M5Stack Cardputer ADV Volume 5 — Cap LoRa-1262, LoRa CSS Theory, and GNSS — M5Stack Cardputer ADV · Hacking

SX1262 sub-GHz radio + AT6668 multi-constellation GNSS + CSS modulation + link budget + regional rules

Section	Topic
1	About this volume
2	The Cap LoRa-1262 module
· 2.1	Block diagram + component function
· 2.2	Authoritative spec table
3	SX1262 — the sub-GHz radio
· 3.1	Frequency coverage and modulation modes
· 3.2	Pin assignments on the EXT bus
· 3.3	Bring-up sequence
· 3.4	RF antenna switch (FM8625H) — must-enable-via-I²C
· 3.5	RadioLib code example
4	LoRa CSS theory of operation
· 4.1	Chirp Spread Spectrum modulation
· 4.2	The four tuning knobs — SF, BW, CR, TX power
· 4.3	Sensitivity by spreading factor
· 4.4	Link budget walkthrough
· 4.5	Time-on-air calculator
5	Regional LoRa rules
· 5.1	EU 868 MHz
· 5.2	US 902–928 MHz
· 5.3	Other regions
· 5.4	The Cap LoRa-1262 EU g1 compliance gotcha
6	GNSS — AT6668 multi-constellation receiver
· 6.1	Constellations supported
· 6.2	Time-to-first-fix (TTFF)
· 6.3	Interface — UART + NMEA + CASIC binary
· 6.4	Code example — Arduino NMEA parsing
7	Cap + Cardputer ADV combined power profile
8	Antenna upgrade options
9	Resources

1. About this volume

Vol 5 covers the Cap LoRa-1262 in full — the SX1262 LoRa radio + AT6668 GNSS daughterboard that turns the Cardputer ADV from “an ESP32-S3 handheld” into “an off-grid Meshtastic + GPS handheld”. It’s the flagship Cap and the single most-impactful add-on for Cardputer ADV utility.

Three components covered: (1) the SX1262 sub-GHz radio, (2) the CSS modulation theory that explains how LoRa achieves its absurd range, (3) the AT6668 multi-constellation GNSS receiver that pairs with the LoRa for position broadcasting.

Combined with Meshtastic firmware (Vol 6 § 5.1), this Cap is what makes the Cardputer ADV a viable Meshtastic node. Without it, the Cardputer ADV is just an ESP32-S3 handheld.

2. The Cap LoRa-1262 module

2.1 Block diagram + component function

       ┌─────────────────────────────────────────────────────┐
       │  Cap LoRa-1262 (M5Stack SKU U214)                   │
       │                                                     │
       │   ┌──────────┐    ┌─────────────┐    ┌──────────┐  │
       │   │ SX1262   │────│ FM8625H     │────│ RP-SMA   │──┼── Antenna
       │   │ LoRa     │    │ ant. switch │    │ female   │  │   (3 dBi whip
       │   │ +22 dBm  │    │ (TX/RX/off) │    │          │  │    included)
       │   └────┬─────┘    └──────┬──────┘    └──────────┘  │
       │        │                 │ enable                  │
       │        │ SPI             │                         │
       │        │                 │                         │
       │   ┌────┴────┐   ┌────────┴────────┐               │
       │   │ EXT     │   │ PI4IOE5V6408    │               │
       │   │ control │   │ I²C GPIO exp.   │               │
       │   │ (4 lines)│   │ (addr 0x43)    │               │
       │   └────┬────┘   └────────┬────────┘               │
       │        │                 │                         │
       │        │                 │ I²C (G8/G9)             │
       │        │                 │                         │
       │   ┌────┴─────────────────┴───────┐                │
       │   │ 14-pin EXT bus header        │ ────────────── To Cardputer ADV
       │   │ (mates Cardputer ADV)        │                │
       │   └──────────────────────────────┘                │
       │                                                     │
       │   ┌──────────┐    ┌─────────────────┐              │
       │   │ AT6668   │────│ on-board patch  │              │
       │   │ GNSS     │    │ antenna (top)   │              │
       │   │ multi-   │    │                 │              │
       │   │ const.   │    └─────────────────┘              │
       │   └────┬─────┘                                     │
       │        │ UART 115200 (G13/G15)                     │
       │        │                                           │
       │   (routes through EXT)                             │
       │                                                     │
       └─────────────────────────────────────────────────────┘

