STM32WLE5 Low-Power LoRa APRS — Tracker + Digipeater

Built around the Ebyte E77-400MBL-01 (STM32WLE5CC, 433 MHz) with support for the ST Nucleo-WL55JC and other STM32WLE5-based modules.

Libraries: RadioLib (jgromes) + APRSPacketLib (richonguzman).

Licensed under the MIT License — free to use, modify, and distribute with attribution.

Verified against the working PicoTrack firmware by K6ATV for RF switch pinout, TCXO voltage, and radio init sequence.

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Repository branches

Branch	Purpose
main	General-purpose LoRa APRS tracker and digipeater. Four operating modes: TRACKER_ONLY, TRACKER_DIGI, DIGI_ONLY, DIGI_CAD.
balloon-digi	Adds MODE_BALLOON_DIGI — a low-power ground station digipeater that autonomously acquires and tracks a specific balloon callsign.

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File overview

platformio.ini        — build/upload config and library deps
include/config.h      — ALL user settings (callsign, mode, pins…)
src/main.cpp          — firmware

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Quick start

1. Install VS Code and PlatformIO

1. Download and install Visual Studio Code.
2. Open VS Code, go to the Extensions panel (Ctrl+Shift+X / Cmd+Shift+X), and search for PlatformIO IDE. Install it and restart VS Code when prompted.
3. PlatformIO will automatically install the STM32 platform and all required libraries the first time you build — no manual library installation needed.

2. Get the code

Option A — Clone with Git:

git clone https://github.com/radiohound/LoRa-Tracker-Digipeater.git

Then in VS Code: File → Open Folder and select the cloned folder.

Option B — Download ZIP:

1. Click the green Code button on GitHub, then Download ZIP.
2. Extract the ZIP to a folder of your choice.
3. In VS Code: File → Open Folder and select the extracted folder.

PlatformIO recognises the project automatically when it sees platformio.ini in the root.

3. Configure

Open include/config.h and set:

• OPERATING_MODE — choose your mode (see table below)
• MY_CALLSIGN, MY_SSID, MY_COMMENT
• LORA_FREQ for your region (433.775 MHz is used almost world wide — check local LoRa APRS frequency)
• Verify RFSWITCH_PINS / RFSWITCH_TABLE matches your board (see RF switch table below)

4. Connect the hardware

Connect the E77 dev board to your computer via the ST-Link USB port (the built-in programmer on the dev board). On first connection, Windows may install ST-Link drivers automatically; if not, install the ST-Link drivers manually.

5. Compile and upload

Using the VS Code UI:

• Click the PlatformIO icon in the left sidebar (the alien head).
• Under PROJECT TASKS → ebyte_e77_dev, click Upload to compile and flash in one step. Click Build to compile only.

Using the terminal:

pio run                        # compile only
pio run --target upload        # compile and flash

6. Monitor serial output

• In VS Code PlatformIO sidebar: Monitor under PROJECT TASKS.
• Or in the terminal:

pio device monitor --baud 115200

You should see [Radio] Init LoRa... OK followed by GPS and TX log lines within a few seconds of powering on.

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Operating modes

Mode	GPS needed	Radio state	Avg current
TRACKER_ONLY	Yes	TX then Stop2 sleep	~2–8 mA (GPS-dominated)
TRACKER_DIGI	Yes	TX then continuous RX	~5–6 mA
DIGI_ONLY	No	Continuous RX always	~6 mA
DIGI_CAD_SOLAR	No	Two-stage: CAD sentinel + full RX window, digipeats all	~0.1 mA idle, ~6 mA active
DIGI_CAD_BATTERY	No	Two-stage: CAD sentinel + RX window for moving stations only	~0.1 mA idle

Both CAD modes use the same two-stage design to maximise detection reliability while minimising power:

Stage 1 — CAD sentinel (~100 µA): The radio performs a 65 ms Channel Activity Detection scan every CAD_SCAN_INTERVAL_MS (default 1000 ms) continuously. Because LoRa CAD detects any chirp energy on the channel — not just a preamble — a scan interval equal to or shorter than the expected transmission length guarantees detection on the first transmission heard.

Stage 2 — Active RX window (~6 mA): On detecting a signal the radio switches to full continuous receive. The window stays open as long as qualifying packets continue to arrive, then returns to Stage 1 after silence. What counts as "qualifying" differs between modes:

• DIGI_CAD_SOLAR — any received packet resets the 6-minute window and is digipeated. Suitable for solar or mains-powered installations where power is not a constraint.

