Cypress STK11C68 manual Device Operation, Sram Read, Sram Write, Software Store, Software Recall

STK11C68

Device Operation

The STK11C68 is a versatile memory chip that provides several modes of operation. The STK16C88 can operate as a standard 8K x 8 SRAM. A 8K x 8 array of nonvolatile storage elements shadow the SRAM. SRAM data can be copied nonvolatile memory or nonvolatile data can be recalled to the SRAM.

SRAM Read

The STK11C68 performs a Read cycle whenever CE and OE are LOW while WE is HIGH. The address specified on pins A0–12determines the 8,192 data bytes accessed. When the Read is initiated by an address transition, the outputs are valid after a delay of tAA (Read cycle 1). If the Read is initiated by CE or OE, the outputs are valid at tACE or at tDOE, whichever is later (Read cycle 2). The data outputs repeatedly respond to address changes within the tAA 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, or WE brought LOW.

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 DQ0–7are written into the memory if it has valid tSD, before the end of 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 tHZWE after WE goes LOW.

Software STORE

Data is transferred from the SRAM to the nonvolatile memory by a software address sequence. The STK11C68 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. 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

4.Read address 0x1FFF, Valid READ

5.Read address 0x10F0, Valid READ

6.Read address 0x0F0E, Initiate RECALL cycle

Internally, RECALL is a two step procedure. First, the SRAM data is cleared; then, the nonvolatile information is transferred into the SRAM cells. After the tRECALL cycle time, the SRAM is again ready for Read and Write operations. The RECALL operation does not alter the data in the nonvolatile elements. The nonvol- atile data can be recalled an unlimited number of times.

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 STK11C68 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.

Hardware Protect

The STK11C68 offers hardware protection against inadvertent STORE operation and SRAM Writes during low voltage condi- tions. When VCAP<VSWITCH, all externally initiated STORE operations and SRAM Writes are inhibited.

Noise Considerations

The STK11C68 is a high speed memory. It must have a high frequency bypass capacitor of approximately 0.1 µF connected between VCC and VSS, using leads and traces that are as short as possible. As with all high speed CMOS ICs, careful routing of power, ground, and signals reduce circuit noise.

Low Average Active Power

CMOS technology provides the STK11C68 the benefit of drawing significantly less current when it is cycled at times longer than 50 ns. Figure 2 shows the relationship between ICC and Read or Write cycle time. Worst case current consumption is shown for both CMOS and TTL input levels (commercial temper- ature range, VCC = 5.5V, 100% duty cycle on chip enable). Only standby current is drawn when the chip is disabled. The overall

Document Number: 001-50638 Rev. ** Page 3 of 16

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