BQ24187 Datasheet, PDF (18/44 Pages) Texas Instruments – 1A USB-OTG Support

bq24187
SLUSBM0 – APRIL 2014
Not Recommended for New Designs
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Device Functional Modes (continued)
8.4.1.1.8 Safety Timer and Watchdog Timer In Charge Mode
At the beginning of charging process, the bq24187 starts the safety timer. This timer is active during the entire
charging process. If charging has not terminated before the safety timer expires, the IC enters suspend mode
where charging is disabled. The safety timer time is selectable using the I2C interface. When a safety timer fault
occurs, a single 128μs pulse is sent on the STAT and INT outputs and the STATx bits of the status registers are
updated in the I²C. The CE bit, HZ_MODE bit, CD terminal or input power must be toggled in order to clear the
safety timer fault. The safety timer duration is selectable using the TMR_X bits in the Safety Timer Register/ NTC
Monitor register. Changing the safety timer duration resets the safety timer.
In addition to the safety timer, the bq24187 contains a 30-second (t_WATCHDOG) watchdog timer that monitors the
host through the I²C interface. Once a write is performed on the I²C interface, a watchdog timer is started. The
watchdog timer is reset by the host using the I2C interface. This is done by writing a “1” to the reset bit
(TMR_RST) in the control register. The TMR_RST bit is automatically set to “0” when the watchdog timer is
reset. This process continues until battery is fully charged or the safety timer expires. If the watchdog timer
expires, the IC enters DEFAULT mode where the default charge parameters are loaded, the safety timer restarts
at 1.25 minutes and charging continues. The I²C may be accessed again to reinitialize the desired values and
restart the watchdog timer as long as the safety timer has not expired. Once the safety timer expires, charging is
disabled. This function prevents continuous charging of a defective battery if the host fails to reset the safety
timer. In the event the watchdog timer expires, the I²C circuit is reset as well. This prevents the I²C from
indefinitely hanging if the I²C master loses control during a read/write. The watchdog timer flow chart is shown in
Figure 9.
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