STK12C68-5 (SMD5962-94599)
Document Number: 001-51026 Rev. **
Page 4 of 18
Figure 4. AutoStore Inhibit Mode
If the power supply drops faster than 20 us/volt before Vcc
reaches VSWITCH, then a 2.2 ohm resistor must be connected
between VCC and the system supply to avoid momentary
excess of current between VCC and VCAP.
AutoStore Inhibit Mode
If an automatic STORE on power loss is not required, then VCC
is tied to ground and +5V is applied to VCAP (Figure 4). This is
the AutoStore Inhibit mode, where the AutoStore function is
disabled. If the STK12C68-5 is operated in this configuration,
references to VCC are changed to VCAP throughout this data
sheet. In this mode, STORE operations are triggered through
software control or the HSB pin. To enable or disable Autostore
using an IO port pin see Preventing Store on page 5. It is not
permissible to change between these three options “on the
fly”.
Hardware STORE (HSB) Operation
The STK12C68-5 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 STK12C68-5 conditionally initiates a STORE
operation after tDELAY. An actual STORE cycle only begins if a
Write to the SRAM takes place since the last STORE or
RECALL cycle. The HSB pin also 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 STK12C68-5 continues SRAM operations for tDELAY.
During tDELAY, multiple SRAM Read operations take place. If
a Write is in progress when HSB is pulled LOW, it allows a
time, tDELAY to complete. However, any SRAM Write cycles
requested after HSB goes LOW are inhibited until HSB returns
HIGH.
During any STORE operation, regardless of how it is initiated,
the STK12C68-5 continues to drive the HSB pin LOW,
releasing it only when the STORE is complete. After
completing the STORE operation, the STK12C68-5 remains
disabled until the HSB pin returns HIGH.
If HSB is not used, it is left unconnected.
Hardware RECALL (Power Up)
During power up or after any low power condition (VCC <
VRESET), an internal RECALL request is latched. When VCC
once again exceeds the sense voltage of VSWITCH, a RECALL
cycle is automatically initiated and takes tHRECALL to complete.
If the STK12C68-5 is in a Write state at the end of power up
RECALL, the SRAM data is corrupted. To help avoid this
situation, a 10 Kohm resistor is connected either between WE
and system VCC or between CE and system VCC.
Software STORE
Data is transferred from the SRAM to the nonvolatile memory
by a software address sequence. The STK12C68-5 software
STORE cycle is initiated by executing sequential CE controlled
Read cycles from six specific address locations in exact order.
During the STORE cycle, an erase of the previous nonvolatile
data is first performed followed by a program of the nonvolatile
elements. When a STORE cycle is initiated, input and output
are disabled until the cycle is completed.
Because a sequence of Reads from specific addresses is
used for STORE initiation, it is important that no other Read or
Write accesses intervene in the sequence. If they intervene,
the sequence is aborted and no STORE or RECALL takes
place.
To initiate the software STORE cycle, the following Read
sequence is performed:
1. Read address 0x0000, Valid READ
2. Read address 0x1555, Valid READ
3. Read address 0x0AAA, Valid READ
4. Read address 0x1FFF, Valid READ
5. Read address 0x10F0, Valid READ
6. Read address 0x0F0F, Initiate STORE cycle
The software sequence is clocked with CE controlled Reads
or OE controlled Reads. When the sixth address in the
sequence is entered, the STORE cycle commences and the
chip is disabled. It is important that Read cycles and not Write
cycles are used in the sequence. It is not necessary that OE
is LOW for a valid sequence. After the tSTORE cycle time is
fulfilled, the SRAM is again activated for Read and Write
operation.
Software RECALL
Data is transferred from the nonvolatile memory to the SRAM
by a software address sequence. A software RECALL cycle is
initiated with a sequence of Read operations in a manner
similar to the software STORE initiation. To initiate the
RECALL cycle, the following sequence of CE controlled Read
operations is performed:
1. Read address 0x0000, Valid READ
2. Read address 0x1555, Valid READ
3. Read address 0x0AAA, Valid READ
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