Component function:

Component	Function	Notes
SX1262	LoRa transceiver — TX up to +22 dBm, RX down to −147 dBm	Semtech. Shielded under RF can.
FM8625H	RF antenna switch	Routes between TX path, RX path, and off. Must be enabled via I²C before use (§ 3.4).
PI4IOE5V6408	I²C GPIO expander	Address 0x43. Drives FM8625H enable + other Cap control.
AT6668	GNSS receiver (ATGM336H-6N module)	CASIC chip in a M-class module package. UART output.
Patch antenna	GNSS antenna on top of Cap	~25 × 25 mm passive patch, ~5 dBi nominal gain.
RP-SMA female	LoRa antenna connector	Industry standard for sub-GHz; supplied antenna mates here.
RF shield can	Over SX1262	Reduces RFI from Cardputer ADV digital traffic.

2.2 Authoritative spec table

Parameter	Value
LoRa radio
Chip	Semtech SX1262
Frequency coverage (chip)	150 MHz - 960 MHz
Frequency tuned (Cap)	868 - 923 MHz (via antenna + matching network)
TX power range	−9 dBm to +22 dBm (1 dBm steps)
TX peak current	~163 mA at +22 dBm, 5V supply
RX sensitivity	−147 dBm at SF12 BW125 (absolute floor)
Modulation modes	CSS (LoRa), FSK, GFSK, MSK, GMSK, OOK
RX current	~12 mA (continuous receive)
Sleep current	<1 µA (with retention)
GNSS
Module	ATGM336H-6N
Chip	CASIC AT6668
Channels	50
Constellations	GPS L1 + GLONASS L1 + Galileo E1 + BeiDou B1I/B1C + QZSS + SBAS
Position accuracy	<1.5 m CEP50 (consumer-grade single-frequency)
TTFF cold	~23 s
TTFF warm (<4h almanac)	~10 s
TTFF hot (<5 min almanac)	~1 s
Update rate	1-10 Hz configurable
Output	NMEA 0183 v4.1 + CASIC binary protocol
UART	115200 bps default (configurable to 9600 / 38400 / 230400 etc.)
Antenna
LoRa connector	RP-SMA female
Included LoRa antenna	3 dBi rubber-duck, 108 × 9.3 mm
GNSS antenna	On-board passive patch (not user-replaceable)
Mechanical
Dimensions	84 × 24 × 15.2 mm
Weight	22.1 g
Mating	14-pin EXT bus (Vol 3 § 4)

3. SX1262 — the sub-GHz radio

3.1 Frequency coverage and modulation modes

The SX1262 silicon covers 150 MHz - 960 MHz natively. The Cap LoRa-1262 is tuned by its antenna network + matching to 868 - 923 MHz. Out-of-band operation (e.g., 433 MHz) is electrically possible — the chip will TX/RX at 433 MHz — but the antenna’s VSWR will be poor, the radiated power will be much lower than the chip output, and the matching network may have detuned passband. For 433 MHz work, re-antenna the Cap (§ 8) or use a separate SX1262 module wired to EXT (Vol 4 § 6.5).

Modulation modes (the SX1262 silicon supports all, the Cap exposes all):

Mode	Use case	Typical Cardputer ADV firmware
CSS (LoRa)	Long-range mesh / point-to-point — the canonical mode	Meshtastic, CardputerLoRaChat
FSK / GFSK	Higher-rate point-to-point (compatible with non-LoRa sub-GHz protocols)	Custom firmware, niche
MSK / GMSK	Some legacy protocols	Rare on Cardputer ADV
OOK	Fixed-code remote replay (433 MHz garage doors, etc.)	Bruce sub-GHz with on-EXT SX1262

For 99% of Cardputer ADV use cases, you’ll be in CSS mode (LoRa-proper).

