STK12C68, STK12C68-5 (SMD5962-94599)
If HSB is not used, it should be left unconnected.
HSB OPERATION
The STK12C68, STK12C68-5 provides the HSB pin
for controlling and acknowledging the STORE opera-
tions. The HSB pin is used to request a hardware
STORE cycle. When the HSB pin is driven low, the
STK12C68, STK12C68-5 will conditionally initiate a
STORE operation after tDELAY; an actual STORE cycle
will only begin if a WRITE to the SRAM took place
since the last STORE or RECALL cycle. The HSB pin
has a very resistive pullup and is internally driven
low to indicate a busy condition while the STORE
(initiated by any means) is in progress.
PREVENTING STORES
The STORE function can be disabled on the fly by
holding HSB high with a driver capable of sourcing
30mA at a VOH of at least 2.2V, as it will have to
overpower the internal pull-down device that drives
HSB low for 20μs at the onset of a STORE. When
the STK12C68, STK12C68-5 is connected for
AutoStore operation (system VCC connected to VCC
and a 68μF capacitor on VCAP) and VCC crosses
VSWITCH on the way down, the STK12C68,
STK12C68-5 will attempt to pull HSB low; if HSB
doesn’t actually get below VIL, the part will stop try-
ing to pull HSB low and abort the STORE attempt.
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 STK12C68,
STK12C68-5 will continue SRAM operations for tDE-
HARDWARE PROTECT
The STK12C68, STK12C68-5 offers hardware pro-
tection against inadvertent STORE operation and
SRAM WRITEs during low-voltage conditions. When
VCAP < VSWITCH, all externally initiated STORE opera-
tions and SRAM WRITEs are inhibited.
. During tDELAY, multiple SRAM READ operations
LAY
may take place. If a WRITE is in progress when HSB
is pulled low it will be allowed a time, tDELAY, to com-
plete. However, any SRAM WRITE cycles requested
after HSB goes low will be inhibited until HSB
returns high.
AutoStore can be completely disabled by tying VCC
to ground and applying + 5V to VCAP. This is the
AutoStore Inhibit mode; in this mode, STOREs are
only initiated by explicit request using either the soft-
ware sequence or the HSB pin.
The HSB pin can be used to synchronize multiple
STK12C68, STK12C68-5s while using a single
larger capacitor. To operate in this mode the HSB
pin should be connected together to the HSB pins
from the other STK12C68, STK12C68-5s. An exter-
nal pull-up resistor to + 5V is required since HSB
acts as an open drain pull down. The VCAP pins from
the other STK12C68, STK12C68-5 parts can be tied
together and share a single capacitor. The capacitor
size must be scaled by the number of devices con-
nected to it. When any one of the STK12C68,
STK12C68-5s detects a power loss and asserts
HSB, the common HSB pin will cause all parts to
request a STORE cycle (a STORE will take place in
those STK12C68, STK12C68-5s that have been
written since the last nonvolatile cycle).
LOW AVERAGE ACTIVE POWER
The STK12C68, STK12C68-5 draws significantly
less current when it is cycled at times longer than
50ns. Figure 4 shows the relationship between ICC
and READ cycle time. Worst-case current consump-
tion is shown for both CMOS and TTL input levels
(commercial temperature range, VCC = 5.5V, 100%
duty cycle on chip enable). Figure 5 shows the
same relationship for WRITE cycles. If the chip
enable duty cycle is less than 100%, only standby
current is drawn when the chip is disabled. The
overall average current drawn by the STK12C68,
STK12C68-5 depends on the following items: 1)
CMOS vs. TTL input levels; 2) the duty cycle of chip
enable; 3) the overall cycle rate for accesses; 4) the
ratio of READs to WRITEs; 5) the operating tempera-
ture; 6) the Vcc level; and 7) I/O loading.
During any STORE operation, regardless of how it
was initiated, the STK12C68, STK12C68-5 will con-
tinue to drive the HSB pin low, releasing it only when
the STORE is complete. Upon completion of the
STORE operation the STK12C68, STK12C68-5 will
remain disabled until the HSB pin returns high.
Rev 2.0
Document Control #ML0008
June, 2008
11
SIMTEK [ SIMTEK CORPORATION ]
