Cypress STK17T88 manual NvSRAM Operation, AutoStore Operation, Hardware Store HSB Operation, 1µF

STK17T88

nvSRAM Operation

The STK17T88 nvSRAM is made up of two functional compo- nents paired in the same physical cell. These are the SRAM memory cell and a nonvolatile QuantumTrap™ cell. The SRAM memory cell operates like a standard fast static RAM. Data in the SRAM can be transferred to the nonvolatile cell (the STORE operation), or from the nonvolatile cell to SRAM (the RECALL operation). This unique architecture allows all cells to be stored and recalled in parallel. During the STORE and RECALL opera- tions SRAM READ and WRITE operations are inhibited. The STK17T88 supports unlimited read and writes like a typical SRAM. In addition, it provides unlimited RECALL operations from the nonvolatile cells and up to 200K STORE operations.

SRAM READ

The STK17T88 performs a READ cycle whenever E and G are low while W and HSB are high. The address specified on pins A0-14determine which of the 32,768 data bytes are accessed. When the READ is initiated by an address transition, the outputs are valid after a delay of tAVQV (READ cycle #1). If the READ is initiated by E and G, the outputs are valid at tELQV or at tGLQV, whichever is later (READ cycle #2). The data outputs repeatedly respond to address changes within the tAVQV access time without the need for transitions on any control input pins, and remain valid until another address change or until E or G is brought high, or W and HSB is brought low.

Figure 13. AutoStore Mode

			VCC
VCAP	VCC	10kOhm	0.1µF
V
CAP
	W

SRAM WRITE

A WRITE cycle is performed whenever E and W are low and HSB is high. The address inputs must be stable prior to entering the WRITE cycle and must remain stable until either E or W goes high at the end of the cycle. The data on the common I/O pins DQ0-7 are written into memory if it is valid tDVWH before the end of a W controlled WRITE or tDVEH before the end of an E controlled WRITE.

It is recommended that G be kept high during the entire WRITE cycle to avoid data bus contention on common I/O lines. If G is left low, internal circuitry turns off the output buffers tWLQZ after W goes low.

AutoStore Operation

The STK17T88 stores data to nvSRAM using one of three storage operations. These three operations are Hardware Store

(activated by HSB), Software Store (activated by an address sequence), and AutoStore (on power down).

AutoStore operation, a unique feature of Cypress QuanumTrap technology that is a standard feature on the STK17T88.

During normal operation, the device draws current from VCC to charge a capacitor connected to the VCAP pin. This stored charge is used by the chip to perform a single STORE operation. If the voltage on the VCC pin drops below VSWITCH, the part automatically disconnects the VCAP pin from VCC. A STORE operation is initiated with power provided by the VCAP capacitor.

Figure 5 shows the proper connection of the storage capacitor (VCAP) for automatic store operation. Refer to the DC Character- istics table for the size of the capacitor. The voltage on the VCAP pin is driven to 5V by a charge pump internal to the chip. A pull up should be placed on W to hold it inactive during power up.

To reduce unneeded nonvolatile stores, AutoStore and Hardware Store operations are 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. The HSB signal can be monitored by the system to detect an AutoStore cycle is in progress.

Hardware STORE (HSB) Operation

The STK17T88 provides the HSB pin for controlling and acknowledging the STORE operations. The HSB pin can be used to request a hardware STORE cycle. When the HSB pin is driven low, the STK17T88 conditionally initiates a STORE operation after tDELAY. An actual STORE cycle only begins if a WRITE to the SRAM took place since the last STORE or RECALL cycle. The HSB pin has a very resistive pull up and is internally driven low to indicate a busy condition while the STORE (initiated by any means) is in progress. This pin should be externally pulled up if it is used to drive other inputs.

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 STK17T88 continues to allow SRAM operations for tDELAY. During tDELAY, multiple SRAM READ operations may take place. If a WRITE is in progress when HSB is pulled low, it is allowed a time, tDELAY, to complete. However, any SRAM WRITE cycles requested after HSB goes low will be inhibited until HSB returns high.

During any STORE operation, regardless of how it was initiated, the STK17T88 will continue to drive the HSB pin low, releasing it only when the STORE is complete. Upon completion of the STORE operation, the STK17T88 will remain disabled until the HSB pin returns high.

If HSB is not used, it should be left unconnected.

Hardware Recall (POWER UP)

During power up or after any low-power condition (VCC<VSWITCH), an internal RECALL request will be latched. When VCC once again exceeds the sense voltage of VSWITCH, a RECALL cycle is automatically initiated and takes tHRECALL to complete.

Document Number: 001-52040 Rev. *A Page 12 of 22

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