3.2 Pin assignments on the EXT bus

The SX1262 connects to the Cardputer ADV via these EXT-bus signals (cross-ref Vol 3 § 4.1):

Signal	EXT pin	GPIO	Function
RESET	3	GPIO 3	Active-low reset
INT (DIO1)	4	GPIO 4	Interrupt — TX done, RX done, timeout, header valid
CS (NSS)	5	GPIO 5	SPI chip-select
BUSY	6	GPIO 6	Busy status — must be low before sending SPI commands
SCK	11	GPIO 40	SPI clock
MISO	10	GPIO 39	SPI data in (to ESP32-S3)
MOSI	12	GPIO 14	SPI data out (from ESP32-S3)

Plus the I²C bus (G8/G9 → EXT pins 7/8) for the PI4IOE5V6408 GPIO expander that controls the FM8625H antenna switch.

3.3 Bring-up sequence

Six steps to bring up SX1262 from cold:

Initialize I²C — Wire.begin(8, 9) (or M5Cardputer’s pre-initialized Wire).
Enable RF switch — set PI4IOE5V6408 (I²C addr 0x43) port P0 HIGH. This connects the FM8625H switch’s enable line. Without this step, the SX1262 will transmit but no RF will emerge from the antenna. Most-common new-user bug.
Initialize SPI — configure MOSI=G14, MISO=G39, SCK=G40 (or use M5Cardputer’s pre-initialized SPI).
Pulse RST — drive G3 (EXT RESET) low for ≥100 µs, then high. Waits for the SX1262 to finish its internal reset.
Wait BUSY low — poll G6 (EXT BUSY) until low. The SX1262 is busy during internal startup; SPI commands must wait.
SetStandby(STDBY_RC) — the standard “ready for commands” SX1262 state. From here, configure modulation, frequency, power, then TX or RX.

All subsequent SPI commands per the SX1262 datasheet (Semtech publishes the full command reference).

3.4 RF antenna switch (FM8625H) — must-enable-via-I²C

The single most-common Cap LoRa-1262 footgun: forgetting to enable the FM8625H antenna switch.

The FM8625H is a small RF SPDT switch that routes between TX path and RX path. It has an enable pin that’s controlled by the PI4IOE5V6408 I²C GPIO expander (I²C address 0x43, port P0). At power-on, P0 is in its default state (low). The SX1262 will accept SPI commands, generate RF, but no RF reaches the antenna because the switch is disabled.

The fix:

// Pseudo-code for FM8625H enable via PI4IOE5V6408
Wire.beginTransmission(0x43);
Wire.write(0x03);              // Output register
Wire.write(0x01);              // P0 = HIGH (P1-P7 = LOW)
Wire.endTransmission();

// Now the SX1262 + FM8625H + antenna path is operational

Meshtastic, Bruce, CardputerLoRaChat all do this automatically. Bare RadioLib code from tutorials sometimes omits it — symptoms: SetTxConfig + transmit succeeds in software (no SPI errors), but received signal strength at the other end is “as if no transmission happened.” Trace to FM8625H enable.

Some firmwares hide this in a M5Stack_LoRa.begin() wrapper; others require explicit configuration. When debugging “I can’t receive my own transmissions,” check the FM8625H first.

