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BQ4845S- Datasheet(PDF) 7 Page - Texas Instruments |
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BQ4845S- Datasheet(HTML) 7 Page - Texas Instruments |
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7 / 22 page  Watchdog Timer The watchdog monitors microprocessor activity through the Watchdog input (WDI). To use the watchdog func- tion, connect WDI to a bus line or a microprocessor I/O line. If WDI remains high or low for longer than the watchdog time-out period (1.5 seconds default), the bq4845 asserts WDO and RST. Watchdog Input The bq4845 resets the watchdog timer if a change of state (high to low, low to high, or a minimum 100ns pulse) occurs at the Watchdog input (WDI) during the watchdog period. The watchdog time-out is set by WD0- WD2 in register B. The bq4845 maintains the watchdog time-out programming through power cycles. The de- fault state (no valid battery power) of WD0-WD2 is 000 or 1.5s on power-up. Table 3 shows the programmable watchdog time-out rates. The watchdog time-out period immediately after a reset is equal to the programmed watchdog time-out. To disable the watchdog function, leave WDI floating. An internal resistor network (100k Ω equivalent impedance at WDI) biases WDI to approximately 1.6V. Internal comparators detect this level and disable the watchdog timer. When VCC is below the power-fail threshold, the bq4845 disables the watchdog function and disconnects WDI from its internal resistor network, thus making it high impedance. Watchdog Output The Watchdog output (WDO) remains high if there is a transition or pulse at WDI during the watchdog time- out period. The bq4845 disables the watchdog function and WDO is a logic high when VCC is below the power fail threshold, battery-backup mode is enabled, or WDI is an open circuit. In watchdog mode, if no transition oc- curs at WDI during the watchdog time-out period, the bq4845 asserts RST for the reset time-out period t1. WDO goes low and remains low until the next transition at WDI. If WDI is held high or low indefinitely, RST will generate pulses (t1 seconds wide) every t3 seconds. Fig- ure 3 shows the watchdog timing. 7 Figure 3. Watchdog Time-out Period and Reset Active Time Aug. 1995 bq4845/bq4845Y |
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