Merge pull request #198 from dhalbert/samd21-bod33

Enable BOD33 protection for SAMD21

src/main.c

@@ -169,6 +169,41 @@ int main(void) {
         }
 
 #if defined(SAMD21)
+    // Check for voltage too low, and set up brownout protection.
+    // Code largely taken from https://blog.thea.codes/sam-d21-brown-out-detector/ (thanks!)
+
+    // Disable the brown-out detector during configuration, otherwise
+    // it might misbehave and reset the microcontroller.
+    SYSCTRL->BOD33.bit.ENABLE = 0;
+    while (!SYSCTRL->PCLKSR.bit.B33SRDY) {};
+
+    // Configure the brown-out detector so that the program can use it to watch the power supply voltage.
+    SYSCTRL->BOD33.reg = (
+        // This sets the minimum voltage level to about 2.9V. See datasheet table 37-21.
+        // Voltage threshold is about 1.5V + LEVEL * 34mV. See "Electrical Characteristics" in datasheet.
+        // 39 is about 2.8V, and is a standard measured value in the datasheet.
+        // External flash chips usually require at least 2.7V.
+        SYSCTRL_BOD33_LEVEL(39) |
+        // Since the program is waiting for the voltage to rise,
+        // don't reset the microcontroller if the voltage is too low.
+        SYSCTRL_BOD33_ACTION_NONE |
+        // Enable hysteresis to better deal with noisy power supplies and voltage transients.
+        SYSCTRL_BOD33_HYST);
+
+    // Enable the brown-out detector and then wait for the voltage level to settle.
+    SYSCTRL->BOD33.bit.ENABLE = 1;
+    while (!SYSCTRL->PCLKSR.bit.BOD33RDY) {}
+
+    // BOD33DET is set when the voltage is *too low*, so wait for it to be cleared.
+    while (SYSCTRL->PCLKSR.bit.BOD33DET) {}
+
+    // Let the brown-out detector automatically reset the microcontroller if the voltage drops too low.
+    SYSCTRL->BOD33.bit.ENABLE = 0;
+    while (!SYSCTRL->PCLKSR.bit.B33SRDY) {};
+
+    SYSCTRL->BOD33.reg |= SYSCTRL_BOD33_ACTION_RESET;
+    SYSCTRL->BOD33.bit.ENABLE = 1;
+
     // If fuses have been reset to all ones, the watchdog ALWAYS-ON is
     // set, so we can't turn off the watchdog.  Set the fuse to a
     // reasonable value and reset. This is a mini version of the fuse

src/selfmain.c

@@ -194,20 +194,14 @@ int main(void) {
     uint32_t zeros[2] = {0, 0};
     flash_write_words((void *)(BOOTLOADER_K * 1024), zeros, 2);
 
-    for (uint32_t i = 0; i < 8; ++i) {
-        LED_MSC_TGL();
-        delay(1000);
-    }
-
-    LED_MSC_OFF();
-
 #ifdef SAMD21
     // Re-enable BOOTPROT
     set_fuses_and_bootprot(2); // 8k
 #endif
     // For the SAMD51, the boot protection will automatically be re-enabled on
     // reset.
 
+    LED_MSC_OFF();
     resetIntoBootloader();
 
     // We should not reach here normally.