3.5 RadioLib code example

#include <SPI.h>
#include <Wire.h>
#include <RadioLib.h>

// EXT-bus pins
#define LORA_CS    5
#define LORA_RST   3
#define LORA_INT   4    // DIO1
#define LORA_BUSY  6

// FM8625H enable
#define IOEXP_ADDR 0x43
#define IOEXP_OUTPUT_REG 0x03
#define IOEXP_P0_HIGH 0x01

SX1262 radio = new Module(LORA_CS, LORA_INT, LORA_RST, LORA_BUSY);

void enable_rf_switch() {
    Wire.beginTransmission(IOEXP_ADDR);
    Wire.write(IOEXP_OUTPUT_REG);
    Wire.write(IOEXP_P0_HIGH);
    Wire.endTransmission();
}

void setup() {
    Serial.begin(115200);
    Wire.begin(8, 9);                  // Primary I²C
    SPI.begin(40, 39, 14, LORA_CS);   // SCK, MISO, MOSI, CS

    enable_rf_switch();                // CRITICAL — enable FM8625H

    int state = radio.begin(915.0,    // Frequency 915 MHz (US ISM)
                            125.0,     // BW 125 kHz
                            9,         // SF 9
                            5,         // CR 4/5
                            0x12,      // Sync word (private network)
                            14,        // +14 dBm TX power
                            8,         // Preamble length
                            1.6,       // TCXO voltage 1.6 V (Cap uses TCXO)
                            false);    // Use DC-DC regulator

    if (state == RADIOLIB_ERR_NONE) {
        Serial.println("SX1262 init OK");
    } else {
        Serial.printf("SX1262 init failed: %d\n", state);
        while (true) delay(10);
    }
}

void loop() {
    Serial.print("Sending packet... ");
    int state = radio.transmit("Hello from Cardputer ADV");
    if (state == RADIOLIB_ERR_NONE) {
        Serial.println("OK");
    } else {
        Serial.printf("failed: %d\n", state);
    }
    delay(5000);
}

This compiles + flashes + runs against the Cap LoRa-1262 directly. For receive, use radio.receive(). For non-blocking operation, use radio.startTransmit() + radio.startReceive() with DIO1-based interrupt handlers.

4. LoRa CSS theory of operation

4.1 Chirp Spread Spectrum modulation

LoRa is a Semtech-proprietary modulation built around Chirp Spread Spectrum (CSS). A LoRa symbol is a frequency sweep — a linear chirp from f₀ to f₁ across the channel bandwidth.

The trick: data is encoded by cyclic-shifting the chirp’s starting frequency. A receiver correlates the incoming signal against a known reference chirp; the cyclic shift that gives maximum correlation = the data bits encoded.

Why this works at extreme low SNR:

Process gain — spreading the symbol across the full bandwidth gives the demodulator gain over noise. SF7 chips at 4× the bit rate but with 4× more bandwidth — net SNR margin improves by ~24 dB vs simple FSK at equivalent power.
Doppler-robust — the chirp’s frequency-varying nature is tolerant of small frequency offsets caused by motion (drone hovering, vehicle passing, antenna swaying).
Multipath-tolerant — the chirp’s time-frequency structure resolves multipath echoes that confuse narrow-bandwidth modulations.

The cost: spectral efficiency is poor compared to OFDM / modern Wi-Fi modulations. LoRa is not for fast bulk data. LoRa is for “is this transmitter reachable from km away?” — which is exactly what Meshtastic needs.

4.2 The four tuning knobs — SF, BW, CR, TX power

LoRa configuration has four primary tunables:

Knob	Range	Effect
Spreading Factor (SF)	5 - 12	+1 SF = +2× range, +2× ToA, halved data rate
Bandwidth (BW)	7.8 - 500 kHz	+1 step = -3 dB sensitivity, +2× data rate, +2× ToA
Coding Rate (CR)	4/5 - 4/8	Higher CR = more FEC overhead = more error tolerance, lower effective rate
TX power	−9 dBm to +22 dBm	Per 1 dB step. +20 dBm = 100 mW; +22 dBm = 158 mW.

The tradeoff matrix is the central LoRa design decision — pick once per channel, configure both ends to match. Meshtastic ships standard “presets” (LongFast, ShortFast, etc., Vol 9 § 3.1) that encode common tradeoff points.

Rule of thumb:

High SF + low BW = maximum range, slowest data, longest ToA.
Low SF + high BW = shortest range, fastest data, shortest ToA.
High CR = more error correction, useful in noisy environments.
Higher TX power = longer range linearly until clip on legal EIRP limits (§ 5).

