Inhero MR2¶

Inhero MR2

Table of Contents¶

Overview
Current Feature Matrix
Power Management Features
Firmware Build
CLI Commands
Diagnostics & Troubleshooting
Regulatory Notes & CE Compliance
See Also

Overview¶

The Inhero MR2 is an application-specific hardware platform for the autonomous, long-term operation of mesh infrastructure at hard-to-reach locations. Active idle consumption is 6.0 mA at 4.2 V and 7.7 mA at 3.3 V (USB off, no radio TX), which gives a full-featured repeater long battery life from a compact cell and a small solar panel. A universal solar input with active MPPT harvests over a wide input range, so a compact, low-profile installation is usually enough and the peripherals need no over-dimensioning. Li-ion, LiFePO4, LTO and Na-ion batteries are supported natively, and RTC-based recovery brings the board back on its own after a low-voltage shutdown, so an empty battery does not force a site visit. This lowers long-term operating costs at sites where a manual service visit is expensive because the location is hard to reach.

Hardware Version: Rev 1.1
Key Features: - Core: Based on RAK4630 (nRF52840 + SX1262). - Power Path: BQ25798 Buck/Boost Charger. Enables energy harvesting even when solar voltage is below battery voltage (critical for low-light conditions). - High-Efficiency Rail: 3.3V rail via TPS62840 for maximum efficiency. - Robust Monitoring: INA228 Coulomb Counter for precise SOC tracking (essential for LiFePO4 chemistry) and long-term energy statistics. - Universal Solar Input: 3.6V – 24V with autonomous MPPT tracking and integrated protection against "stuck states" (hardware watchdog logic). - Environmental Sensing & Timekeeping: Integrated BME280 and RV-3028 RTC for autonomous wake-up management and precise time base. See FAQ #23 for why a correct clock matters. - Form Factor: Only 45 × 40 mm – optimized for low-profile enclosures and minimal mechanical stress.

⚠ WARNING — No Reverse Polarity Protection: The board has no hardware reverse polarity protection on the battery or solar input. Connecting with reversed polarity will cause immediate, irreversible damage. Always verify correct polarity before connecting any power source.

Current Feature Matrix¶

Feature	Status	Notes
INA228 ALERT → Low-Voltage System Sleep	Active	ISR on P1.02 → volatile flag → tickPeriodic() → System Sleep with GPIO latch + RTC Wake
RTC Wakeup (Low-Voltage Recovery)	Active	60 min (periodic)
BQ CE Pin Safety (FET-inverted)	Active	GPIO HIGH → FET ON → CE LOW → charge ON (BQ25798 CE active-low), Dual-Layer: GPIO + I2C
System Sleep with latched CE	Active	< 500µA, GPIO4 latch preserved HIGH → FET ON → CE LOW → solar charging possible
SOC 0% after Low-Voltage Recovery	Active	SOC initialized to 0% on recovery, auto-sync on "Charging Done"
SOC via INA228 + manual battery capacity	Active	`set board.batcap` available
SOC→Li-ion mV Mapping (workaround)	Active	Will be removed when MeshCore transmits SOC% natively
MPPT Recovery + Stuck-PGOOD Handling	Active	Cooldown logic active
PFM Forward Mode	Active (chip default)	Enabled by BQ25798 power-on default (PFM_FWD_DIS=0, REG0x12); the firmware does not modify it. Improves efficiency at low solar currents

Power Management Features¶

Low-Voltage Handling (Flag/Tick Architecture)¶

INA228 ALERT fires at lowv_sleep_mv (hardware interrupt on P1.02)
ISR sets lowVoltageAlertFired = true (volatile flag only, no FreeRTOS call)
tickPeriodic() (main loop, next tick()) checks flag → shutdown:
CE pin → HIGH (FET ON → CE LOW → charging active)
P0.04 excluded from disconnectLeakyPullups() → GPIO latch preserved in sleep
RTC wake configured (LOW_VOLTAGE_SLEEP_MINUTES = 60 min)
sd_power_system_off() → System Sleep with GPIO latch (< 500µA)
RTC Wake (hourly) → system boots, early-boot checks VBAT:
Below lowv_wake_mv → immediately back to System Sleep (CE remains latched LOW)
Above lowv_wake_mv → normal boot, SOC starts at 0%

Note: All I2C operations (MPPT, SOC, Hourly Stats) run in main loop context via tickPeriodic() — no FreeRTOS tasks for I2C, no mutex needed.

