Common Issues

Solutions to frequently encountered problems with Multiflexmeter 3.7.0.

Device Won’t Join TTN

Symptoms

Device powers on
LED blinks showing valid configuration
No join success on TTN Console
Serial shows “EV_JOINING” repeatedly

Solutions

1. Verify Credentials

// Check EEPROM values match TTN Console exactly
- AppEUI (8 bytes)
- DevEUI (8 bytes)
- AppKey (16 bytes)

2. Check Byte Order

All values are MSB-first (big-endian)
Copy directly from TTN Console (no byte swapping needed)

3. Verify Frequency Plan

// In platformio.ini
-DCFG_eu868=1  // Must match your region

4. Check Gateway Coverage

Use TTN Mapper to verify coverage
RSSI should be better than -120 dBm
Try closer to known gateway

5. Reset Device

# Trigger device reset
avrdude -c usbasp -p m1284p -U flash:w:firmware.hex:i

No Sensor Data

Symptoms

Device joins TTN successfully
Uplinks are received
All sensor channels show zero or invalid values

Solutions

1. Check Sensor Power

// Verify sensor power pin is configured
board_sensor_power(true);
delay(50);  // Wait for stabilization

2. Verify I²C Connection

// Check sensor address
#define SENSOR_ADDRESS 0x36  // Default

// Test I²C communication
Wire.beginTransmission(SENSOR_ADDRESS);
if (Wire.endTransmission() != 0) {
    // Sensor not responding
}

3. Check Pullup Resistors

SDA and SCL need 4.7kΩ pullups
Measure resistance to VCC
Add external resistors if missing

4. Test Sensor Commands

// Send perform command
sensorPerform(SENSOR_ADDRESS);
delay(100);  // Wait for measurement

// Read data
int16_t data[8];
if (sensorRead(SENSOR_ADDRESS, data) == 0) {
    // Success
}

5. Verify Cable Length

I²C maximum recommended: 2 meters
Use shielded cable for longer runs
Check for cable damage

Upload Fails

Symptoms

avrdude: error: program enable: target doesn't answer
avrdude: initialization failed, rc=-1

Solutions

1. Check Connections

Verify all 6 ISP pins connected
Check for loose wires
Ensure correct pin orientation

2. Slow Down SCK Clock

# In platformio.ini
upload_flags =
    -Pusb
    -B100  # Very slow, more reliable

3. Check Power Supply

Device needs 5V during programming
Verify voltage at VCC pin
Try external power supply

4. Verify Programmer

# Test USBasp connection
avrdude -c usbasp -p m1284p

5. Check Fuses

# Read current fuses
avrdude -c usbasp -p m1284p -U lfuse:r:-:h -U hfuse:r:-:h

# Should show: lfuse=0xFF, hfuse=0xD1

Incorrect Message Interval

Symptoms

Device sends too frequently or too slowly
Interval doesn’t match configuration

Solutions

1. Check EEPROM Interval

# Interval is in SECONDS, not hours!
interval = 300  # 300 seconds = 5 minutes
eeprom_bytes = struct.pack('>H', interval)  # Big-endian

2. Verify Interval Range

// Valid range
#define MIN_INTERVAL 20      // 20 seconds
#define MAX_INTERVAL 4270    // 71 minutes

3. Check Fair Use Policy

// If enabled, minimum interval may be enforced
uint8_t fair_use = EEPROM.read(40);
// 0x00 = disabled, 0x01 = enabled

4. Update via Downlink

Port: 1
Payload: 0x00 0x10 0x01 0x2C  // Set to 300 seconds
         |    |    |    |
         |    |    +----+--- Interval (big-endian)
         |    +------------- Command code
         +------------------ Reserved

Poor Signal Quality

Symptoms

RSSI < -120 dBm
SNR < -10 dB
Messages not received by TTN
High packet loss

Solutions

1. Improve Antenna

Use higher gain antenna (5dBi vs 3dBi)
Ensure antenna is vertical
Move antenna outside enclosure
Check antenna connection (RP-SMA)

2. Relocate Device

Higher mounting position
Clear line-of-sight to gateway
Away from metal obstacles
Test different locations

3. Adjust Spreading Factor

// In LMIC configuration
// Higher SF = longer range, longer airtime
LMIC_setDrTxpow(DR_SF7, 14);  // SF7, 14dBm
// Change to DR_SF9 or DR_SF10 for longer range