• DIGI_CAD_BATTERY — only packets from stations that have moved since they were last heard reset the window and are digipeated. Fixed ground stations are received but silently ignored, saving significant power at sites with regular beaconing ground stations nearby. The RX window duration is set by CAD_BEACON_INTERVAL_MS plus a 15-second buffer (see configuration below).

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Power budget (Ebyte E77 @ 3.3 V)

State	Current
Stop2 — MCU only, radio off	~2 µA
MCU idle / light sleep	~1.5 mA
LoRa RX (continuous)	~5 mA
Channel Activity Detection (CAD) scan (~65 ms at SF12)	~1.5 mA
GPS acquiring fix	~20–30 mA
LoRa TX at 22 dBm (peak)	~100 mA

Mode-by-mode averages

TRACKER_ONLY (120 s interval, 30 s GPS warm start): Average ~2–3 mA, dominated by GPS. With GPS power gating and a fast-fix module this can drop below 1 mA average.

DIGI_ONLY (continuous RX): Flat ~6 mA regardless of traffic. Suitable for mains or large solar.

DIGI_CAD_SOLAR (1 s CAD scan interval, 6 min RX window, all stations):

• Idle with no traffic: ~0.1 mA average (continuous CAD sentinel)
• With moderate traffic (1 packet/min): ~0.5 mA average
• With heavy traffic (1 packet/10 s): ~6 mA (effectively continuous RX)

DIGI_CAD_BATTERY (1 s CAD scan interval, moving stations only):

• Quiet site, no moving stations: ~0.1 mA (same as solar idle)
• Fixed ground station transmitting every 20 min: ~0.32 mA
• Moving station active (balloon pass, vehicle): ~6 mA while in range

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Battery life estimates

Two AA Energizer Ultimate Lithium (L91) — 3000 mAh each in series

Mode	Average current	Estimated life
DIGI_ONLY continuous RX	~6 mA	~3 weeks
DIGI_CAD_SOLAR quiet site	~0.1 mA	~3.4 years
DIGI_CAD_SOLAR moderate traffic	~0.5 mA	~8 months
DIGI_CAD_BATTERY quiet site	~0.1 mA	~3.4 years
DIGI_CAD_BATTERY fixed ground station nearby	~0.32 mA	~1 year
TRACKER_ONLY 120 s interval	~2.5 mA	~7 weeks
TRACKER_ONLY 600 s interval	~0.8 mA	~5 months

Battery life = 3000 mAh ÷ average current. Actual life depends on temperature, traffic load, and GPS fix time. L91 cells have a ~10-year shelf life and perform well at temperature extremes, making them ideal for remote installations.

Solar operation

DIGI_CAD mode at idle (~0.1 mA) is easily sustained by even a small salvaged solar panel — a typical lawn light panel produces far more current than the CAD sentinel consumes. When active traffic triggers Stage 2, RX current rises to ~6 mA, still well within the output of any small panel in reasonable daylight.

For continuous RX (DIGI_ONLY), a panel delivering at least 600–700 mWh per day is needed, which is marginal for small panels in poor weather. DIGI_CAD is strongly recommended for solar/battery installations.

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Voltage and battery wiring

E77 module voltage limits

The E77-400M22S module requires 1.8–3.6 V. Fresh AA lithium cells in series measure ~3.58 V open circuit — uncomfortably close to the 3.6 V maximum. Under load the voltage drops quickly into the safe range, but it is good practice to add protection.

Recommended: 1N5817 Schottky diode in series

Place a single 1N5817 (or equivalent Schottky diode) in series between the battery positive terminal and the module VDD pin.

[Battery +] ──── Anode | 1N5817 | Cathode ──── [E77 VDD]
[Battery −] ──────────────────────────────────── [E77 GND]

The 1N5817 drops approximately 0.2 V at low currents, bringing fresh-battery voltage from ~3.58 V down to ~3.38 V — well within the safe operating range. At TX peaks (~100 mA) the drop increases slightly to ~0.3 V, still safe.

Energy cost of the diode: negligible. At 0.1 mA average current the power lost in the diode is ~0.02 mW — less than one day's battery life over a multi-year deployment.

Alternative: A low-dropout regulator (LDO) such as the MCP1700-3302 gives a precise 3.3 V output and handles the full input voltage range of two AA cells (fresh at 3.58 V down to ~1.8 V depleted). Efficiency is ~92% at low currents, acceptable for this use case.