BQ CE Pin (Rev 1.1 — FET-inverted)¶

DMN2004TK-7 N-FET: Gate ← GPIO4 (ext. pull-down), Drain → CE, Source → GND
GPIO HIGH → FET ON → CE LOW → charging ON (BQ25798 CE active-low)
GPIO LOW / High-Z → ext. pull-down on gate → FET OFF → pull-up on CE → CE HIGH → charging OFF
System Sleep: GPIO4 latch preserved HIGH (excluded from disconnectLeakyPullups()) → FET ON → CE LOW → solar charging active
Safety default: RAK unpowered/unflashed → pull-down on gate → FET OFF → CE HIGH → charging disabled
Dual-Layer: CE pin (hardware FET) + setChargeEnable() (I2C register)

Voltage Thresholds (all chemistries)¶

Chemistry	lowv_sleep_mv	lowv_wake_mv	Hysteresis
Li-ion 1S	3100	3300	200mV
LiFePO4 1S	2700	2900	200mV
LTO 2S	3900	4100	200mV
Na-ion 1S	2500	2700	200mV

lowv_sleep_mv: INA228 ALERT threshold → triggers System Sleep with GPIO latch
lowv_wake_mv: RTC wake threshold → boot only when VBAT is above, also 0% SOC marker

System Sleep Power Consumption¶

< 500µA total consumption (nRF52840 System-Off + RTC + quiescent currents of all components)
GPIO4 latch preserved HIGH → FET ON → CE LOW → solar charging active

Active Idle Power Consumption¶

6.0 mA @ VBAT 4.2 V (USB off, no radio TX)
7.7 mA @ VBAT 3.3 V (USB off, no radio TX)
+0.8–1.0 mA with USB peripheral enabled
USB is auto-managed: enabled on VBUS detect, disabled on removal

Power Saving Measures¶

WFE Idle (board.sleep(0)): CPU enters Wait-For-Event between loop iterations. Wakes on any interrupt (radio, SysTick, USB, I2C) — typically within 1 ms. Reduces nRF52840 CPU current from ~3 mA (busy-loop) to ~0.5–0.8 mA.
USB Auto-Disable: nRF52840 USB peripheral is automatically disabled when no VBUS is detected, saving ~0.8–1.0 mA. Re-enabled automatically when USB cable is connected.

Coulomb Counter & SOC Tracking¶

Real-time SOC tracking via INA228 (±0.1% accuracy)
100mΩ shunt resistor (1.6A max current)
200mV uniform hysteresis for all chemistries (lowv_sleep_mv → lowv_wake_mv)
Manual capacity: set board.batcap for fixed capacity

SOC→Li-ion mV Mapping (Workaround)¶

Problem: MeshCore only transmits getBattMilliVolts(), not SOC%. The MeshCore app uses a Li-ion curve for SOC calculation — incorrect display for LiFePO4/LTO.
Solution: When valid coulomb-counting SOC is available, an equivalent Li-ion 1S OCV (3000–4200 mV) is returned, so the app displays the correct SOC%. See TELEMETRY.md for details on how this affects the app display.
This workaround will be removed once MeshCore supports native SOC% transmission.
→ FAQ #11 — SOC shows 0% or N/A?

Batt-TTL Prediction¶

Batt-TTL is short for battery time-to-live — the estimated remaining runtime on battery. It is not the packet hop limit that "TTL" denotes in mesh networking. - Time base: 7-day moving average (avg_7day_daily_net_mah) of daily net energy consumption - Data source: 168-hour ring buffer (7 days) with hourly INA228 coulomb counter samples (charged/discharged/solar mAh) - Formula: TTL_hours = max(0, SOC% × capacity_mah / 100 − capacity_mah × (1 − f(T))) / |avg_7day_daily_net_mah| × 24 - f(T): Cold-temperature derating factor (trapped-charge model) — at low temperatures part of the stored charge is unusable and is subtracted first; f(T) = 1 at warm temperatures - Prerequisites: living_on_battery == true (24h deficit), min. 24h data, capacity known - Batt-TTL = 0: Solar surplus, no 24h data available, or capacity unknown - CLI: Batt-TTL is shown in get board.stats (BAT mode only, e.g. BT:12d0h) - Telemetry: Transmitted as days via CayenneLPP Distance field (max. 990 days for "infinite"). See TELEMETRY.md for channel details.