4.3 Sensitivity by spreading factor

Receiver sensitivity table (BW 125 kHz, the Meshtastic default):

SF	Sensitivity (dBm)	Data rate (bps)
7	−120	5470
8	−123	3125
9	−126	1758
10	−129	977
11	−132	537
12	−147	292

(Sensitivity at SF12 jumps to −147 dBm — the absolute floor. This isn’t a typo; CSS achieves this via the spreading gain.)

For BW 250 kHz, subtract 3 dB from each sensitivity number (less spreading). For BW 500 kHz, subtract another 3 dB.

Why −147 dBm is significant: this is roughly 6 dB below the natural thermal noise floor of a 125 kHz receiver. LoRa demodulates below the noise floor via spreading gain. Conventional FSK / FM demodulators cannot do this.

4.4 Link budget walkthrough

For a hypothetical Cardputer ADV → Cardputer ADV link at +22 dBm TX, 3 dBi antennas, SF12 BW125 sensitivity −147 dBm:

   Link budget = TX power + TX antenna gain + RX antenna gain − RX sensitivity
              = +22 dBm + 3 dBi + 3 dBi + 137 dB (sensitivity expressed as positive)
              = +22 + 6 − (−147)
              = +22 + 6 + 147
              = 175 dB total link budget at SF12 BW125

(Actually: link budget = +22 dBm + 3 + 3 − (−147 dBm) = +22 + 3 + 3 + 147 = 175 dB.)

Free-space path loss at 915 MHz vs distance:

Distance	Path loss (dB)	Margin (dB at 175 dB budget)
1 km	92	83 dB
10 km	112	63 dB
100 km	132	43 dB
1000 km	152	23 dB

In free space (no obstacles), SF12 BW125 LoRa at +22 dBm has over 1000 km of theoretical range. In practice — obstacles, multipath, antenna inefficiencies, RF noise — real-world performance is much worse:

Environment	Realistic range (SF12 BW125, +22 dBm, 3 dBi antennas)
Urban (buildings)	200 m - 2 km
Suburban (line-of-sight broken by trees / houses)	5 - 15 km
Rural / open field	15 - 30 km
Air-to-ground (drone TX, ground station RX)	100+ km (record: 700+ km in optimal conditions)
Boat-to-shore over water	50+ km

Higher-gain antennas extend these proportionally. A 6 dBi omni doubles range vs 3 dBi (+3 dB on each end = +6 dB link budget, ~2× distance in free space).

4.5 Time-on-air calculator

LoRa packet airtime depends on SF, BW, CR, payload length, and preamble length. For a typical 50-byte Meshtastic position packet:

Configuration	ToA
SF7 BW125 CR4/5	~85 ms
SF9 BW125 CR4/5	~250 ms
SF11 BW250 CR4/5 (Meshtastic LongFast)	~510 ms
SF12 BW125 CR4/8	~2000 ms (2 seconds!)

Implication for EU duty cycle (§ 5.1):

EU 868 g1 allows 1% duty cycle = 36 seconds of TX per hour
SF12 BW125 CR4/8 packet at 2 s ToA = 18 packets per hour maximum before the duty cycle locks you out
After hitting the 1% limit, the radio must stay silent for the remaining hour

For SF12 use in EU, packet rate is severely limited. Meshtastic’s region-default LongFast (SF11/BW250) at 510 ms ToA enables ~7 packets/min sustained — practical for mesh chat.

5. Regional LoRa rules

LoRa hardware operates in ISM bands per region. Rules differ on:

Frequency band allocated for unlicensed sub-GHz use
Maximum EIRP (Effective Isotropic Radiated Power = TX power + antenna gain)
Duty cycle or dwell-time restrictions
Channel plan (specific frequencies vs free-form)

5.1 EU 868 MHz

Regulator: ETSI EN 300 220.

