The boot program implements the algorithm below.
Figure 1. Boot Program Algorithm Flow
Diagram
The SAM-BA boot program looks for a source clock, either from the embedded main oscillator with external crystal (main oscillator enabled) or from a supported frequency signal applied to the XIN pin (Main oscillator in bypass mode).
If a clock is supplied by one of the two sources, the boot program checks that the frequency is one of the supported external frequencies. If the frequency is supported, USB activation is allowed. If no clock is supplied, or if a clock is supplied but the frequency is not a supported external frequency, the internal 12 MHz RC oscillator is used as the main clock. In this case, the USB is not activated due to the frequency drift of the 12 MHz RC oscillator.