Device Operation, Sram Read, Sram Write

Device Operation

The STK12C68 nvSRAM is made up of two functional compo- nents paired in the same physical cell. These are an SRAM memory cell and a nonvolatile QuantumTrap cell. The SRAM memory cell operates as a standard fast static RAM. Data in the SRAM is transferred to the nonvolatile cell (the STORE operation) or from the nonvolatile cell to SRAM (the RECALL operation). This unique architecture enables the storage and recall of all cells in parallel. During the STORE and RECALL operations, SRAM Read and Write operations are inhibited. The STK12C68 supports unlimited reads and writes similar to a typical SRAM. In addition, it provides unlimited RECALL opera- tions from the nonvolatile cells and up to one million STORE operations.

SRAM Read

The STK12C68 performs a Read cycle whenever CE and OE are LOW while WE and HSB are 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 or HSB is brought LOW.

SRAM Write

A Write cycle is performed whenever CE and WE are LOW and HSB is HIGH. 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.

AutoStore Operation

The STK12C68 stores data to nvSRAM using one of three storage operations:

1.Hardware store activated by HSB

2.Software store activated by an address sequence

3.AutoStore on device power down

AutoStore operation is a unique feature of QuantumTrap technology and is enabled by default on the STK12C68.

Document Number: 001-51027 Rev. **

STK12C68

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 2 shows the proper connection of the storage capacitor (VCAP) for automatic store operation. A charge storage capacitor between 68 µF and 220 µF (+20%) rated at 6V should be provided. The voltage on the VCAP pin is driven to 5V by a charge pump internal to the chip. A pull up is placed on WE to hold it inactive during power up.

Figure 2. AutoStore Mode

9&$3	9FF	N2KP	N2KP
	:(	N2KP	N2KP
	+6%

5) Y				5) %\SDVV

9VV

In system power mode, both VCC and VCAP are connected to the +5V power supply without the 68 μF capacitor. In this mode, the AutoStore function of the STK12C68 operates on the stored system charge as power goes down. The user must, however, guarantee that VCC does not drop below 3.6V during the 10 ms STORE cycle.

To reduce unnecessary 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. An optional pull up resistor is shown connected to HSB. The HSB signal is monitored by the system to detect if an AutoStore cycle is in progress.

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