Set MANW bit of NVMCTRL to 1 at startup (default is 0).

This prevents accidental overwrites of the bootloader if a NULL
pointer is used for writing by mistake.

Author: cmaglie

CHANGELOG

@@ -1,3 +1,7 @@
+SAMD CORE 1.6.5
+
+* Set NVMCTRL.MANW bit to 1 (default is 0). This prevents accidental writes on program memory.
+
 SAMD CORE 1.6.4 2016.02.19
 
 * Fixed Wire address setup when running as slave.

cores/arduino/startup.c

@@ -223,4 +223,9 @@ void SystemInit( void )
   linearity |= ((*((uint32_t *) ADC_FUSES_LINEARITY_1_ADDR) & ADC_FUSES_LINEARITY_1_Msk) >> ADC_FUSES_LINEARITY_1_Pos) << 5;
 
   ADC->CALIB.reg = ADC_CALIB_BIAS_CAL(bias) | ADC_CALIB_LINEARITY_CAL(linearity);
+
+  /*
+   * 9) Disable automatic NVM write operations
+   */
+  NVMCTRL->CTRLB.bit.MANW = 1;
 }