AP-701x - Battery Monitoring and Charging Scheme
At startup, the battery status is checked for 'Battery critical'. If that is the case, the pen will enter low power-mode and halt the startup of the rest of the system until the battery level is deemed sufficiently charged.
During this time, the LED indicator blinks red. The pen will not present itself for enumeration on the USB bus while in this initial charging state.
During Operation and Stand-by
The battery status is checked every 10 seconds, and reported to the LED.
When the battery voltage is too low, the pen will shut down into shipment mode in order to preserve power for the RTC (real time clock). Prior to this, the pen will write a 'battery critical' entry in the STF2 file. The pen will then only wake up once connected to USB.
When connected to a USB port, after it has been enumerated the device may charge 170mA from the USB port. If no enumeration is done, the pen may (in accordance with USB standard) draw 100mA from the USB port. The pen will perform a start up sequence if it has previously been placed into shipment mode.
If the USB port is suspended, the pen may not draw more than 2.5mA from the USB port. This typically happens when a PC host is set to sleep mode - at this point, the LED will be left off.