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 is deemed fit to continue.
During this time, the display is blank. The pen will not present itself for enumeration on the USB bus while in this initial charging state.
The battery status is checked every second, and reported to MMI.
The pen is wakened every 30 seconds to retrieve fuel gauge status.
When the battery voltage is too low, the pen will shut down into shipment mode in order to preserve power for the RTC. Prior to this, the pen will write a 'battery critical' entry in the STF2 file. The pen will then only wake up if docked.
When connected to a USB port, the pen will perform a start up sequence, and allow for enumeration of the device. When enumerated, the device may charge 170 mA from the USB port. If no enumeration is done, the pen may (in accordance with USB standard) draw 100 mA from the USB port.
If the USB port is suspended, the pen may not draw more than 2.5 mA from the USB port. This typically happens when a PC host is set into sleep mode. At this point, the display will be left blank.