What should I consider if my system is experiencing voltage irregularities?
It depends; some you can ignore, while for some you need to do Power-On Reset (POR) or an alternate recovery that works like POR.
Usually, overshoots and undershoots taking less than 20 ns are not visible to the flash memory. However, for other types of voltage drops and power-down, specific recoveries need to be done.
Power-Down / Voltage drop requirements
If voltage fluctuations stay above VLKO, the part is considered to be powered on so it will work correctly when VCC returns to the normal operating range.
However any time the flash supply voltage drops below VCC Lockout (VLKO), the flash is considered to be powered off. In this case, the best practice is to perform a full POR. For Cypress flash devices, this means that the supply voltage, VCC, must drop below VRST for a period of time tPD for the part to initialize correctly when VCC again rises to the normal operating range. Other forms of reset, like toggling the hardware reset line RESET# or sending the software reset command – are different from POR so they are not appropriate recoveries for this situation.
Some Cypress NOR flash memories, like the 65-nm HyperFlash™ and HyperRAM™ memories, also support the Deep Power Down (DPD) mode, which can be used as part of an alternative method for POR. Therefore, for a power dip where VCC drops below VLKO, but where full POR is not achieved before VCC returns to the normal operating range, you can use the following workaround:
While VCC > VCC(MIN), do the following:
- Toggle the RESET# signal to initiate Hardware Reset.
- Execute the DPD Entry command.
- Exit DPD mode via RESET#.
This recovery is fully equivalent to a valid POR for any parallel NOR, serial NOR, HyperFlash or HyperRAM that supports the DPD mode, except for the S25FS-S family of serial NOR flash because the DPD Exit does not perform a full power-up like POR.