Cypress STK17T88 manual Software Store, Noise Considerations, Preventing AutoStore, Best Practices

STK17T88

Software STORE

Data can be transferred from the SRAM to the nonvolatile memory by a software address sequence. The STK17T88 software STORE cycle is initiated by executing sequential E controlled READ cycles from six specific address locations in exact order. During the STORE cycle, previous data is erased and then the new data is programmed into the nonvolatile elements. Once a STORE cycle is initiated, further memory inputs and outputs are disabled until the cycle is completed.

To initiate the software STORE cycle, the following READ sequence must be 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

If the STK17T88 is in a WRITE mode (both E and W low) at power up, after a RECALL, or after a STORE, the WRITE is inhibited until a negative transition on E or W is detected. This protects against inadvertent writes during power up or brown out conditions.

Noise Considerations

The STK17T88 is a high-speed memory and so must have a high-frequency bypass capacitor of 0.1 µF connected between both VCC pins and VSS ground plane with no plane break to chip VSS. Use 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.

Preventing AutoStore

Because of the use of nvSRAM to store critical RTC data, the AutoStore function can not be disabled on the STK17T88.

Best Practices

Once the sixth address in the sequence has been entered, the STORE cycle commences and the chip is disabled. It is important that READ cycles and not WRITE cycles be used in the sequence. After the tSTORE cycle time has been fulfilled, the SRAM is again activated for READ and WRITE operation.

Software RECALL

Data can be 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 man- ner similar to the software STORE initiation. To initiate the RECALL cycle, the following sequence of E controlled READ operations must be 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 second, the nonvolatile information is trans- ferred into the SRAM cells. After the tRECALL cycle time, the SRAM is again ready for READ or WRITE operations. The RECALL operation in no way alters the data in the nonvolatile storage elements.

Data Protection

The STK17T88 protects data from corruption during low-voltage conditions by inhibiting all externally initiated STORE and WRITE operations. The low-voltage condition is detected when

VCC<VSWITCH.

nvSRAM products have been used effectively for over 15 years. While ease-of-use is one of the product’s main system values, experience gained working with hundreds of applications has resulted in the following suggestions as best practices:

■The nonvolatile cells in an nvSRAM are programmed on the test floor during final test and quality assurance. Incoming inspection routines at customer or contract manufacturer’s sites sometimes reprograms these values. Final NV patterns are typically repeating patterns of AA, 55, 00, FF, A5, or 5A. End product’s firmware should not assume an NV array is in a set programmed state. Routines that check memory content values to determine first time system configuration, cold or warm boot status, etc. should always program a unique NV pattern (e.g., complex 4-byte pattern of 46 E6 49 53 hex or more random bytes) as part of the final system manufacturing test to ensure these system routines work consistently.

■Power up boot firmware routines should rewrite the nvSRAM into the desired state (autostore enabled, etc.). While the nvSRAM is shipped in a preset state, best practice is to again rewrite the nvSRAM into the desired state as a safeguard against events that might flip the bit inadvertently (program bugs, incoming inspection routines, etc.).

■The OSCEN bit in the Calibration register at 0x7FF8 should be set to 1 to preserve battery life when the system is in storage (see Stopping and Starting the RTC Oscillator on page 14.

■The VCAP value specified in this datasheet includes a minimum and a maximum value size. Best practice is to meet this requirement and not exceed the max VCAP value because the nvSRAM internal algorithm calculates VCAP charge time based on this max Vcap value. Customers that want to use a larger VCAP value to make sure there is extra store charge and store time should discuss their Vcap size selection with Cypress to understand any impact on the VCAPvoltage level at the end of a tRECALL period.

Document Number: 001-52040 Rev. *A Page 13 of 22

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