EEPROM Configuration

The Multiflexmeter stores its configuration in the ATmega1284P’s EEPROM. This includes LoRaWAN credentials and operational settings.

EEPROM Structure

Configuration starts at address 0x00 and occupies 42 bytes.

struct __attribute__((packed)) rom_conf_t {
  uint8_t MAGIC[4];                // "MFM\0" (validation)
  struct {
    uint8_t MSB;
    uint8_t LSB;
  } HW_VERSION;                    // Hardware version
  uint8_t APP_EUI[8];              // Application EUI (LSB first)
  uint8_t DEV_EUI[8];              // Device EUI (LSB first)
  uint8_t APP_KEY[16];             // Application Key (MSB first)
  uint16_t MEASUREMENT_INTERVAL;   // Interval in seconds
  uint8_t USE_TTN_FAIR_USE_POLICY; // 0 or 1
  uint8_t WHEEL_TEETH_COUNT;       // For RPM calculation
};

Memory Map

Offset	Size	Field	Default
0x00	4	Magic bytes	”MFM\0”
0x04	2	Hardware version	-
0x06	8	APP_EUI	-
0x0E	8	DEV_EUI	-
0x16	16	APP_KEY	-
0x26	2	Measurement interval	300 (5 min)
0x28	1	TTN Fair Use Policy	1 (enabled)
0x29	1	Wheel teeth count	91

Total: 42 bytes (0x2A)

Field Details

Magic Bytes

The magic bytes "MFM\0" validate the EEPROM contents. If magic doesn’t match, the firmware uses default values.

Hardware Version

16-bit encoded version number:

Bit 15:     Protocol (0=dev, 1=release)
Bits 14-10: Major (0-31)
Bits 9-5:   Minor (0-31)
Bits 4-0:   Patch (0-31)

LoRaWAN Credentials

TTN Console shows:

AppEUI:  70B3D57ED0000001
DevEUI:  0004A30B001C1234
AppKey:  2B7E151628AED2A6ABF7158809CF4F3C

EEPROM storage:

const uint8_t APP_EUI[8] = { 0x01, 0x00, 0x00, 0xD0, 0x7E, 0xD5, 0xB3, 0x70 }; // LSB
const uint8_t DEV_EUI[8] = { 0x34, 0x12, 0x1C, 0x00, 0x0B, 0xA3, 0x04, 0x00 }; // LSB
const uint8_t APP_KEY[16] = { 0x2B, 0x7E, 0x15, 0x16, 0x28, 0xAE, 0xD2, 0xA6,
                              0xAB, 0xF7, 0x15, 0x88, 0x09, 0xCF, 0x4F, 0x3C }; // MSB

Measurement Interval

Range: 20 - 4270 seconds
Default: 300 seconds (5 minutes)
Format: 16-bit big-endian

Can be changed via downlink command 0x10.

TTN Fair Use Policy

0: Disabled
1: Enabled (default)

When enabled, the firmware tracks daily airtime usage and adjusts transmission frequency to stay within TTN’s 30-second/day limit.

Wheel Teeth Count

Number of teeth on the mill wheel. Used for RPM calculation.

Default: 91

Can be changed via downlink command 0x12.

Programming EEPROM

Using Arduino Sketch

Create a separate sketch to program EEPROM:

#include <EEPROM.h>

// Replace with your TTN credentials (see byte order notes above)
const uint8_t APP_EUI[8] = { /* LSB first */ };
const uint8_t DEV_EUI[8] = { /* LSB first */ };
const uint8_t APP_KEY[16] = { /* MSB first */ };

void setup() {
  int addr = 0;

  // Magic
  EEPROM.write(addr++, 'M');
  EEPROM.write(addr++, 'F');
  EEPROM.write(addr++, 'M');
  EEPROM.write(addr++, 0);

  // HW Version (example: 1.0.0 release)
  EEPROM.write(addr++, 0x90);  // MSB: proto=1, major=1
  EEPROM.write(addr++, 0x00);  // LSB: minor=0, patch=0

  // APP_EUI
  for (int i = 0; i < 8; i++) {
    EEPROM.write(addr++, APP_EUI[i]);
  }

  // DEV_EUI
  for (int i = 0; i < 8; i++) {
    EEPROM.write(addr++, DEV_EUI[i]);
  }

  // APP_KEY
  for (int i = 0; i < 16; i++) {
    EEPROM.write(addr++, APP_KEY[i]);
  }

  // Measurement interval: 300 seconds (0x012C)
  EEPROM.write(addr++, 0x01);
  EEPROM.write(addr++, 0x2C);

  // TTN Fair Use: enabled
  EEPROM.write(addr++, 1);

  // Wheel teeth: 91
  EEPROM.write(addr++, 91);

  Serial.begin(115200);
  Serial.println("EEPROM programmed!");
}

void loop() {}

Using AVRDUDE

Read current EEPROM:

avrdude -c usbasp -p m1284p -U eeprom:r:eeprom_backup.bin:r

Write EEPROM from file:

avrdude -c usbasp -p m1284p -U eeprom:w:eeprom_data.bin:r

Runtime Configuration API

The firmware provides functions to access configuration:

// Load configuration from EEPROM
bool conf_load(void);

// Save configuration to EEPROM
void conf_save(void);

// Get LoRaWAN credentials
void conf_getAppEui(uint8_t *buf);
void conf_getDevEui(uint8_t *buf);
void conf_getAppKey(uint8_t *buf);

// Measurement interval
uint16_t conf_getMeasurementInterval();
void conf_setMeasurementInterval(uint16_t interval);

// Wheel teeth
uint8_t conf_getWheelTeethCount();
void conf_setWheelTeethCount(uint8_t count);

// Version info
version conf_getHardwareVersion(void);
version conf_getFirmwareVersion(void);

// Fair use policy
uint8_t conf_getUseTTNFairUsePolicy(void);

OTA Configuration Updates

Some settings can be changed via LoRaWAN downlinks:

Setting	Command	Persistent
Measurement interval	`0x10 <HI> <LO>`	Yes
Wheel teeth count	`0x12 <count>`	Yes

Changes are saved to EEPROM and persist across power cycles.