Sub-band	Frequency	EIRP limit	Duty cycle
g1	868.0 - 868.6 MHz	+14 dBm / 25 mW	1% (36 sec/hour)
g2	868.7 - 869.2 MHz	+14 dBm / 25 mW	0.1%
g3	869.4 - 869.65 MHz	+27 dBm / 500 mW	10%
g4	869.7 - 870.0 MHz	+7 dBm / 5 mW	1%

Meshtastic EU868 preset uses 869.525 MHz (in g3 sub-band) for +14 dBm EIRP and 10% duty.

Strict 1% duty cycle compliance in g1: a 2-second TX requires 200 seconds of silence before next TX. Meshtastic implements this — burst-then-silent pattern is normal behavior for EU nodes.

5.2 US 902–928 MHz

Regulator: FCC §15.247.

Parameter	Value
Frequency band	902 - 928 MHz
Maximum EIRP	+30 dBm / 1 W
Duty cycle	None (replaced by dwell-time rules)
Dwell-time rule	If using FHSS: must dwell ≤400 ms on each channel; if DSSS: must channel-hop within 50 channels; LoRa CSS gets §15.247(c) coverage

US rules are the most permissive of major regions. LoRa nodes can transmit continuously (subject to FCC §15.249 max EIRP and antenna gain limits). The 1 W EIRP limit is well above the Cap LoRa-1262’s +22 dBm + 3 dBi = +25 dBm EIRP, so the stock configuration is legal.

5.3 Other regions

Region	Frequency	Max EIRP	Duty cycle
AU/NZ (AS/NZS 4268)	915 - 928 MHz	+30 dBm / 1 W	None
JP (ARIB STD-T108)	920.6 - 928 MHz	+13 dBm / 20 mW	10% (sub-band varies)
CN (SRRC)	470 - 510 MHz / 779 - 787 MHz	varies	varies
IN (TEC)	865 - 867 MHz	+30 dBm / 1 W	None
RU	varies	varies	varies

For travel: set your firmware’s region code to match the country you’re operating in. Operating with the wrong region code is technically a regulatory violation regardless of whether anyone notices.

5.4 The Cap LoRa-1262 EU g1 compliance gotcha

The Cap LoRa-1262 ships at +22 dBm by default. With the included 3 dBi antenna:

EIRP = TX power + antenna gain = +22 dBm + 3 dBi = +25 dBm

This is 11 dB over the EU g1 limit (+14 dBm). Running the stock Cap at +22 dBm in EU g1 frequencies is non-compliant.

Compliance options:

Firmware-clamp TX to ≤+11 dBm (so +11 + 3 = +14 dBm EIRP). Meshtastic does this automatically when EU868 region is selected.
Run g3 only (+27 dBm EIRP cap, 10% duty) — operate in 869.4 - 869.65 MHz, allow +22 dBm.
Use a 0 dBi antenna — odd choice for LoRa but allows +14 dBm TX with +14 dBm EIRP.

Bare RadioLib code without region-aware configuration is non-compliant by default. Meshtastic’s region presets handle this automatically.

US operators don’t have this problem — +25 dBm EIRP is well under the +30 dBm FCC limit.

6. GNSS — AT6668 multi-constellation receiver

6.1 Constellations supported

The ATGM336H-6N module wraps a CASIC AT6668 chip — a 50-channel multi-constellation GNSS receiver.

Constellation	Frequency	Active satellites visible	Coverage
GPS L1	1575.42 MHz	~31 worldwide	Global
GLONASS L1	1602 MHz	~24 worldwide	Global
Galileo E1	1575.42 MHz	~30 worldwide	Global
BeiDou B1I + B1C	1561 / 1575.42 MHz	~45 worldwide	Global with stronger Asia-Pacific coverage
QZSS L1	1575.42 MHz	4 satellites	Asia-Pacific only (regional augmentation)
SBAS	1575.42 MHz	Various (WAAS, EGNOS, MSAS, GAGAN)	Region-specific augmentation for higher accuracy

Typical visibility outdoors clear sky: 20-30 satellites in view, 10-15 tracked simultaneously for position fix. Indoors / urban canyon: degraded but still typically functional.

