STK12C68
DEVICE OPERATION
The STK12C68 has two separate modes of opera-
tion: SRAM mode and nonvolatile mode. In SRAM
mode, the memory operates as a standard fast
static RAM. In nonvolatile mode, data is transferred
from SRAM to Nonvolatile Elements (the STORE
operation) or from Nonvolatile Elements to SRAM
(the RECALL operation). In this mode SRAM func-
tions are disabled.
POWER-UP RECALL
During power up, or after any low-power condition
(VCAP < VRESET), an internal RECALL request will be
latched. When VCAP once again exceeds the sense
voltage of VSWITCH, a RECALL cycle will automatically
be initiated and will take tRESTORE to complete.
If the STK12C68 is in a WRITE state at the end of
power-up RECALL, the SRAM data will be corrupted.
To help avoid this situation, a 10K Ohm resistor
should be connected either between W and system
NOISE CONSIDERATIONS
The STK12C68 is a high-speed memory and so
must have a high-frequency bypass capacitor of
approximately 0.1μF connected between VCAP and
VSS, using leads and traces that are as short as pos-
sible. As with all high-speed CMOS ICs, normal care-
ful routing of power, ground and signals will help
prevent noise problems.
VCC or between E and system VCC.
SOFTWARE NONVOLATILE STORE
The STK12C68 software STORE cycle is initiated by
executing sequential E controlled READ cycles from
six specific address locations. During the STORE
cycle an erase of the previous nonvolatile data is
first performed, followed by a program of the nonvol-
atile elements. The program operation copies the
SRAM data into nonvolatile memory. Once a STORE
cycle is initiated, further input and output are dis-
abled until the cycle is completed.
SRAM READ
The STK12C68 performs a READ cycle whenever E
and G are low and W and HSB are high. The
address specified on pins A0-12 determines which of
the 8,192 data bytes will be accessed. When the
READ is initiated by an address transition, the out-
puts will be valid after a delay of tAVQV (READ cycle
#1). If the READ is initiated by E or 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 E or G is brought high, or W or HSB is
brought low.
Because a sequence of READs from specific
addresses is used for STORE initiation, it is impor-
tant that no other READ or WRITE accesses inter-
vene in the sequence, or the sequence will be
aborted and no STORE or RECALL will take place.
To initiate the software STORE cycle, the following
READ sequence must be performed:
1. Read address
2. Read address
3. Read address
4. Read address
5. Read address
6. Read address
0000 (hex)
1555 (hex)
0AAA (hex)
1FFF (hex)
10F0 (hex)
0F0F (hex)
Valid READ
Valid READ
Valid READ
Valid READ
Valid READ
Initiate STORE cycle
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 the memory if it is valid tDVWH
before the end of a W controlled WRITE or tDVEH
before the end of an E controlled WRITE.
The software sequence must be clocked with E con-
trolled READs.
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.
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.
March 2006
8
Document Control # ML0008 rev 0.5
CYPRESS [ CYPRESS ]
