STK22C48
July 1999 3-27
The STK22C48 has two separate modes of operation:
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 EEPROM (the STORE operation) or
from
EEPROM to SRAM (the RECALL operation). In
this mode
SRAM functions are disabled.
NOISE CONSIDERATIONS
The STK22C48 is a high-speed memory and so
must have a high-frequency bypass capacitor of
approximately 0.1µF connected between V
CAP
and
V
SS
, using leads and traces that are as short as pos-
sible. As with all high-speed
CMOS ICs, nor ma l car e-
ful routing of power, ground and signals will help
prevent noise problems.
SRAM READ
The STK22C48 performs a READ cycle whenever E
and G are low and W and HSB are high. The
address specified on pins A
0-10
determines which of
the 2,048 data bytes will be accessed. When the
READ is initiated by an address transition, the out-
puts will be valid after a delay of t
AVQV
(READ cycle
#1). If the
READ is initiated by E or G, the output s will
be valid at t
ELQV
or at t
GLQV
, whichever is later (READ
cycl e #2 ). The data outputs wi ll repe atedly re spond
to address changes within the t
AVQV
access time without the need for transitions on any control input pins,
and will remain val id unti l another address change or
until E
or G is b rou ght hi gh, o r W or HSB is brou ght
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
DQ
0-7
will be written into the memory if it is valid t
DVWH
before the end of a W controlled WRITE or t
DVEH
before the end of an E c ontrolled 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 t
WLQZ
after W goes low.
POWER-UP RECALL
During power up, or after any low-power condition
(V
CAP
< V
RESET
), an internal RECALL request will be
latched. When V
CAP
once again exceeds the sense
voltage of V
SWITCH
, a RECALL cycle will automatically
be initiated and will take t
RESTORE
to compl e te .
If the STK22C48 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
V
CC
or between E and system VCC.
AutoStore™ OPERATION
The STK22C48 can be powered in one of three
modes.
During normal AutoStore™ operation, the
STK22C48 will draw current from V
CCX
to charge a
capacitor connected to the V
CAP
pin. This stored
charge will be used by the chip to perform a single
STORE operation. After power up, when the voltage
on the V
CAP
pin drops below V
SWITCH
, the part will
automatically disconnect the V
CAP
pin from V
CCX
and
initiate a
STORE operation.
Figure 2 shows the proper connection of capacitors
for automatic store operation. A charge storage
capacitor having a capacity of between 68µF and
220µF (± 20%) rated at 6V should be provided.
In system power mode (Figure 3), both V
CCX
and
V
CAP
are connected to the + 5V power supply without
the 68µF capacitor. In this mode the AutoStore™
function of the STK22C48 will operate on the stored
system charge as power goes down. The us er must ,
however, guarantee that V
CCX
does not drop below
3.6V during the 10ms
STORE cycle.
If an automatic
STORE on power loss is not required,
then V
CCX
can be tied to ground and + 5V applied to
V
CAP
(Figure 4). This is the AutoStore™ Inhibit
mode, in which the AutoStore™ function is disabled.
If the STK22C48 is operated in this configuration,
references to V
CCX
should be changed to V
CAP
throughout this data sheet. In this mode, STORE
operations may be triggered with the HSB pin. It is
not permissable to change between these three
options “on the fly”.
DEVICE OPERATION