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| DS1557 |
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DS1557 4 of 19 BATTERY LONGEVITY The DS1557 has a lithium power source that is designed to provide energy for the 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 DS1557 continuously for the life of the equipment in which it is installed. For specification purposes, the life expectancy is 10 years at 25 °C with the internal clock oscillator running in the absence of VCC. INTERNAL BATTERY MONITOR The DS15573 constantly monitors the battery voltage of the internal batter. The Battery Low Flag (BLF) bit of the Flags Register (B4 of 7FFF0h) is not writable and should always be a 0 when read. If a 1 is ever present, an exhausted lithium energy source is indicated and both the contents of the RTC and RAM are questionable. POWER-ON RESET A temperature compensated comparator circuit monitors the level of VCC. When VCC falls to the power fail trip point, the RST signal (open drain) is pulled low. When VCC returns to nominal levels, the RST signal continues to be pulled low for a period of 40 ms to 200 ms. The power-on reset function is independent of the RTC oscillator and thus is operational whether or not the oscillator is enabled. CLOCK OPERATIONS Table 2 and the following paragraphs describe the operation of RTC, alarm, and watchdog functions. |
