How to disable the write protection of the USART mode register?
Under Windows, Arduino puts the source code for hardware specific stuff in User\AppData\Local\Arduino, and not in the installation folder. 

/* -------- US_WPMR : (USART Offset: 0xE4) Write Protect Mode Register -------- */
#define US_WPMR_WPEN (0x1u &lt;&lt; 0) /**&lt; \brief (US_WPMR) Write Protect Enable */
#define US_WPMR_WPKEY_Pos 8
#define US_WPMR_WPKEY_Msk (0xffffffu &lt;&lt; US_WPMR_WPKEY_Pos) /**&lt; \brief (US_WPMR) Write Protect KEY */
#define US_WPMR_WPKEY(value) ((US_WPMR_WPKEY_Msk &amp; ((value) &lt;&lt; US_WPMR_WPKEY_Pos)))


I would guess unlocking / locking would take the format:

????-&gt;US_WPMR = (US_WPMR_WPEN | US_WPMR_WPKEY(0x555341));
????-&gt;US_WPMR = (~US_WPMR_WPEN &amp; US_WPMR_WPKEY(0x555341));