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Non Volatile RAM (F-RAM & NVSRAM)


The nvSRAM has a specified tHRECALL (Power up RECALL duration) time for boot up / RECALL of data. When user powers up the device, at the end of the tHRECALL period, the device is ready for access. Which means user can perform read/writes. The tHRECALL completion is signaled at the HSB# (Hardware Store Busy) pin and user can start read/write based on HSB# going HIGH in case user is monitoring HSB# pin. HSB# will go HIGH in <= tHRECALL time. Since there is no restriction on what user can do after the part is ready, let us say user performs a write as soon as the part completes the power up RECALL and immediately powers down the part (planned on unplanned power down). The device will perform an AutoStore using the charge from the VCAP pin and it is guaranteed that the AutoStore will be successful under this condition also.


If user now has a capacitor on VCAP pin which is outside the spec, consider a 330uF which is higher than the datasheet spec limit of 180uF max, the capacitor would have charged to a lesser voltage than what would be the level if 180uF was on the pin. This would affect the AutoStore operation completion as described below.


During power down, when the VCC crosses VSWITCH, the power to the internal store circuit switches to the VCAP pin. From then on VCAP charge provides the power required to complete the AutoStore. Assume VSWITCH for the part is 2.4V and the store circuit needs to have a minimum voltage of 2.0V on VCAP to complete the STORE, then the capacitor value should be such that it accumulates enough charge on it to complete the AutoStore operation in tSTORE time (8 ms) while it discharges from 2.4 V to 2.0 V. In the case of higher capacitor, the starting voltage would be lower (since it was charged for the same tHRECALL time from the same internal charging circuit). However it still may accumulate sufficient charges to complete the AutoStore in time which is determined by the value of capacitor and its voltage level above 2V. On the worst case side, assume the capacitor did not even reach 2V because the capacitor value is too high. Then there will be a complete failure of AutoStore. Therefore, the higher value spec for VCAP is specified to guarantee that AutoStore will be successful if it is required to be done as soon as at the end of tHRECALL (= 20ms). If user still needs to use 330uF, user needs to ensure that AutoStore is not going to be required immediately after power up and ensure that there is sufficiently longer time before performing a write operation. (If there is no write operation after a NV operation, there will be no initiation of AutoStore since the NV data is same as the SRAM data). That is, give additional power up time before starting access, say add 20ms more before starting access.


Also note that the typical value mentioned in the datasheet is the minimum cap value with 10% tolerance which will ensure AutoStore will always be completed for any part across PVT (Process, Voltage, Temperature) range. And minimum value is the value net of tolerance (the minimum absolute value) which will ensure that AutoStore is successful across PVT. Please note that the performance of the part is the same if any value of capacitor across the min to max specification is used.

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