Solar Power Management¶

Solar current display: The BQ25798 IBUS ADC is inaccurate at low currents (~±30mA error). Therefore solar current is displayed in steps:
0mA — ADC reports exactly 0 (no solar current)
<50mA — 1–49mA (ADC unreliable in this range)
~72mA — 50–100mA with rounding symbol ~ (limited accuracy)
385mA — >100mA without rounding symbol (sufficiently accurate)
Always integer without decimal places (no pseudo-precision)
PFM Forward Mode: PFM forward mode is enabled by BQ25798 power-on default (PFM_FWD_DIS=0, REG0x12); the firmware does not modify it. Improves efficiency at low currents.
MPPT VOC_PCT 81.25%: The BQ25798 MPPT is configured to VOC_PCT=81.25% (instead of the chip default 87.5%). This value matches the typical Vmp/Voc ratio of crystalline silicon solar cells (~80-83%).
MPPT Recovery: Re-enables MPPT on PowerGood=1 (readback check: only on actual change)
BQ INT pin not used: No interrupt — pure polling every 60s in runMpptCycle()
Error monitoring: Diagnostic commands show FAULT_STATUS registers (0x20, 0x21) for detailed analysis incl. VBAT_OVP, VBUS_OVP and temperature conditions
VREG display: Shows the actually configured battery regulation voltage in diagnostics for threshold verification

JEITA Temperature Zone Configuration¶

The BQ25798 uses the TS pin (NTC thermistor) for JEITA-compliant temperature-dependent charge control. The Inhero MR2 uses a voltage divider (RT1=5.6 kΩ pullup to REGN, RT2=27 kΩ parallel to GND) that shifts TS thresholds lower than the TI reference design (5.24 kΩ / 30.31 kΩ). The shift is temperature-dependent: ~5–6 °C in the cold range, ~2–3 °C in the warm/hot range (because at low temperatures the NTC resistance is large relative to RT2, amplifying the divider mismatch).

JEITA Zone	BQ25798 Threshold	TI Reference	Inhero MR2 (actual)	Shift	Firmware Config
T-Cold (charge suspend)	VT1 = 72.0% REGN	+3.7 °C	−2.0 °C	−5.7 °C	— (not configurable)
T-Cool (reduced current)	VT2 = 69.8% REGN	+7.9 °C	+2.8 °C	−5.1 °C	`set board.fmax`
T-Warm start	VT3 = 37.7% REGN	+54.5 °C	+52.2 °C	−2.3 °C	`TS_WARM = 55°C` register setting
T-Hot (charge suspend)	VT5 = 34.2% REGN	+59.9 °C	+57.7 °C	−2.2 °C	— (not configurable)

Calculation based on: NTC 103AT (B25/50=3435) for TI reference, NCP15XH103F03RC (B25/85=3380) for Inhero. Typical %REGN values from BQ25798 datasheet.

Key firmware settings in configureBaseBQ():

TS_WARM = 55°C (BQ register value): Moves the WARM zone threshold from the default 45 °C setting (44.8% REGN, ~41.8 °C with Inhero divider) up to 37.7% REGN (~52.2 °C with Inhero divider). This prevents premature WARM zone entry at moderate temperatures.
JEITA_VSET = UNCHANGED: No battery regulation voltage reduction in the WARM zone. The POR default (VREG−400 mV) would reduce VREG to 3.1 V for LiFePO4, causing VBAT_OVP at normal battery voltages (3.3–3.5 V).
JEITA_ISETH = ICHG unchanged (POR default, retained): No charge current reduction in the WARM zone. Combined with JEITA_VSET=UNCHANGED, the WARM zone is effectively neutralized — charging continues at full voltage and full current.
AUTO_IBATDIS = disabled: Disables the BQ25798's automatic 30 mA battery discharge during VBAT_OVP. The POR default actively drains the battery at ~30 mA (IBAT_LOAD) when OVP is triggered, which is counterproductive for solar-powered systems.