Position accuracy: <1.5 m CEP50 (Circular Error Probable, 50% of fixes within this radius). Consumer-grade. For RTK-level <10 cm accuracy, you need a dedicated RTK module (not the AT6668).

6.2 Time-to-first-fix (TTFF)

Start type	Almanac state	Typical TTFF
Cold	No almanac, no recent fix	~23 seconds
Warm	Almanac valid (<4 hours since last fix)	~10 seconds
Hot	Almanac valid + recent fix (<5 minutes)	~1 second

The almanac is stored in volatile chip RAM by default; loss of power = cold start on next boot. Some firmwares use the backup battery on the Atomic GPS Kit (NEO-M8N) to retain almanac — the Cap LoRa-1262’s AT6668 does not have a backup battery, so every cold boot = ~23 s TTFF.

For Meshtastic users: this matters at trail-head start-up — your first GPS fix takes 20-30 seconds after power-on. Plan accordingly.

6.3 Interface — UART + NMEA + CASIC binary

The AT6668 module communicates with the Cardputer ADV via UART:

Baud rate: 115200 default (configurable to 9600 / 38400 / 230400)
Protocol: NMEA 0183 v4.1 (sentences $GNGGA, $GPGGA, $GNRMC, $GLGSV, etc.) by default
Alternative: CASIC binary protocol — proprietary higher-rate / lower-overhead alternative

EXT-bus pin map:

Direction	EXT pin	GPIO	AT6668 pin
Cardputer RX (AT6668 → Cardputer)	13	GPIO 13	TX
Cardputer TX (Cardputer → AT6668)	9	GPIO 15	RX

The TX path is mostly for configuration commands (set baud rate, set update rate, select constellations, enable/disable specific NMEA sentences). For passive position reading, only the RX path is used.

6.4 Code example — Arduino NMEA parsing

#include <HardwareSerial.h>
#include <TinyGPSPlus.h>

HardwareSerial GPS(2);          // Use UART2
TinyGPSPlus gps;

void setup() {
    Serial.begin(115200);
    GPS.begin(115200, SERIAL_8N1, 13, 15);   // baud, config, RX pin, TX pin
}

void loop() {
    while (GPS.available()) {
        gps.encode(GPS.read());
    }

    if (gps.location.isValid()) {
        Serial.printf("Lat %.6f Lng %.6f Alt %.1f m Sats %u Time %02d:%02d:%02d UTC\n",
                      gps.location.lat(), gps.location.lng(),
                      gps.altitude.meters(),
                      gps.satellites.value(),
                      gps.time.hour(), gps.time.minute(), gps.time.second());
    } else {
        Serial.println("Waiting for fix...");
    }
    delay(1000);
}

TinyGPSPlus library handles NMEA parsing. SparkFun also publishes SparkFun_u-blox_GNSS_Arduino_Library — works against the AT6668 since the NMEA protocol is standard.

Configuration commands (to change update rate, etc.) use the $PCAS sentence prefix (CASIC-vendor-specific). Example: $PCAS02,1000*1E\r\n sets update rate to 1 Hz. Full command reference in CASIC documentation.

7. Cap + Cardputer ADV combined power profile

When the Cap LoRa-1262 is mated, the combined power draw is the sum of Cardputer ADV base + Cap operations:

State	Cardputer ADV	Cap LoRa-1262	Total
Idle (display backlight on, both radios sleeping)	~120 mA	+1 µA	~120 mA
GNSS tracking only	~120 mA	+20-30 mA	~150 mA
Meshtastic listen (RX + GNSS)	~120 mA	+12 mA (LoRa RX) + 25 mA (GNSS)	~157 mA
Meshtastic TX +14 dBm (LongFast burst)	~120 mA	+63 mA peak	~183 mA peak (during ~510 ms burst)
Meshtastic TX +22 dBm	~120 mA	+163 mA peak	~283 mA peak
Display backlight off (sleep variant)	~30 mA	+12 mA	~42 mA

Battery life estimates for the 1750 mAh stock battery:

Meshtastic LongFast continuous listen (no TX): ~10 hours
Meshtastic LongFast with periodic TX every 10s: ~8 hours
Meshtastic + GPS active continuously: ~6 hours
Display backlight off, listen-only: ~30 hours

For 24-hour deployment, either replace battery with 3000+ mAh upgrade (Vol 9 § 4.1) or use solar-harvesting (Vol 9 § 3.3) or USB power.

