boardloader: enable PVD programmable voltage detector and interrupt

embed/boardloader/lowlevel.c

@@ -64,6 +64,19 @@ void periph_init(void)
     __HAL_RCC_GPIOC_CLK_ENABLE();
     __HAL_RCC_GPIOD_CLK_ENABLE();
 
+    // enable the PVD (programmable voltage detector).
+    // select the "2.7V" threshold (level 5). the typical electrical
+    // characteristic values are similar to BOR level 3.
+    // this detector will be active regardless of the
+    // flash option byte BOR setting.
+    __HAL_RCC_PWR_CLK_ENABLE();
+    PWR_PVDTypeDef pvd_config;
+    pvd_config.PVDLevel = PWR_PVDLEVEL_5;
+    pvd_config.Mode = PWR_PVD_MODE_IT_RISING_FALLING;
+    HAL_PWR_ConfigPVD(&pvd_config);
+    HAL_PWR_EnablePVD();
+    NVIC_EnableIRQ(PVD_IRQn);
+
     // Clear the reset flags
     PWR->CR |= PWR_CR_CSBF;
     RCC->CSR |= RCC_CSR_RMVF;