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FM3808 Datasheet(PDF) 7 Page - Ramtron International Corporation |
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FM3808 Datasheet(HTML) 7 Page - Ramtron International Corporation |
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7 / 28 page  FM3808 Rev 1.1 May 2003 Page 7 of 28 Real-Time Clock Operation The real-time clock (RTC) consists of an oscillator, a clock divider, and a register system to access the information. It divides down the 32.768 kHz time- base to provide the user timekeeping resolution of one second (1Hz). The RTC will not run until the oscillator is enabled. The ocillator enable bit is bit 7 of register 7FF8h and is automatically set to a one (disabled) when the device powers up without a backup supply. Static registers provide the user with read/write access to the time values. The synchronization of these registers with the timekeeper core is performed using R and W bits in register 7FF0h. Setting the R bit from 0 to 1 causes a transfer of the timekeeping information to holding registers that can be read by the user. If a timekeeper update is in progress when the R is set, the update will be completed prior to loading the registers. Another update cannot be performed unless the R bit is first cleared to 0 again. Setting the W bit causes the timekeeper to freeze updates. Clearing it to 0 causes the values in the time registers to be written into the timekeeper core. Users should be sure not to load invalid values, such as FFh to any of the timekeeping registers. Updates to the timekeeping core occur continuously except when frozen. A diagram of the timekeeping core is shown below. Backup Power The real-time clock/calendar is intended for permanently powered operation. When primary system power fails, the VDD voltage will drop. When it crosses the voltage on the VBAK supply pin, the clock/calendar power will switch to the backup power supply VBAK. The supervisor function, described below, controls the switchover process as part of a more complete power management circuit. The clock uses very little current, which maximizes battery life. Although a backup battery may be used with the FM3808, the key advantage to combining a clock function with FRAM is that the configuration data (shaded regsiter bits in Table 2) is nonvolatile and does not require a battery backup power source. Therefore, it is more practical to use a capacitor as a backup energy source than a battery-backed RAM/clock combo. With the FM3808, the user has the choice of using a battery or a capacitor as the backup source. Some of the considerations in the backup source decision include: The expected duration of power outages, the difficulty of resetting the time if lost, and the cost tradeoff of using a small battery versus a capacitor. The following functions are powered from the backup power source when VDD < VBAK (backup mode) : • Clock/calendar core • Alarm interrupt/comparator • INT pin (determined by ABE & AIE bits); active low only • Flags connected to related functions The following functions are not powered and are disabled when VDD < VLO : • User interface • Watchdog timer • Power monitor & band-gap (V DD < ≈ 2.0V) • Flags connected to related functions • All FRAM access & updates • Calibration operation • INT pin if programmed as active-high |
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