Background: With default BQ25798 settings, the combination of the Inhero divider offset and LiFePO4 chemistry caused a failure chain at ~42 °C: WARM zone entry → VREG reduced to 3.1 V → VBAT_OVP (battery at 3.47 V > 104% × 3.1 V) → active 30 mA discharge → net −45 mA drain despite solar input. The settings above prevent this entirely. The WARM zone (52–58 °C with Inhero divider) now has no effect on charging behavior.

Firmware Build¶

# Repeater (default)
platformio run -e Inhero_MR2_repeater

# Repeater with RS232 bridge (Serial2 on P0.19/P0.20)
platformio run -e Inhero_MR2_repeater_bridge_rs232

# Sensor
platformio run -e Inhero_MR2_sensor

CLI Commands¶

Get Commands¶

get board.bat       # Query current battery type
                    # Output: liion1s | lifepo1s | lto2s | naion1s | none

get board.fmax      # Query frost charge behavior
                    # Output: 0% | 20% | 40% | 100%
                    # Value = max charge current in T-Cool range
                    # (approx. -2 °C to +3 °C, see JEITA table in README),
                    # relative to board.imax
                    # 40% at imax=500mA → max. 200mA charge current in T-Cool range
                    # 0% = charging blocked in T-Cool range
                    # 100% = no reduction (full current even in cold)
                    # Below approx. -2 °C (T-Cold): charging always completely blocked (JEITA)
                    # Note: Only charging is restricted. With sufficient
                    # solar, the board continues to run on solar power —
                    # the battery is neither charged nor discharged.
                    # LTO / Na-ion batteries: N/A (JEITA disabled, charges even in frost)

get board.imax      # Query maximum charge current
                    # Output: <current>mA (e.g. 200mA)

get board.mppt      # Query MPPT status
                    # Output: MPPT=1 (enabled) | MPPT=0 (disabled)

get board.telem     # Query real-time telemetry with SOC
                    # Output: B:<V>V/<I>mA/<T>C SOC:<percent>% S:<V>V/<solar current>
                    # Examples:
                    #   B:3.85V/125.4mA/22C SOC:68.5% S:5.12V/385mA      (>100mA: accurate)
                    #   B:3.85V/-8.2mA/18C SOC:72.0% S:4.90V/~72mA      (50-100mA: ~estimate)
                    #   B:3.30V/-45.0mA/5C SOC:40.1% S:0.00V/<50mA      (<50mA: ADC inaccurate)
                    # Output variants:
                    # - SOC:N/A — no valid coulomb-counting SOC available
                    # - SOC:68.5% (52%) — second value = cold-derated SOC,
                    #   shown when the temperature derating factor is < 1
                    # - <T>C becomes N/A when the NTC temperature is unavailable
                    # Components:
                    # - B: Battery (Voltage/Current/Temperature/SOC)
                    # - S: Solar (Voltage/Current — accuracy depends on BQ25798 IBUS ADC)

get board.stats     # Query energy statistics (balance + MPPT)
                    # Output: <24h>/<3d>/<7d>mAh C:<24h> D:<24h> 3C:<3d> 3D:<3d> 7C:<7d> 7D:<7d> <SOL|BAT> M:<mppt>% BT:<ttl>
                    # Example: +125/+45/+38mAh C:200 D:75 3C:150 3D:105 7C:140 7D:102 SOL M:85% BT:N/A
                    # Example: -30/-45/-40mAh C:10 D:40 3C:5 3D:50 7C:8 7D:48 BAT M:45% BT:72h
                    # Components:
                    # - +125: Last 24h net balance (charge - discharge) in mAh
                    # - +45: 3-day average net balance in mAh
                    # - +38: 7-day average net balance in mAh
                    # - C/D: Charged/Discharged mAh (24h)
                    # - 3C/3D: 3-day average charged/discharged mAh
                    # - 7C/7D: 7-day average charged/discharged mAh
                    # - SOL: Running on solar (self-sufficient)
                    # - BAT: Living on battery (deficit mode)
                    # - M:85%: MPPT enabled percentage (7-day average)
                    # - BT:72h: Batt-TTL (only shown if BAT mode, 7d-avg basis)
                    #   Format: BT:12d5h (≥24h) or BT:72h (<24h) or BT:N/A

