Meshtastic Watcher

Ultra-low-power MSP430 firmware that emits an active-LOW pulse to simulate a Meshtastic button press.

• Schedule; one pulse every PULSE_INTERVAL_MIN minutes; default is 24 hours.
• Pulse width; default 500 ms.
• Output style; open-drain behavior on PULSE_PIN_BIT; idle Hi-Z; only driven LOW during the pulse.
• Power; LPM3 between ~30 s timer ticks; ~0.1 µA typical in sleep excluding the brief pulse.

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Motivation

Some Heltec V3 nodes in solar setups fail to rejoin the mesh after a deep discharge; a manual button press usually recovers them. This firmware schedules that press using a low-power MSP430 so the node periodically gets a wake-nudge without user interaction.

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How it works

• Timer_A runs from ACLK = VLO; interrupts about every 30 s. The ISR accumulates ticks until the requested interval elapses; then it emits a single LOW pulse.
• Open-drain style output; PULSE_PIN_BIT is an input when idle; for the pulse it becomes output-LOW for PULSE_MS, then returns to input.
• Low power; CPU sleeps in LPM3 between interrupts; DCO at 1 MHz is only enabled briefly to time the pulse with a cycle delay.
• Board hygiene; all unused pins are outputs driven LOW to minimize leakage.

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Features

• Automatic simulated press on a fixed cadence; default every 24 hours.
• Active-LOW pulse; 500 ms by default; adjustable at build time.
• No external crystal required; uses VLO; cadence is approximate without calibration.

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State machine diagram

stateDiagram-v2
    [*] --> Init
    Init --> Sleep : Configure clocks & GPIO
    Sleep --> Tick : Timer_A interrupt (~30 s)
    Tick --> Sleep : Accumulate time < Interval
    Tick --> Pulse : Interval reached
    Pulse --> Sleep : Drive PULSE_PIN_BIT LOW for PULSE_MS then return to Hi-Z

Loading

This state machine shows the minimal cycle: after initialization, the MSP430 sleeps in LPM3, wakes briefly on ~30 s ticks, and generates a LOW pulse when the target interval is reached.

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Hardware

• MCU; MSP430G2553 by default; portable to similar MSP430s.

• Power; 1.8–3.6 V; use a low-IQ regulator if needed.

• Connections:

  • PULSE_PIN_BIT → Meshtastic button GPIO; the target must provide a pull-up; add a 220–1 kΩ series resistor if desired.
  • GND → common ground with the Meshtastic node.

• If the Meshtastic input must be open-drain, this firmware already idles Hi-Z; if strict open-drain is required at all times, a small NPN or MOSFET works as a buffer.

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Build & Upload

1. Install PlatformIO.
2. Connect MSP430 LaunchPad or compatible programmer.
3. Build:

   pio run

4. Upload:

   pio run --target upload

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Configuration

Edit constants at the top of src/main.c.

• PULSE_INTERVAL_MIN; minutes between pulses; default 60 * 24.

• PULSE_MS; pulse width in milliseconds; default 500.

• PULSE_PIN_BIT; output pin bit.

• Timing base:

  • ACLK_VLO_HZ; nominal VLO frequency; default 12000 Hz; used to derive a ~30 s ISR tick.
  • BASE_PERIOD_S; tick period in seconds; default 30.

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Low-power design

• LPM3 between interrupts; short wake for the ISR and the pulse.
• Unused pins configured as outputs driven LOW.
• No always-on LEDs.
• Target sleep current; ~0.1 µA typical at 3 V on a clean board; excludes the pulse window and any target pull-ups.

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Limitations

• VLO drifts with temperature and voltage; expect cadence variation unless calibrated.
• Heltec V3 GPIO behavior may change with firmware; if Meshtastic adds a reliable wake source, this watcher may become unnecessary.

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License

MIT License; see LICENSE.