Pololu Wixel - Manual (Page 64)

When the Wixel resets, it uses this procedure to decide
whether to run the bootloader or the app.
Startup Procedure
Every time the Wixel powers on, the
bootloader’s startup code runs. This code
decides whether to launch the bootloader or
the application by using the following
procedure: First, it configures the three LED
lines to have internal pull-down resistors and
disables the internal pull-up and pull-down
resistors of the other Port 2 pins. Second, if
the application is not valid (first byte is
0xFF), it goes into bootloader mode. Third, if
the Wixel has USB power and the yellow
LED (P2_2) line is high, it goes into
bootloader mode. Finally, if none of these
tests have caused it to go into bootloader
mode, it runs the application.
Pololu Wixel User's Guide © 2001–2014 Pololu Corporation
11. The Wixel USB Bootloader Page 64 of 64