4. Check Gateway

Verify gateway is online (TTN Console)
Check gateway location (TTN Mapper)
Consider adding gateway if no coverage

High Power Consumption

Symptoms

Battery drains faster than expected
Device runs hot
Short battery life

Solutions

1. Enable Sleep Mode

// In main loop
void loop() {
    os_runloop_once();

    // Add sleep
    Watchdog.sleep();  // ~8s sleep cycles
}

2. Reduce TX Power

// Lower transmission power
LMIC_setDrTxpow(DR_SF7, 2);  // 2dBm instead of 14dBm
// Use if close to gateway

3. Increase Interval

// Longer interval = less frequent TX
uint16_t interval = 900;  // 15 minutes instead of 5

4. Power Off Peripherals

// Turn off sensor between readings
board_sensor_power(false);

// Disable unnecessary peripherals
// Check board-specific power management

5. Check for Software Loops

// Avoid busy-wait loops
while (condition) {
    delay(10);  // Better than tight loop
}

EEPROM Configuration Invalid

Symptoms

LED blinks error code on startup
Device doesn’t join network
Serial shows “Invalid configuration”

Solutions

1. Verify Magic Bytes

# Must be exactly: 'M', 'F', 'M', '\0'
MAGIC = b'MFM\x00'

2. Check EEPROM Write

# Read back EEPROM
avrdude -c usbasp -p m1284p -U eeprom:r:eeprom_check.bin:r

# Verify with hex dump
xxd eeprom_check.bin | head -n 3

3. Reprogram EEPROM

# Write fresh configuration
python3 generate_eeprom.py
avrdude -c usbasp -p m1284p -U eeprom:w:eeprom.bin:r

4. Verify Structure

Offset | Field      | Size | Value
-------|------------|------|-------
0x00   | Magic      | 4    | MFM\0
0x04   | Version    | 2    | 0x0845
0x06   | AppEUI     | 8    | From TTN
0x0E   | DevEUI     | 8    | From TTN
0x16   | AppKey     | 16   | From TTN
0x26   | Interval   | 2    | 20-4270
0x28   | Fair Use   | 1    | 0 or 1

Downlinks Not Working

Symptoms

Device receives uplinks successfully
Downlinks sent from TTN Console
Device doesn’t respond to commands

Solutions

1. Check FPort

// Downlinks must be on FPort 1
if (LMIC.txrxFlags & TXRX_PORT) {
    uint8_t port = LMIC.frame[LMIC.dataBeg - 1];
    if (port == 1) {
        // Process command
    }
}

2. Verify Timing

Class A: Downlinks only after uplink
Wait for next measurement cycle
Check “Schedule downlink” is confirmed

3. Check Payload Format

Command: Update interval to 300s
Bytes: [0x00, 0x10, 0x01, 0x2C]
       |     |     |     |
       |     |     +-----+-- Value (big-endian)
       |     +-------------- Command code
       +--------------------- Reserved

4. Enable Confirmed Uplinks

// Request downlink confirmation
LMIC_setTxData2(1, payload, sizeof(payload), 1);
//                                            ^ confirmed

Build Errors

Symptoms

fatal error: lmic.h: No such file or directory

Solutions

1. Install Dependencies

pio lib install

2. Clean Build

pio run --target clean
pio run

3. Update Platform

pio pkg update
pio platform update atmelavr

4. Check platformio.ini

lib_deps =
    mcci-catena/MCCI Arduino LoRaWAN Library @ ^0.9.2
    adafruit/Adafruit SleepyDog Library @ ^1.6.4

Decoder Issues

Symptoms

Uplinks received but not decoded
Empty decoded_payload in TTN
JavaScript errors in console

Solutions

1. Check FPort

// Decoder must handle correct FPort
if (input.fPort === 1) {
    // Measurements (16 bytes)
}

2. Verify Payload Length

if (input.bytes.length !== 16) {
    return { errors: ["Invalid length"] };
}

3. Test Decoder

Use TTN Console “Test” tab
Input known payload
Check output matches expected

4. Check Byte Order

// Big-endian (MSB first)
var value = (input.bytes[i*2] << 8) | input.bytes[i*2 + 1];

Next Steps

Debugging Guide - Advanced debugging
FAQ - Frequently asked questions
Development Guide - Code-level debugging