Device Operation
The STK14C88-3 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 STK14C88-3 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 STK14C88-3 performs a READ cycle whenever CE and OE are LOW while WE and HSB are HIGH. The address specified on pins A0–14determines the 32,768 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 transitions 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 STK14C88-3 can be powered in one of three storage opera- tions:
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 having a capacity of between 68 uF and 220 uF (+20%) rated at 4.7V should be provided.
To reduce unnecessary nonvolatile stores, AutoStore and Hardware Store operations are ignored, unless at least one
Figure 2. AutoStore Mode
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.
If the power supply drops faster than 20 us/volt before Vcc reaches VSWITCH, then a 1 ohm resistor should be connected between VCC and the system supply to avoid momentary excess of current between VCC and VCAP.
AutoStore Inhibit Mode
If an automatic STORE on power loss is not required, then VCC is tied to ground and +3.3V is applied to VCAP (Figure 3). This is the AutoStore Inhibit mode, where the AutoStore function is disabled. If the STK14C88-3 is operated in this configuration, references to VCC are changed to VCAP throughout this data sheet. In this mode, STORE operations are triggered through software control. It is not permissible to change between these options “On the fly”.
