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DS1644L-120 Datasheet(PDF) 5 Page - Dallas Semiconductor |
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DS1644L-120 Datasheet(HTML) 5 Page - Dallas Semiconductor |
5 / 11 page DS1644LPM 041697 5/11 DATA RETENTION MODE When VCC is within nominal limits (VCC > 4.5 volts) the DS1644L can be accessed as described above with read or write cycles. However, when VCC is below the power fail point VPF (point at which write protection oc- curs) the internal clock registers and RAM are blocked from access. This is accomplished internally by inhibit- ing access via the CE signal. At this time the power–fail output signal (PFO) will be driven active low and will remain active until VCC returns to nominal levels. When VCC falls below the level of the internal battery supply, power input is switched from the VCC pin to the internal battery and clock activity, RAM, and clock data are maintained from the battery until VCC is returned to nominal level. INTERNAL BATTERY LONGEVITY The DS1644L has a self contained lithium power source that is designed to provide energy for clock activity, and clock and RAM data retention when the VCC supply is not present. The capability of this internal power supply is sufficient to power the DS1644L continuously for the life of the equipment in which it is installed. For specifi- cation purposes, the life expectancy is 10 years at 25 °C with the internal clock oscillator running in the absence of VCC power. The DS1644L is shipped from Dallas Semiconductor with the clock oscillator turned off, so the expected life should be considered to start from the time the clock oscillator is first turned on. Actual life ex- pectancy of the DS1644L will be much longer than 10 years since no internal lithium battery energy is con- sumed when VCC is present. In fact, in most applica- tions, the life expectancy of the DS1644L will be approx- imately equal to the shelf life (expected useful life of the lithium battery with no load attached) of the lithium bat- tery which may prove to be as long as 20 years. |
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