STK12C68
HSB OPERATION
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 is connected for AutoStore™ opera-
tion (system VCC connected to VCCX and a 68μF
capacitor on VCAP) and VCC crosses VSWITCH on the
way down, the STK12C68 will attempt to pull HSB
low; if HSB doesn’t actually get below VIL, the part
will stop trying to pull HSB low and abort the STORE
attempt.
The STK12C68 provides the HSB pin for controlling
and acknowledging the STORE operations. The HSB
pin is used to request a hardware STORE cycle.
When the HSB pin is driven low, the STK12C68 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 acts as an open drain
driver that is internally driven low to indicate a busy
condition while the STORE (initiated by any means)
is in progress.
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 will
HARDWARE PROTECT
The STK12C68 offers hardware protection against
inadvertent STORE operation and SRAM WRITEs
continue SRAM operations for tDELAY. During tDELAY
,
during low-voltage conditions. When VCAP < VSWITCH
,
multiple SRAM READ operations may take place. If a
WRITE is in progress when HSB is pulled low it will
be allowed a time, tDELAY, to complete. However, any
SRAM WRITE cycles requested after HSB goes low
will be inhibited until HSB returns high.
all externally initiated STORE operations and SRAM
WRITEs are inhibited.
AutoStore™ can be completely disabled by tying
VCCX 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
STK12C68s while using a single larger capacitor.
To operate in this mode the HSB pin should be con-
nected together to the HSB pins from the other
STK12C68s. An external pull-up resistor to + 5V is
required since HSB acts as an open drain pull down.
The VCAP pins from the other STK12C68 parts can
be tied together and share a single capacitor. The
capacitor size must be scaled by the number of
devices connected to it. When any one of the
STK12C68s 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
STK12C68s that have been written since the last
nonvolatile cycle).
LOW AVERAGE ACTIVE POWER
The STK12C68 draws significantly less current
when it is cycled at times longer than 50ns. Figure 5
shows the relationship between ICC and READ cycle
time. Worst-case current consumption is shown for
both CMOS and TTL input levels (commercial tem-
perature range, VCC = 5.5V, 100% duty cycle on chip
enable). Figure 6 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 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
During any STORE operation, regardless of how it
was initiated, the STK12C68 will continue to drive
the HSB pin low, releasing it only when the STORE is
complete. Upon completion of the STORE operation
the STK12C68 will remain disabled until the HSB
pin returns high.
temperature; 6) the V level; and 7) I/O loading.
cc
If HSB is not used, it should be left unconnected.
March 2006
10
Document Control # ML0008 rev 0.5
CYPRESS [ CYPRESS ]
