Inhero MR2 — Datasheet¶
Inhero MR2 – Smart Solar Mesh Board Hardware Revision 1.1
Board Overview¶
The Inhero MR2 is a LoRa mesh repeater board based on the RAK4630 module (nRF52840 + SX1262) with integrated smart solar charging, power monitoring, and low-voltage protection. Supported battery configurations are 1S Li-ion, 1S LiFePO4, 2S LTO, and 1S Na-ion. The board was specifically designed for autonomous long-term deployment at remote or hard-to-reach locations. In Central Europe, uninterrupted continuous repeater operation is possible with unshaded solar panels ≥ 1 W and battery capacities ≥ 9 Ah.
The charge and discharge cutoff voltages (see table Supported Battery Chemistries) are chosen to avoid excessive stress on the batteries during summer while ensuring that sleep mode can be reliably initiated when energy is low.
In low-voltage sleep, current consumption is < 500 µA. Once the battery voltage has risen above the respective low-V wake threshold (see table Supported Battery Chemistries) through solar charging, the board boots normally. The 200 mV hysteresis between sleep and wake thresholds prevents motorboating – an uncontrolled, rapid on/off cycling of the system that would occur if the sleep and wake thresholds were too close together.
Safety & Protection Features¶
| Feature | Description |
|---|---|
| Watchdog Timer (WDT) | nRF52840 hardware watchdog. Automatically reboots the board if the firmware hangs – essential for unattended long-term operation. |
| Low-Voltage Protection | INA228 ALERT interrupt on chemistry-specific threshold → controlled System Sleep with RTC wake. Solar charging remains active during sleep (CE pin latched). |
| Charger requires active firmware | The BQ25798 only charges when the firmware is actively running. Without flashed firmware or with the 3.3V off switch engaged, charging remains disabled. The nRF52840 must be able to monitor the charger at all times as host. |
| JEITA Temperature Protection | Temperature-dependent charge current reduction via the NTC sensor (TS pin). Frost charge protection configurable via set board.fmax. JEITA is disabled for LTO and Na-ion. The Inhero voltage divider (RT1=5.6 kΩ, RT2=27 kΩ) shifts TS thresholds lower than TI reference (~2–3 °C; effective T-Cool range approx. −2 °C to +3 °C, see JEITA table in README). WARM zone configured to start at ~52 °C (register: 55 °C), effectively neutralized (VREG + ICHG unchanged in WARM), auto battery discharge disabled — see README.md — JEITA for details. Note: JEITA thresholds are evaluated by the BQ25798 directly in hardware. The set board.tccal calibration corrects only the CLI/telemetry temperature readout and does not affect JEITA behavior — see FAQ #12. |
⚠ WARNING — No Reverse Polarity Protection: The board has no hardware reverse polarity protection on the battery or solar input. Connecting a battery or solar panel with reversed polarity will cause immediate, irreversible damage to the board. Always double-check the polarity before connecting any power source.
Solar Power Management¶
| Feature | Description |
|---|---|
| MPPT (Maximum Power Point Tracking) | The BQ25798 optimizes solar harvesting via MPPT (VOC_PCT = 81.25%, matched for crystalline silicon solar cells). Automatic recovery on power-good loss and stuck-PGOOD detection with HIZ toggle. |
| PFM Forward Mode | Enabled by BQ25798 power-on default (PFM_FWD_DIS=0, REG0x12); the firmware does not modify it. Improves efficiency at low solar currents. |
Specifications¶
| Parameter | Value |
|---|---|
| MCU | nRF52840 (ARM Cortex-M4, 64 MHz) |
| Radio | Semtech SX1262 (via RAK4630) |
| Frequency | LoRa Sub-GHz (region-dependent) |
| Connectivity | LoRa, BLE 5.0, USB-C |
| Supply Voltage | 1S Li-ion / 1S LiFePO4 / 2S LTO / 1S Na-ion (via firmware config) |
| Solar Input | 3.6 V – 24 V (MPPT) |
| Max. Solar Voc | 25 V |
| USB Charging | 5 V via USB-C (Schottky diode to VBUS-BQ, same charger path as solar) |
| Charger | BQ25798 (MPPT, JEITA) |
| Max. Charge Current | 50 – 1500 mA (configurable) |
| Power Monitor | INA228 (Coulomb Counter, ALERT) |
| RTC | RV-3028-C7 (time base / wake-up timer). See FAQ #23 |
| Buck Converter | TPS62840 (3.3 V rail, max. 750 mA) |
