Initial public release of WII5 Buoy firmware

Firmware for an autonomous wave-measurement buoy (ATmega2560-based
WII5 v2 board). Reads wave motion from a Sparton AHRS-M1/M2 IMU,
samples GPS and battery state, and reports back over Iridium SBD
satellite telemetry. Originally developed 2012-2024.

This is the first public release. Code, documentation, and field-tested
operating modes (Capture, Sleep, Position, ManualTest, SelfTest,
LowBattery) are licensed under Apache 2.0 — see LICENSE and NOTICE.

See README.md for an overview and build instructions, CONTRIBUTING.md
for how to contribute, and DEPLOYMENTS.md for the field-deployment log.

WII5ModePosition.h

@@ -0,0 +1,68 @@
+// SPDX-License-Identifier: Apache-2.0
+// Copyright (c) 2012-2024 Scott Penrose <scottp@dd.com.au> and WII5 Buoy contributors
+//
+// This file is part of WII5 Buoy firmware.
+// See LICENSE for full terms.
+
+/**
+ * @file WII5ModePosition.h
+ * @brief Declares WII5ModeLowBattery (low-battery mode).
+ *
+ * @note The WII5ModePosition.h / WII5ModeLowBattery.h header pair are
+ * swapped relative to their filenames in this codebase: this file
+ * (WII5ModePosition.h) declares class WII5ModeLowBattery, and
+ * WII5ModeLowBattery.h declares class WII5ModePosition. The corresponding
+ * .cpp files match their filenames. TODO: untangle and rename.
+ */
+
+#ifndef WII5ModeLowBattery_h
+#define WII5ModeLowBattery_h
+
+#include <Arduino.h>
+#include <WII5.h>
+#include <TimeLib.h>
+
+enum WII5LOWBATTERY_STEPS {
+  WII5LOWBATTERY_START,
+  WII5LOWBATTERY_STROBE,
+  WII5LOWBATTERY_TIME,
+  WII5LOWBATTERY_GPS_START,
+  WII5LOWBATTERY_GPS_WAIT,
+  WII5LOWBATTERY_IRIDIUM_WAIT,
+  WII5LOWBATTERY_END
+};
+
+/**
+ * @brief Low-battery mode: degraded operation when supply is depleted.
+ *
+ * Suppresses Capture mode's heavy operations (IMU, SD writes), keeps
+ * a minimum-viable position+telemetry cadence, and pesters Iridium
+ * with a battery-low alert. The buoy returns to its default mode once
+ * battery voltage recovers (or via an explicit Iridium override).
+ */
+class WII5ModeLowBattery : public WII5Mode {
+  public:
+		WII5ModeLowBattery() {}
+    /** @brief Reset to WII5LOWBATTERY_START. */
+    void reset();
+    /** @brief One-time bring-up. */
+    void begin();
+    /** @brief State-machine tick. */
+    void loop();
+    /** @brief Begin a low-battery alert cycle. */
+    void start();
+    /** @brief Abort the current cycle. */
+    void stop();
+
+  protected:
+    bool first;
+    elapsedMillis displayWait;
+    elapsedMillis wait;
+    WII5LOWBATTERY_STEPS step;
+    WII5LOWBATTERY_STEPS lastStep;
+    uint32_t minutes; // since midnight.
+};
+
+extern WII5ModeLowBattery wii5ModeLowBattery;
+
+#endif