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BQ4285LP Datasheet(PDF) 7 Page - Texas Instruments |
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BQ4285LP Datasheet(HTML) 7 Page - Texas Instruments |
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7 / 32 page  Alarm Interrupt The alarm interrupt request is valid in battery-backup mode, providing a “wake-up” capability. During each up- date cycle, the RTC compares the hours, minutes, and seconds bytes with the three corresponding alarm bytes. If a match of all bytes is found, the alarm interrupt event flag bit, AF in register C, is set to 1. If the alarm event is enabled, an interrupt request is generated. An alarm byte may be removed from the comparison by setting it to a “don’t care” state. An alarm byte is set to a “don’t care” state by writinga1toeachofitstwo most- significant bits. A “don’t care” state may be used to select the frequency of alarm interrupt events as follows: n If none of the three alarm bytes is “don’t care,” the frequency is once per day, when hours, minutes, and seconds match. n If only the hour alarm byte is “don’t care,” the frequency is once per hour, when minutes and seconds match. n If only the hour and minute alarm bytes are “don’t care,” the frequency is once per minute, when seconds match. n If the hour, minute, and second alarm bytes are “don’t care,” the frequency is once per second. Update Cycle Interrupt The update cycle ended flag bit (UF) in register C is set to a 1 at the end of an update cycle. If the update interrupt enable bit (UIE) of register B is 1, and the update transfer inhibit bit (UTI) in register B is 0, then an interrupt re- quest is generated at the end of each update cycle. Accessing RTC bytes Time and calendar bytes read during an update cycle may be in error. Three methods to access the time and calendar bytes without ambiguity are: n Enable the update interrupt event to generate interrupt requests at the end of the update cycle. The interrupt handler has a maximum of 999ms to access the clock bytes before the next update cycle begins (see Figure 3). n Poll the update-in-progress bit (UIP) in register A. If UIP = 0, the polling routine has a minimum of tBUC time to access the clock bytes (see Figure 3). n Use the periodic interrupt event to generate interrupt requests every tPI time, such that UIP = 1 always occurs between the periodic interrupts. The interrupt handler will have a minimum of tPI/2 + tBUC time to access the clock bytes (see Figure 3). Oscillator Control When power is first applied to the bq4285E/L and VCC is above VPFD, the internal oscillator and frequency divider are turned on by writing a 010 pattern to bits 4 through 6 of register A. A pattern of 011 behaves as 010 but addi- tionally transforms register C into a read/write register. This allows the 32.768kHz output on the square wave pin to be turned on. A pattern of 11X turns the oscillator on, but keeps the frequency divider disabled. Any other pat- tern to these bits keeps the oscillator off. 7 bq4285E/L Figure 3. Update-Ended/Periodic Interrupt Relationship |
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