| System-Off Current | via 3.3V off switch ~15 µA |
| System Sleep Current | < 500 µA (firmware sleep with GPIO latch, CE active, RTC wake) |
| Idle Current (active) | 6.0 mA @ 4.2 V / 7.7 mA @ 3.3 V (USB off, no radio TX) |
| USB Peripheral | ~0.8–1.0 mA additional (auto-enabled on VBUS detect, auto-disabled on removal) |
| CPU Idle Mode | WFE (Wait-For-Event) between loop iterations, reduces CPU current from ~3 mA to ~0.5–0.8 mA |
| PCB Size | 45 × 40 mm |
| Mounting Holes | 4× M2.5, hole spacing 40 × 35 mm |
| Operating Temperature | –40 °C to +85 °C (MCU spec) |
| Bootloader | Adafruit nRF52 OTA-Fix Bootloader (factory-installed), UF2-capable |
PCB – Front Side (Component Side)¶
Connectors, Buttons & LEDs – Front Side¶
| Label (→ image) | Name | Description |
|---|---|---|
| Ble-Conn | U.FL – BLE | Antenna connector for Bluetooth Low Energy (top left on RAK4630) |
| LoRa-Conn | U.FL – LoRa | Antenna connector for LoRa Sub-GHz (left center on RAK4630) |
| USB-C | USB-C Port | USB interface for power supply, charging, firmware flashing and CLI access (top right). CC1/CC2 pulled to GND via 4.7 kΩ (USB sink). VBUS-USB is connected to VBUS-BQ (solar input) via a Schottky diode — USB power feeds the same charger input as the solar panel. |
| Reset | Reset Button | Single click: reset the nRF52840. Double click: enter USB mass storage mode for UF2 firmware updates (right side, below USB-C) |
| Led 1+2 | Status LEDs | LED1 + LED2 = RAK4630 user LEDs (heartbeat / boot indicator, right side, stacked) |
| Chrg. Led | Charge LED | BQ25798 STAT output – indicates charge status (bottom right, next to solar connector) |
| 3.3V off | Power Switch | Slide switch to disconnect the 3.3 V supply (bottom left). ⚠ Caution: Inverted logic! Switch position "ON" = EN pin low = board off. Switch position "OFF" = EN pin high = board on. |
| Bat-Conn (JST PH2.0-3P) | Battery Connector | 3-pin JST PH2.0 connector: Batt+, Batt−, TS (bottom left) |
| Solar-Conn (JST PH2.0-2P) | Solar Connector | 2-pin JST PH2.0 connector: Solar+, Solar− (bottom right) |
| Ø 2.5mm | Mounting Holes | 4× M2.5 mounting holes in the corners |
Key Components – Front Side¶
| Component | Name | Description |
|---|---|---|
| RAK4630 | Core Module | nRF52840 SoC + SX1262 LoRa transceiver (center, shielded) |
| BME280 | Environmental Sensor | Temperature, humidity, pressure |
| BQ25798 | Battery Charger | MPPT, JEITA temperature protection, 15-bit ADC |
| INA228 | Power Monitor | Coulomb counter with ALERT interrupt |
| TPS62840 | Buck Converter | DC/DC, 750 mA, EN switched via 3.3V off switch |
Pinout – Battery Connector (JST PH2.0-3P, left to right)¶
| Pin | Signal | Description |
|---|---|---|
| 1 | Batt + | Battery positive terminal |
| 2 | Batt − | Battery negative terminal (GND) |
| 3 | TS | Temperature sensor (NTC) for JEITA charge protection. Required type: NCP15XH103F03RC (10 kΩ @ 25 °C, Beta 3380) or compatible |
⚠ WARNING: No reverse polarity protection. Verify correct polarity before connecting.
Pinout – Solar Connector (JST PH2.0-2P, left to right)¶
| Pin | Signal | Description |
|---|---|---|
| 1 | Solar + | Solar panel positive (3.6 V – 24 V, max. Voc 25 V) |
| 2 | Solar − | Solar panel negative (GND) |
⚠ WARNING: No reverse polarity protection. Verify correct polarity before connecting.
USB Charging Path¶
USB-C VBUS is connected to the BQ25798 VBUS input (same single input as solar) via a Schottky diode. The BQ25798 has only one VBUS input and does not distinguish between USB and solar. CC1 and CC2 are pulled to GND via 4.7 kΩ resistors, advertising the board as a USB power sink (5 V default). The Schottky diode prevents backflow from the solar panel to the USB bus, but current can flow from USB-VBUS out through the solar connector.
USB Auto-Management¶
The nRF52840 USB peripheral is automatically managed based on VBUS detection:
- VBUS detected → USB peripheral enabled (Serial available)
- VBUS removed → USB peripheral disabled (saves ~0.8–1.0 mA)
- Boot without USB → USB disabled on first loop iteration
No manual CLI commands are required. USB is always available when a cable is connected. See also FAQ #7 — USB charging.