get board.cinfo     # Charger info + last PG-stuck HIZ toggle
                    # Output: <state> + flags
                    # States: !CHG, PRE, CC, CV, TRICKLE, TOP, DONE

get board.selftest  # I²C hardware probe (all on-board devices)
                    # Output: INA:<state> BQ:<state> RTC:<state> BME:<state>
                    # States: OK | NACK | WR_FAIL (RTC only, write-verify mismatch)

get board.conf      # Query all configuration values
                    # Output: B:<bat> F:<fmax> M:<mppt> I:<imax> Vco:<voltage> V0:<0%SOC>

get board.batcap    # Query battery capacity
                    # Output: <capacity> mAh (set) or <capacity> mAh (default)
                    # Shows whether capacity was manually set or chemistry default

get board.tccal     # Query NTC temperature calibration offset
                    # Output: TC offset: <+/-offset> C (0.00=default)

get board.leds      # Query LED enable status
                    # Output: "LEDs: ON (Heartbeat + BQ Stat)" or "LEDs: OFF (Heartbeat + BQ Stat)"
                    # Shows whether heartbeat LED and BQ25798 stat LED are enabled

Set Commands¶

set board.bat <type>           # Set battery type
                               # Options: lto2s | lifepo1s | liion1s | naion1s | none
                               # none = no battery / unknown (charging disabled)

set board.fmax <behavior>      # Set frost charge behavior
                               # Options: 0% | 20% | 40% | 100%
                               # Limits charge current in T-Cool range
                               # (approx. -2 °C to +3 °C, see JEITA table in README)
                               # to X% of board.imax
                               # 0% = charging blocked in T-Cool range
                               # 20% = max. 20% of imax in T-Cool range
                               # 40% = max. 40% of imax in T-Cool range
                               # 100% = no reduction
                               # Below approx. -2 °C (T-Cold): charging always blocked (JEITA)
                               # Note: Only charging is restricted. With sufficient
                               # solar, the board continues to run on solar power —
                               # the battery is neither charged nor discharged.
                               # N/A for LTO / Na-ion batteries (JEITA disabled)

set board.imax <current>       # Set maximum charge current in mA
                               # Range: 50-1500mA (BQ25798 minimum: 50mA)

set board.mppt <1|0>           # Enable/disable MPPT
                               # 1 = enabled, 0 = disabled

set board.batcap <capacity>    # Set battery capacity in mAh
                               # Range: 100-100000 mAh
                               # Used for accurate SOC calculation

set board.tccal                # Calibrate NTC temperature
                               # Two modes:
                               # 1) set board.tccal         → auto-calibration via BME280
                               #    Output: TC auto-cal: BME=<temp> offset=<+/-offset> C
                               # 2) set board.tccal reset   → reset offset to 0.00
                               #    Output: TC calibration reset to 0.00 (default)

set board.leds <on|off>        # Enable/disable heartbeat + BQ stat LED
                               # on/1 = enable, off/0 = disable
                               # Boot LEDs follow this setting; only the
                               # low-voltage recovery flash (3 blue blinks)
                               # is always active

set board.soc <percent>        # Manually set SOC
                               # Range: 0-100
                               # Note: INA228 must be initialized

Diagnostics & Troubleshooting¶

I²C Hardware Self-Test¶

get board.selftest

Probes all I²C devices on the board and reports their status in one line:

INA:OK BQ:OK RTC:OK BME:OK

Device	Address	Test
`INA`	`0x40`	INA228 power monitor — address ACK
`BQ`	`0x6B`	BQ25798 charger — address ACK
`RTC`	`0x52`	RV-3028 RTC — address ACK plus user-RAM (`0x1F`) write/readback verification with two patterns (`0xA5`, `0x5A`); original byte is restored
`BME`	`0x76`	BME280 environment sensor — address ACK

Possible per-device states:

OK — device responds (and, for the RTC, persists writes correctly)
NACK — device does not acknowledge on the I²C bus
WR_FAIL — RTC only — chip ACKs but write/readback mismatched. The same write-verify runs in BoardConfigContainer::begin() and triggers the slow red error LED on failure, so the board is flagged as faulty before deployment.