Two AA cells — wiring summary

[AA+][AA−][AA+][AA−]
  Series = ~3.0–3.58 V, 3000 mAh

With diode:
  [+]──1N5817──[E77 VDD]   ~3.38 V fresh, ~2.8 V depleted
  [−]──────────[E77 GND]

Module operates down to 1.8 V, so cells are used to near full depletion.

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RF switch pin configurations

The STM32WLE5's internal radio is connected to an external RF switch. Pinout varies by module — uncomment the right block in config.h.

Module	Pins	Notes
Ebyte E77-400MBL-01	PA6, PA7, PB3	Verified against PicoTrack
Seeed LoRa-E5 / E5-mini	PA4, PA5	Different logic from E77
ST Nucleo-WL55JC	PC3, PC4, PC5	Both LP and HP paths
RAK3172-E (no TCXO)	PB8, PC13	LP path only

Important: radio.setRfSwitchTable() must be called before radio.begin().

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Digipeater behaviour

• DIGI_ONLY: Listens continuously. Digipeats any packet with an unactivated WIDE1-1 or WIDE2-N path.
• DIGI_CAD_SOLAR / DIGI_CAD_BATTERY: Both run CAD scans every CAD_SCAN_INTERVAL_MS (default 1 s) continuously. On detection, Stage 2 switches to full continuous RX. DIGI_CAD_SOLAR digipeats all stations and resets the 6-minute window on every packet. DIGI_CAD_BATTERY digipeats only stations that have moved since last heard, and only resets the window for those stations — fixed ground stations are silently ignored. See the battery mode configuration section for setting the RX window.
• Both modes apply a random 80–450 ms delay before retransmitting to reduce collision probability with other digipeaters.
• An 8-element CRC32 ring buffer suppresses duplicate packets.
• Own beacon packets are never digipeated.

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DIGI_CAD_BATTERY — configuring the RX window

In battery mode the active RX window is set by specifying the expected beacon interval of the moving stations you want to capture. The firmware automatically adds a 15-second buffer so timing variation never causes a missed packet.

In config.h, uncomment the line that best matches your use case:

// Uncomment ONE of these to match the expected moving station beacon interval:

//#define CAD_BEACON_INTERVAL_MS    360000  // 6 min  — infrequent tracker
#define CAD_BEACON_INTERVAL_MS      180000  // 3 min  — typical vehicle or balloon (default)
//#define CAD_BEACON_INTERVAL_MS     60000  // 1 min  — fast vehicle
//#define CAD_BEACON_INTERVAL_MS     30000  // 30 sec — high-rate balloon or tracker

The actual RX window will be your chosen interval plus 15 seconds:

CAD_BEACON_INTERVAL_MS	Actual RX window	Best for
360000 (6 min)	6 min 15 sec	Infrequent trackers
180000 (3 min)	3 min 15 sec	Typical vehicle / balloon — covers any interval ≤ 3 min, including 30 s (default)
60000 (1 min)	1 min 15 sec	Fast vehicle — covers any interval ≤ 1 min
30000 (30 sec)	45 sec	High-rate balloon / tracker — tightest window

How to choose: set this to match the slowest beacon interval you expect from stations you want to digipeat. If a balloon transmits every 30 seconds and you set 3 minutes, you will still catch every packet — the window simply stays open longer than necessary. If you set 30 seconds and a station transmits every 3 minutes, the window will expire between transmissions and the station will be re-detected by the CAD sentinel on its next transmission, typically within one beacon cycle.

Fixed ground stations are never digipeated regardless of this setting.

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BMP280 barometric altimeter (optional)

A BMP280 pressure sensor can be added to the same I2C bus as the GPS (PA9 = SCL, PA10 = SDA) to improve altitude accuracy. GPS altitude is typically accurate to ±10–20 m; a barometer with a good sea-level reference is significantly more stable.

Enable in config.h:

#define BMP280_ENABLED     1       // 1 = use if present, 0 = disable
#define BMP280_I2C_ADDR    0x76    // 0x76 (SDO→GND) or 0x77 (SDO→VCC)
#define BMP280_CAL_SAMPLES 8       // GPS altitude readings to average

How calibration works: immediately after each GPS fix, the firmware takes BMP280_CAL_SAMPLES altitude readings from the GPS at 1 Hz, averages them to reduce GPS noise, then back-calculates the sea-level pressure reference the BMP280 needs to report accurate absolute altitude. From that point until the next fix, all transmitted altitude values come from the barometer.

