or control logic changes. The ability of
Simtek's nvSRAM devices to run at processor bus
speeds eliminates the need for wait states, bus con-
trol logic, or specialized decoder circuitry. This com-
bination of speed and versatility in one package
serves to simplify board layout and reduce device
count.
SRAM Reads are performed whenever E and G are
low and W is high. The output data on lines DQ
0
-
DQ
7
corresponds to the address specified on pins
A
0
- A
14
.
SRAM Writes are performed whenever E and W are
low. The data on pins DQ
0
- DQ
7
will be written into
memory at the location specified by the address
present on lines A
0
- A
14
. The G (Output Enable)
8-21
Applying the STK11C/15C/16C88
line should preferably be held high during the entire
Write cycle to avoid bus contention on the data
lines.
The STK11C88, STK15C88 and STK16C88 are
high speed memories and therefore must have a
high frequency bypass capacitor of approximately
0.1µF connected between V
CC
and ground. As with
all high speed CMOS integrated circuits, careful
routing of power, ground and signals will help to
reduce noise induced problems.
the system to stabilize before attempting nvSRAM
accesses.
Data can also be recalled under software control in
a manner similar to the
STORE
operation. Six
addresses are read in sequence to initiate the trans-
fer from nonvolatile memory to SRAM. The following
address sequence is used to start a memory
RECALL:
1. Read Address 0E38H
2. Read Address 31C7H
3. Read Address 03E0H
4. Read Address 3C1FH
5. Read Address 303FH
6. Read Address 0C63H (Initiate Recall Cycle)
It takes a maximum of 20µs for data that was Stored
in nonvolatile memory to be Recalled to SRAM.
STK11C88 Nonvolatile Operations
nvSRAM Store
The STK11C88 is a member of Simtek's “Software
Store” nvSRAM family, and is designed for easy in-
circuit programmability. Data that has been written
to the STK11C88's SRAM is Stored to nonvolatile
memory by reading six specific SRAM addresses in
sequence. The Store operation is completely auton-
omous from that point, and requires no processor
support nor external glue logic to complete. Once
the
STORE
process is initiated, the STK11C88 is
disabled and no SRAM accesses are allowed until
the Store operation is completed (<10 ms). It is
important that Read operations be used instead of
Writes. SRAM Writes will invalidate the sequence.
The following address sequence is used to start a
memory
STORE
operation:
1. Read Address 0E38H
2. Read Address 31C7H
3. Read Address 03E0H
4. Read Address 3C1FH
5. Read Address 303FH
6. Read Address 0FC0H (Initiate
STORE
Cycle)
The chance of inadvertently reading the correct six
addresses in sequence, and accidentally corrupting
the data Stored in the STK11C88's nonvolatile sec-
tion, is less than 1 chance in 10
25
.
STK15C88 Nonvolatile Operations
The STK15C88 is very similar in operation to the
STK11C88. Its software
STORE
and
RECALL
oper-
ations, and its power-up
RECALL
capabilities are
identical. The STK15C88 adds the capability of
Simtek's
AutoStore™
technology that allows it to
sense power loss and
STORE
data to nonvolatile
memory autonomously without external circuitry or
processor intervention. As with the STK11C88, once
data is stored into nonvolatile memory, no batteries,
capacitors or external power sources are required to
safely maintain the data for a minimum of 10 years.
The only requirement for full
AutoStore™
operation
is that power supply voltage remain above 3.6 volts
for a minimum of 10ms after V
SWITCH
(approxi-
mately 4.25 volts, see Figure 2).
5
4
V
cc
AutoStore
TM
Cycle in Progress
Volts (V)
3
2
1
nvSRAM Recall
STK11C88 data is Recalled from nonvolatile mem-
ory to SRAM during power-up. A
RECALL
is auto-
matically initiated when internal circuitry senses
VCC levels rising above approximately 4.25 volts
(V
SWITCH
).
Embedded
processor
wake-up
sequences (power-on reset), should be delayed for
a minimum of 550µs (t
RESTORE
) after V
SWITCH
to
assure that data is completely recalled and to allow
Store
Time
15
30
45
60
75
90
105
120
135
Time (ms)
Figure 2:
STK15C88
AutoStore™
Vcc Requirements
Even if the design engineer cannot guarantee the
10ms power supply voltage requirement, it is still
possible to use the STK15C88 in many applications.
8-22
Applying the STK11C/15C/16C88
The addition of a low forward voltage drop diode,
and a 100µF capacitor will supply the energy neces-
sary for the
STORE
operation to complete indepen-
dent of the Power Supply decay characteristics
(Figure 3). The optional resistor on W prevents acci-
dental writes to the SRAM in systems whose power
supply is unreliable on power-up above V
SWITCH
.
Note that this alternate configuration effectively
raises the V
SWITCH
level by the diode drop V
f
.
Applications
Simtek's family of nvSRAMs is the ideal comple-
ment to many of today's most advanced technolo-
gies. By increasing the level of integration, designs
can be achieved that are smaller (more compact),
lower in power, higher reliability, lower cost, and
much higher performance than possible just a few
years ago.
FPGA/PGA
Many manufacturers of programmable gate arrays
require high speed nonvolatile memory to load con-
figuration table data into their programmable arrays.
Combining the high speed and density of Gate Array
devices from Xylinx and Altera, with the advanced
capability of the Simtek 32K x 8 family of nonvolatile
memories, opens the door to many new embedded
applications. Adaptive security systems, video/tele-
com Digi-Cypher, and automotive theft protection
and recovery systems are just a few of the potential
applications for embedded PGA/ nvSRAM systems
(Figure 4).