BQ25798 Register Verification¶

The diagnostic functions enable precise verification of BQ25798 registers against the datasheet:

Key Registers: - 0x0F (CHARGER_CONTROL_0): EN_CHG (Bit 5) - 0x15 (MPPT_CONTROL): EN_MPPT (Bit 0), VOC_PCT (Bits 7-5), VOC_DLY (Bits 4-3), VOC_RATE (Bits 2-1) - 0x1B (CHARGER_STATUS_0): PG_STAT (Bit 3), VINDPM (Bit 6), IINDPM (Bit 7) - 0x1C (CHARGER_STATUS_1): CHG_STAT (Bits 7-5), VBUS_STAT (Bits 4-1) - 0x1F (CHARGER_STATUS_4): Temperature status (Bits 3-0)

Known Issues: 1. MPPT disabled: BQ25798 automatically sets MPPT=0 when PG=0 - Solution: checkAndFixSolarLogic() re-enables MPPT on PG=1 2. PG stuck at sunrise: VBUS rises slowly, BQ fails to qualify the source - Solution: checkAndFixSolarLogic() toggles HIZ when VBUS ≥ 4.5V + PG=0 (5min cooldown)

→ FAQ #9 — Red LED blinks / battery not charging

Regulatory Notes & CE Compliance (RED 2014/53/EU)¶

The Inhero MR2 is shipped as a hardware platform (development module) with a pre-installed bootloader. The hardware is CE-marked and conforms to the European Radio Equipment Directive (RED 2014/53/EU); the corresponding tests were carried out by an accredited test laboratory and an EU Declaration of Conformity is on file. Radiated power certification was performed using the designated reference antennas (RAK FPCB antenna 863–870 MHz, MHF1 connector, antenna gain: 0.7 dBi).

Requirements for legally compliant operation of radio firmware: Since the final transmission characteristics (TX power, frequency, duty cycle) are largely determined by the software installed by the user (e.g. MeshCore) and the chosen antenna, the following European limits (per EN 300 220 and EN 300 328, ERC/REC 70-03 Annex 1) must be strictly observed:

Standard LoRa (868 MHz band):
Frequency range: 865.0 – 868.6 MHz
Max. radiated TX power (ERP): 25 mW (14 dBm)
Max. duty cycle: 1% (or LBT+AFA per EN 300 220)
Note: Different duty cycle requirements per ERC/REC 70-03 apply in the 863.0 – 865.0 MHz sub-band.
High-Power LoRa (special band 869.5 MHz):
Frequency range: 869.40 – 869.65 MHz (MeshCore default channel)
Max. radiated TX power (ERP): 500 mW (27 dBm)
Max. duty cycle: 10%
Hardware note: The onboard LoRa transceiver (SX1262) provides a maximum conducted TX power of 22 dBm. To fully utilize the legal limit of 500 mW ERP (equivalent to 29.15 dBm EIRP), an antenna with a gain of approx. +7 dBi is required (minus any cable losses). With the included FPCB antenna (0.7 dBi), max. approx. 114 mW ERP is achieved.
Bluetooth Low Energy (2.4 GHz):
Max. radiated TX power (EIRP): 100 mW (20 dBm)

Antennas & operator responsibility (EIRP/ERP limit): The user is obligated to match the configured TX power in the chip with the antenna gain. If an antenna is used whose gain, in combination with the configured TX power, exceeds the legal EIRP/ERP limits stated above, the TX power must be reduced in software.

Disclaimer: The Inhero MR2 is a module intended for professional developers and qualified users. If the legal parameters are operated outside EU norms due to the choice of firmware, antenna, or manual configuration, the CE compliance of the device is void. In this case, all legal responsibility for operation transfers to the integrator or end user.