⚠ Warning: Since VBUS-USB and VBUS-BQ (solar input) are connected via the Schottky diode, a short circuit on the solar connector will also short VBUS-USB. Never short-circuit the solar input while USB is connected.
See also FAQ #16 — 3.3V off switch for practical use cases.
PCB – Back Side¶
Headers & Pads – Back Side¶
UART/I2C – Header Row 1 (top row, castellated pads)¶
| Pin | Signal | Description |
|---|---|---|
| 1 | GND | Ground |
| 2 | RX | UART Receive |
| 3 | TX | UART Transmit |
| 4 | SDA | I2C Data |
| 5 | SCL | I2C Clock |
| 6 | 3.3V | 3.3 V output (max. 500 mA, shared with board consumption) |
SWD – Header Row 2 (bottom row, castellated pads)¶
| Pin | Signal | Description |
|---|---|---|
| 1 | RESET | nRF52840 Reset |
| 2 | GND | Ground |
| 3 | SWCLK | SWD Clock (debug interface) |
| 4 | SWDIO | SWD Data (debug interface) |
| 5 | 3.3V | 3.3 V output (max. 500 mA, shared with board consumption) |
Solder Bridge – Onboard Temperature Sensor (bottom right)¶
| Label (→ image) | Description |
|---|---|
| Solder-Bridge (close for onboard Temp-Sensor) | Solder bridge for the onboard NTC temperature sensor (NCP15XH103F03RC, 10 kΩ @ 25 °C, Beta 3380). Closed = onboard NTC active. Open = external NTC of type NCP15XH103F03RC (10 kΩ @ 25 °C, Beta 3380) or compatible required via TS pin on the battery connector. See FAQ #2. |
I2C Bus – Address Map¶
| Address | Component | Function |
|---|---|---|
| 0x40 | INA228 | Power Monitor / Coulomb Counter |
| 0x52 | RV-3028-C7 | Real-Time Clock (RTC) |
| 0x6B | BQ25798 | Battery Charger (MPPT, JEITA) |
| 0x76 | BME280 | Environmental Sensor (T, H, P) |
Pin Assignment (Key GPIOs)¶
| nRF52840 Pin | RAK Module Pin | Function |
|---|---|---|
| P0.04 | WB_IO4 | BQ CE pin (via DMN2004TK-7 N-FET, inverted) |
| P1.02 | WB_IO2 | INA228 ALERT (low-voltage interrupt) |
| P0.17 | WB_IO1 | RV-3028 RTC interrupt |
| P0.21 | WB_IO3 | BQ25798 INT (unused, polled; pulled up) |
Supported Battery Chemistries¶
| Type | Nominal Voltage | Charge Voltage | Low-V Sleep | Low-V Wake | Hysteresis |
|---|---|---|---|---|---|
| Li-ion 1S | 3.7 V | 4.1 V | 3100 mV | 3300 mV | 200 mV |
| LiFePO4 1S | 3.2 V | 3.5 V | 2700 mV | 2900 mV | 200 mV |
| LTO 2S | 4.6 V (2× 2.3 V) | 5.4 V | 3900 mV | 4100 mV | 200 mV |
| Na-ion 1S | 3.1 V | 3.9 V | 2500 mV | 2700 mV | 200 mV |
| none | — | — | — | — | — |
Choosing the right chemistry: See BATTERY_GUIDE.md for a detailed comparison of pros, cons, and deployment recommendations. A brief summary is also in FAQ #1.
Firmware Environments¶
| Build Target | Description |
|---|---|
Inhero_MR2_repeater |
Standard repeater |
Inhero_MR2_repeater_bridge_rs232 |
Repeater with RS232 bridge (Serial2 on P0.19/P0.20) |
Inhero_MR2_sensor |
Sensor firmware |
Absolute Maximum Ratings¶
| Parameter | Min | Max | Unit |
|---|---|---|---|
| Solar input voltage (Voc) | — | 25 | V |
| Charge current (configurable) | 50 | 1500 | mA |
| Shunt current (INA228, 100 mΩ) | — | 1600 | mA |
| Ambient temperature (operating) | –40 | +85 | °C |
See Also¶
- README.md – Overview, feature matrix and diagnostics
- TELEMETRY.md — Telemetry channels explained (what the app displays)
- QUICK_START.md – Quick start for commissioning and CLI setup
- BATTERY_GUIDE.md – Battery chemistry comparison and deployment guide
- FAQ.md – Frequently asked questions
- CLI_CHEAT_SHEET.md – All board-specific CLI commands at a glance
- POWER_MANAGEMENT.md – Complete technical documentation