Cypress Semiconductor STK14C88-3 Manual do Utilizador Página 3
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AN55662
October 5, 2009 Document No. 001-55662 Rev. ** 3
Critical Considerations
The impact of the differences in CY14B256LA with respect
to the STK14C88-3 in existing applications are discussed
as follows. Board designers are recommended to review the
detailed datasheets when converting to the new part.
DC Parameters
The power supply design in most applications with
STK14C88-3 would require no changes when replacing the
nvSRAM with the CY14B256LA in spite of the higher values
in the operating / higher standby current. The critical
parameters to consider are the V
CAP
and the V
SWITCH
V
CAP
While most of the differences do not impact the application,
the difference in V
CAP
is a critical consideration while
converting from the older rev parts. V
CAP
is the capacitor
which provides the required charge for AutoStore to
complete NV store of the SRAM data during power down.
The required capacitor range is different in the two parts.
Table 6. V
CAP
Comparison
Description
STK14C88-3 CY14B256LA
V
CAP
54 uF to 264 uF
61 uF to 180 uF
Therefore, any existing application using a capacitor value
outside the overlapping range for the capacitor values needs
to consider the impact of capacitor dimensions while
changing to the new capacitor. Note: The capacitor range is
the absolute value of the capacitor, net of tolerence.
V
SWITCH
The STK14C88-3 operates from 3V while the CY14B256LA
operates from 2.7V. Hence the V
SWITCH
levels are different.
Table 7. V
CC
, V
SWITCH
Comparison
Description
STK14C88-3 CY14B256LA
V
CC
3.0V to 3.6V
2.7V to 3.6V
V
SWITCH
2.7V to 2.95V
<2.65V
The difference in levels do not affect most applications
except where the controller has operating range from 3.0V
and above. In such applications, if CY14B256LA is used to
replace STK14C88-3, at voltage levels below 3V, the
controller could be tristated and hence CY14B256LA could
write invalid data into the SRAM. This is because the
CY14B256LA would be active up to 2.7V. In such
application, it is necessary that the controller issues a reset
when the V
CC
crosses below 3V and remains LOW as long
as the controller is inactive. This would initiate a Hardware
STORE in the CY14B256LA and the nvSRAM outputs would
be in tristate as long as the
HSB
is held LOW by the
controller.
AC Switching Parameters
There are a few minor differences in switching parameters
between the CY14B256LA and the STK14C88-3, as listed in
the Table 5. However, these differences do not impact most
applications. For replacing 35ns speed parts, choose the
25ns speed parts as replacement (since 35ns speed grade
is not available in the CY14B256LA).
AutoStore/Power Up RECALL Parameters
t
HRECALL
The power up RECALL is much different in the
CY14B256LA compared to the STK14C88-3 because of
architecture differences.
Table 8. t
HRECALL
Comparison
Description
STK14C88-3 CY14B256LA
t
HRECALL
550 us
20 ms
This difference is not likely to affect applications since the
initialization of the controller on the board happens at the
same time. However, this should be taken into consideration
when replacing the STK14C88-3 with CY14B256LA.
Software Controlled STORE/RECALL Cycle
Parameters
The Software cycle parameter t
RECALL
is different in
CY14B256LA as described in the following section.
t
RECALL
Software RECALL time (t
RECALL
) is higher in CY14B256LA.
Table 9. t
RECALL
Comparison
Description
STK14C88-3 CY14B256LA
t
RECALL
20 us
200 us
This difference could require firmware change in the
application to increase the wait state when software
RECALL is initiated.
Software Sequence
The CY14B256LA is designed to be compatible with
STK14C88-3 in the software sequence modes. Hence, the
same Software STORE and RECALL address sequences in
STK14C88-3 works in CY14B256LA, requiring no firmware
change.
Hardware STORE Cycle Parameters
The Hardware STORE parameters are much improved in
the CY14B256LA. The improvements are listed under the
following section on Details of Improvement. No changes
are required in applications.
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