Choice of SRAM

The Supervisor is a self-contained device which converts standard low-power SRAMs into non-volatile memory. Most low power SRAMs on the market today can be used with both NVRAM and TIMEKEEPER® Supervisors, although there are some criteria for making the final choice of an SRAM. The SRAM must be designed so that the chip enable input disables all other inputs to the SRAM. This allows inputs to the external SRAMs to be "Don't care" once Vcc falls below Vpfd (min). The SRAM should also guarantee data retention down to Vcc = 2.0V. When using the either Supervisor, the chip enable access time must also be sufficient to meet the system needs with the chip enable (and output enable) output propagation delays included.

Data Retention Lifetime

If data retention lifetime is a critical parameter for the system, it is important to review the data retention current specifications for the particular SRAMs being evaluated. Most SRAMs specify data retention current (IccDR) at 3.0V. Manufacturers generally specify a typical condition for room temperature along with a worst case condition (generally at elevated temperatures.) The system level requirements will determine the choice of which value to use. The data retention current value of the SRAMs can then be added to the Ibat value of the Supervisor to determine the total current requirements for data retention. The available battery capacity for the SNAPHAT® of your choice can then be divided by this current to determine the amount of data retention available (48mAh or 120mAh).

Supervisor Data Retention Calculation

To calculate the typical and worse case data retention lifetimes, select the Supervisor Device and Battery Type in the form below. Enter the typical and worse case IccDR currents (nA) for the SRAM being evaluated and click on Calculate. The calculated typical and worse case Data Retention Lifetime in years will be then be displayed.

External SRAM ICCDR (in nA)
Data Retention Lifetime (in Years)