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The first low-battery thresholds are defined in software as part of the 
power management driver running on the PXA255.  Temperature-
compensated battery level sampled through PSC U38 (see “Battery Status 
Monitoring” on page 60) is averaged over 10 samples. When this value 
falls below the software threshold, the power management code displays 
the appropriate battery status icon in the icon tray. When the battery level 
drops below the 0% threshold, the power management code also sends an 
I2C command to the PSC to turn on the red “low-battery” LED 
indication. Note that because the battery level is averaged over 10 samples 
at 3-second intervals, it may take up to 30 seconds for a threshold crossing 
to take effect. 

The red LED remains lit until the CK30 enters the Suspend state, or the 
battery level (averaged over 10 samples) rises above the software threshold. 
See the next section “Battery Status LED,” for more details on red LED 
states. 

As the battery level declines further, or the battery is removed, the level 
crosses the hardware “critical low battery” threshold set by temperature 
compensated comparator U36. This interrupts the PXA255 through signal 
BATT_FAULT_IRQ* to start a Suspend and interrupts the PSC so that it 
knows the PXA255 should be suspending. When the system suspends, the 
processor’s RESET_OUT* output is driven low, causing U42 output 
BATT_FAULT* to go low. This inhibits the system from resuming until 
the battery is replaced or its level rises above U36 high-going threshold 
(BATT_FAULT_IRQ* goes high). 

PSC U38 drives red LED D16 through Q11 D16 to display the following 
battery and boot states. The power management driver running on the 
PXA255 sends I2C commands to the PSC to use the red LED for low 
battery indication. The PSC itself directly controls the red LED to indicate 
cold boot and other power management events.