firmware: Add missing gem_bod.{h,c}

firmware/src/hw/gem_bod.c

@@ -0,0 +1,40 @@
+/*
+    Copyright (c) 2021 Alethea Katherine Flowers.
+    Published under the standard MIT License.
+    Full text available at: https://opensource.org/licenses/MIT
+*/
+
+#include "gem_bod.h"
+#include "sam.h"
+
+void gem_wait_for_stable_voltage() {
+    /*
+        Configure the BOD to monitor for at least 3.0 volts and wait for
+        VDD to reach that level.
+    */
+    SYSCTRL->BOD33.bit.ENABLE = 0;
+    while (!SYSCTRL->PCLKSR.bit.B33SRDY) {};
+
+    SYSCTRL->BOD33.reg = (
+        /* VBOD-: 3.0, VBOD+: 3.3 See datasheet Table 37-21. */
+        SYSCTRL_BOD33_LEVEL(48) |
+        /* Don't reset */
+        SYSCTRL_BOD33_ACTION_NONE |
+        /* Enable hysteresis */
+        SYSCTRL_BOD33_HYST);
+
+    /* Wait for the voltage to stabilize. */
+    SYSCTRL->BOD33.bit.ENABLE = 1;
+    while (!SYSCTRL->PCLKSR.bit.BOD33RDY) {}
+    while (SYSCTRL->PCLKSR.bit.BOD33DET) {}
+
+    /*
+        Everything is good, let the BOD33 reset the microcontroller if the
+        voltage falls below 3.0 volts.
+    */
+
+    SYSCTRL->BOD33.bit.ENABLE = 0;
+    while (!SYSCTRL->PCLKSR.bit.B33SRDY) {};
+    SYSCTRL->BOD33.reg |= SYSCTRL_BOD33_ACTION_RESET;
+    SYSCTRL->BOD33.bit.ENABLE = 1;
+}

firmware/src/hw/gem_bod.h

@@ -0,0 +1,11 @@
+/*
+    Copyright (c) 2021 Alethea Katherine Flowers.
+    Published under the standard MIT License.
+    Full text available at: https://opensource.org/licenses/MIT
+*/
+
+#pragma once
+
+/* Routines for interacting with the SAM D21's brown out detector (BOD33) */
+
+void gem_wait_for_stable_voltage();