Altera
EPF8150
0
(EPLD)
V
f
< 0.3V
I
f
> 500mA
1
28
27
68µF
6v, ±20%
+
0.1µF
Bypass
10kΩ
14
Data Bus
DQ
0
-DQ
7
Simtek
STK11C88
nvSRAM
Address Bus
A
0
-A
14
G
Figure 3
Alternate STK15C88
AutoStore™
Implementation
CONF_DONE
nCONFIG
STK16C88 Nonvolatile Operations
The STK16C88 is a close relation to the STK15C88.
Its software
STORE
and
RECALL
operations, and
its power-up
RECALL
capabilities are identical. The
AutoStorePlus™
feature of the STK16C88 provides
automatic
STORE
of SRAM data to EEPROM on
power-down that is independent of the power supply
decay rate. The
AutoStorePlus™
is initiated 1µs
after the detection of power failure (the transition
through V
SWITCH
) and does not depend upon the
power supply to complete the
STORE
cycle. It is the
ideal standard pin-out solution in systems whose
power supply decays very quickly. To a design engi-
neer who cannot guarantee the 10ms required by
the STK15C88 to execute a
STORE
on power-
down, the STK16C88 is a simpler solution requiring
no additional components.
W
E
System Reset
System Reset must be long
enough for STK11C88 to
RECALL data (550us max)
Processor Reset must not be
released until EPLD has
completed load
R/W
Embedded
Processor
I/O
Processor Board
Figure 4
Altera EPF81500 EPLD in a parallel load
configuration with the SIMTEK STK11C88
256K “Fast” nvSRAM
By using a single STK11C88 nvSRAM to parallel
load configuration data to an Altera EPLD, and by
using the same memory as a fast SRAM for proces-
sor scratch pad RAM memory, the design engineer
can realize a significant reduction in parts count and
cost. At system power-up, the EPF81500 is held in a
8-23
Applying the STK11C/15C/16C88
reset condition for a minimum of 550µs after V
CC
reaches V
SWITCH
(about 4.25V). This allows suffi-
cient time for the STK11C88 to Recall data from its
nonvolatile memory into SRAM. After system reset
is released the EPF81500 will request data from the
STK11C88 (pull CONF_DONE line low) and begin
sending addresses in a sequential count up or count
down manner. After data is loaded into the EPLD, it
will release the CONF_DONE line allowing the
STK11C88 to be accessed by the embedded micro-
processor. It is required that the microprocessor be
held in reset until the EPF81500 is finished loading.
This simple approach solves the problem of needing
both fast SRAM scratch pad memory and nonvola-
tile memory to load the Altera device.
tile SRAMs efficiently replaces Battery Backed
RAM, and results in higher reliability and higher per-
formance products. The “C88” nvSRAM family also
replaces the combination of SRAM and EEPROM
presently used in many stand-alone applications to
get around Battery Backed RAM's inherent limita-
tions. The resulting savings in board space, power,
and cost makes the nvSRAM an attractive design
option.
RD
WE
16 Bit Data Bus
W
G
W
G
nvSRAM and TI-TMS320C52
Use of Simtek’s 256K parts allows the TMS320C52
to run at its fully rated 80 megahertz with no wait-
states. This design approach also assures that all
calculated or measured control variables are saved
in the event of power failure. No glue logic or inter-
face components are required for systems of up to
96K words of nonvolatile memory, and only minimal
external logic for systems of up to 192K words. This
compact, efficient memory design simplifies PCB
layout, improves system reliability, and reduces
cost. Since memory data is safe when power fails,
short VCC drop-outs will not necessarily result in
loss of control or require system re-boot. If the on-
board real-time clock maintains nonvolatile elapsed
time knowledge, recovery may be possible without
loss of control or unstable operation.
Use of the STK11C88 for program memory guaran-
tees that control code will not be lost during power
dropouts, but still allows code changes to be made
in-circuit without special equipment or hardware
modifications. This is of special value in applications
such as security systems where periodic updates to
password files or access lists are required, but
where nonvolatility is a necessity (Figure 5).
Texas
Instruments
TMS320C52
(DSP)
Simtek
STK15C88
AutoStore
TM
Data
Memory
E
Simtek
STK11C88
Program
Memory
E
Address Bus (A0-A14)
DS
PS
IS
A15
DECODER
Figure 5
High Performance Embedded DSP System Using
the TMS320C52 with the Simtek STK11C/15C88
Simtek's nvSRAMs also have many advantages
over fast FLASH memory, including a single byte
write, single operating voltage (5V), faster access
times, and unlimited SRAM Write cycles. nvSRAMs
also have the ability to perform Store operations
autonomously without processor intervention or the
requirement to run FLASH store algorithms. This
results in less processor overhead, reduced parts
count and higher product reliability.
The Simtek “C88” family of nonvolatile memories is
fast, versatile, and cost effective for many types of
modern high performance designs. The combination
of speed and nonvolatility in one package allows
high technology processing engines to operate at
their full potential. The added advantage of remote
programmability without the need to modify hard-
ware, simplifies field upgrades and lengthens prod-
uct lifetimes.
Conclusion
Applications for Battery Backed RAM are limited by
the temperature range over which batteries operate
reliably. Lithium batteries have shortened life spans
at high temperature and electro-chemical inefficien-
cies at low temperature. Battery Backed RAMs are
also slower than nvSRAMs, and therefore are not
suited to many high performance embedded proces-
sor applications. Use of Simtek’s family of nonvola-
8-24
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