8. Antenna upgrade options

The included 3 dBi rubber-duck antenna is adequate for short-range Meshtastic. For longer range:

Antenna	Gain	Form factor	Approximate price
Stock rubber-duck	3 dBi	108 × 9.3 mm whip	Included
SignalPlus 6 dBi 915 MHz dipole	6 dBi	Cargo-pocket foldable handheld	~$25
Comet GP-3 / GP-9 (omni, fixed)	6-8 dBi	Long fiberglass rod, fixed-mount	~$80-120
14 dBi 5-element Yagi (directional, fixed)	14 dBi	~600 mm boom	~$80 (L-Com)
J-pole DIY (homebrew)	~6 dBi	12 AWG wire, ~400 mm	Free

EIRP recalculation with antenna upgrade:

TX power	Antenna gain	EIRP	EU g1 limit	Compliant?
+22 dBm	3 dBi	+25 dBm	+14 dBm	No
+22 dBm	6 dBi	+28 dBm	+14 dBm	No (even further over)
+14 dBm	6 dBi	+20 dBm	+14 dBm	No
+8 dBm	6 dBi	+14 dBm	+14 dBm	Yes (clamp TX further)

Always recalculate EIRP for your region after an antenna change. Higher-gain antennas + EU g1 = lower allowed TX power.

Always attach an antenna before powering on. SX1262 PA can be damaged by open or short load. FM8625H protection helps but isn’t a guarantee. Vol 11 § 6 has the RF-safety details.

9. Resources

Datasheets

Semtech SX1262: https://www.semtech.com/products/wireless-rf/lora-connect/sx1262
ATGM336H-6N (AT6668) — CASIC vendor docs (search “AT6668 datasheet”)
PI4IOE5V6408 GPIO expander: Diodes Inc.
FM8625H RF switch: vendor docs

Libraries

RadioLib (jgromes — canonical SX1262 driver): https://github.com/jgromes/RadioLib
TinyGPSPlus (NMEA parsing): https://github.com/mikalhart/TinyGPSPlus
SparkFun u-blox GNSS (works with AT6668 NMEA): https://github.com/sparkfun/SparkFun_u-blox_GNSS_Arduino_Library

Regulatory

ETSI EN 300 220 (EU rules): https://www.etsi.org/
FCC §15.247 (US rules): https://www.fcc.gov/general/title-47-code-federal-regulations
ARIB STD-T108 (Japan): https://www.arib.or.jp/english/std_tr/telecommunications/desc/std-t108.html
Meshtastic region preset docs: https://meshtastic.org/docs/configuration/region-by-country/

Community

Meshtastic Cardputer ADV setup guide: https://meshtastic.org/docs/hardware/devices/
The Things Network LoRa coverage maps: https://www.thethingsnetwork.org/community

Forward references in this series

Meshtastic firmware configuration: Vol 6 § 5.1
LoRa-APRS for licensed amateurs: Vol 6 § 5.2
Range optimization recipes (per environment): Vol 9 § 3.1
Solar-harvested fixed relay: Vol 9 § 3.3
Antenna safety / never-TX-unloaded: Vol 11 § 6

This is Volume 5 of a twelve-volume series. Next: Vol 6 is the firmware ecosystem — M5Launcher, Bruce, NEMO, Marauder, Evil-M5, MicroHydra, Meshtastic, ESPHome, retro emulators, and niche apps.

M5Stack Cardputer ADV Volume 5 — Cap LoRa-1262, LoRa CSS Theory, and GNSS

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