If no BMP280 is detected at boot the firmware falls back to GPS altitude automatically — no configuration change is needed.

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Cutdown (tracker modes only)

A GPIO pin can be pulsed HIGH at a set altitude to trigger a cutdown mechanism (nichrome wire, pyro charge, servo, etc.). Intended for balloon payloads running TRACKER_ONLY or TRACKER_DIGI.

Enable and configure in config.h:

#define CUTDOWN_ENABLED        1
#define CUTDOWN_PIN            PA0
#define CUTDOWN_ALTITUDE_M     30000   // meters MSL
#define CUTDOWN_ARM_ASCENT_M   500     // meters above launch before armed
#define CUTDOWN_CONFIRM_COUNT  5       // consecutive readings required
#define CUTDOWN_PULSE_MS       5000    // pin HIGH duration in milliseconds

Safeguards:

• Arming delay — the system records altitude at the first valid GPS fix (launch altitude) and will not arm until the balloon has risen CUTDOWN_ARM_ASCENT_M above that baseline. Prevents triggering at a high-altitude launch site or from a noisy reading on the ground.
• Confirmation count — once armed, CUTDOWN_CONFIRM_COUNT consecutive altitude readings must all be at or above CUTDOWN_ALTITUDE_M. The counter resets if any reading falls below the threshold, so a single spike cannot trigger the cutdown.
• Timed pulse — the pin is held HIGH for exactly CUTDOWN_PULSE_MS milliseconds (blocking), then driven LOW before any further code runs. The pin is guaranteed to return LOW regardless of what happens next.
• One-shot — fires once only. The system will not trigger again even if altitude readings remain above the threshold.

Altitude source follows the same priority as the beacon: BMP280 if detected and calibrated, otherwise GPS altitude.

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Adjusting the beacon rate

Change BEACON_INTERVAL_S in config.h. Recommended minimums:

• Fixed station: 600 s (10 min)
• Slow vehicle: 120 s (2 min)
• Fast vehicle / balloon: 30–60 s

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Horus Binary 4FSK — not supported on this hardware

Horus Binary v1/v2 4FSK (100 baud, 270 Hz tone spacing) was investigated for this platform and is not feasible on the STM32WLE5 with a TCXO-equipped E77 module.

The STM32WLE5 SubGhz IP generates 4FSK by switching between four CW frequencies using SET_TX_CONTINUOUS_WAVE. Each call to that command forces a full TCXO startup sequence (~5 ms) enforced in hardware, regardless of prior radio state. At 100 baud (10 ms/symbol), this creates ~50% dead time per symbol — effectively transmitting OOK at 100 Hz rather than 4FSK. The resulting OOK sidebands fall directly on adjacent tones (270 Hz spacing) and prevent the modem from distinguishing symbols.

Approaches tried and confirmed not to work:

• Setting fallback mode to STDBY_XOSC or FS before transmission
• Calling SetRfFrequency while staying in TX CW mode
• Writing the PLL frequency register directly via writeRegister
• Reducing tcxoDelay to 0 or 1 ms
• Single vs multiple fsk4.write() calls

The root cause is a hardware limitation: the STM32WLE5 SubGhz IP always enforces the TCXO startup delay on every SET_TX_CONTINUOUS_WAVE command. This cannot be worked around in software.

Possible hardware solutions (not implemented):

• Always-on TCXO: solder the TCXO VCC line directly to 3.3V, bypassing DIO3 control. Tone transitions would then only require PLL relock (~52 µs).
• Separate radio: add a Si4032 or Si4463 with native 4FSK hardware modulation on SPI alongside the E77. This is how RS41 sondes work.

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Known caveats

• APRSPacketLib field names — the library evolves quickly. If APRSPacket struct fields differ from main.cpp, check the library header and adjust buildAPRSPacket() accordingly.
• Stop2 and Serial — serial output is flushed before entering Stop2. The USART restarts automatically on wakeup under the STM32 Arduino core.
• GPS power switching — GPS_POWER_PIN logic assumes a high-side P-FET (HIGH = GPS on). Invert if using an N-FET or active-low load switch.
• TCXO vs passive crystal — E77 modules with SN batch ≥ 3202995 have a TCXO. Modules below this batch have a passive crystal. Both work with TCXO_VOLTAGE 1.7f in config.h; passive crystal modules are tolerant of the TCXO init call.