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Device Operation

The STK15C88 is a versatile memory chip that provides several

modes of operation. The STK15C88 can operate as a standard

32K x 8 SRAM. It has a 32K x 8 nonvolatile element shadow to

which the SRAM information can be copied, or from which the

SRAM can be updated in nonvolatile mode.

SRAM Read

The STK15C88 performs a READ cycle whenever CE and OE

are LOW while WE is HIGH. The address specified on pins A

determines the 32,768 data bytes accessed. When the READ is

initiated by an address transition, the outputs are valid after a

delay of t

(READ cycle 1). If the READ is initiated by CE or OE,

the outputs are valid at t

or at t

ACE

cycle 2). The data outputs repeatedly respond to address

changes within the t

access time without the need for transi-

tions on any control input pins, and remains valid until another

address change or until CE or OE is brought HIGH.

SRAM Write

A WRITE cycle is performed whenever CE and WE are LOW.

The address inputs must be stable prior to entering the WRITE

cycle and must remain stable until either CE or WE goes HIGH

at the end of the cycle. The data on the common IO pins DQ

are written into the memory if it has valid t

a WE controlled WRITE or before the end of an CE controlled

WRITE. Keep OE HIGH during the entire WRITE cycle to avoid

data bus contention on common IO lines. If OE is left LOW,

internal circuitry turns off the output buffers t

LOW.

AutoStore Operation

The STK15C88 uses the intrinsic system capacitance to perform

an automatic STORE on power down. As long as the system

power supply takes at least t

STORE

to 3.6V, the STK15C88 will safely and automatically store the

SRAM data in nonvolatile elements on power down.

In order to prevent unneeded STORE operations, automatic

STOREs will be ignored unless at least one WRITE operation

has taken place since the most recent STORE or RECALL cycle.

Software initiated STORE cycles are performed regardless of

whether a WRITE operation has taken place.

Hardware RECALL (Power Up)

During power up or after any low power condition (V

), an internal RECALL request is latched. When V

RESET

once again exceeds the sense voltage of V

cycle is automatically initiated and takes t

If the STK15C88 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 V

or between CE and system V

Document Number: 001-50593 Rev. **

0–14

, whichever is later (READ

DOE

0–7

, before the end of

after WE goes

HZWE

to decay from V

down

SWITCH

, a RECALL

SWITCH

to complete.

HRECALL

Software STORE

Data is transferred from the SRAM to the nonvolatile memory by

a software address sequence. The STK15C88 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 0x0E38, Valid READ

2. Read address 0x31C7, Valid READ

3. Read address 0x03E0, Valid READ

4. Read address 0x3C1F, Valid READ

5. Read address 0x303F, Valid READ

6. Read address 0x0FC0, Initiate STORE cycle

The software sequence is clocked with CE 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

cycle time is fulfilled, the SRAM is again activated for

STORE

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 0x0E38, Valid READ

2. Read address 0x31C7, Valid READ

3. Read address 0x03E0, Valid READ

4. Read address 0x3C1F, Valid READ

5. Read address 0x303F, Valid READ

6. Read address 0x0C63, Initiate RECALL cycle

Internally, RECALL is a two step procedure. First, the SRAM data

is cleared, and then the nonvolatile information is transferred into

the SRAM cells. After the t

RECALL

again ready for READ and WRITE operations. The RECALL

operation does not alter the data in the nonvolatile elements. The

nonvolatile data can be recalled an unlimited number of times.

STK15C88

cycle time, the SRAM is once

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