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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. |
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| 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). |
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