1 Reply Latest reply on Jul 1, 2011 1:55 AM by ravip_71

    Software Store as the power fails





      I have a very old circuit using UL634H256 nvSRAM's that has been laid out to use the hardware store and has a 68uf capacitor for Vcap. If I start a software store using a power fail signal I am getting corruption of the EEdata.     



      If I monitor the Vcap line it never gets disconnected from the 3.3 volt supply so drops quickly. The chip does not get the time to complete. See scope plot. The pink trace falls at the start of the store. Blue is supply and green is Vcap   


      Is this correct that Vcap only gets disconnected from the supply if a hardware store is going to be performed or every time Vcc drops below Vswitch?



            I have tested this on a few boards and some corrupt and some do not but all the signal timings look identical. Any thoughts on why some could be ok.     







        • 1. Re: Software Store as the power fails

          Hi Colin,


          It has been seen in the legacy parts that if power fails during an ongoing store (software or hardware store) the VCAP charging circuit does not get disconnected from VCC and thus tracks VCC. This would result in VCAP getting discharged and consequently, in incomplete store. This is what you are seeing. Hence, you should allow the device to perform autostore instead of initiating a store based on power fail detect.


          Alternatively, you can change over to the Cypress nvSRAMs in 130nm technology which are in production. The 256Kb part, CY14B256LA is pin compatible with the legacy part.


          In these parts, an ongoing store will be powered from VCC and if power fails, from VCAP - when VCC crosses below VSWITCH, VCAP gets disconnected from VCC and powers the store circuit for completion of the ongoing store. 


          Please refer to App note AN55662 when converting from old technology parts to CY14B256LA