FEP 004

The FujiNet Protocol

Name	Value
FEP ID	004
Title	FujiNet Protocol
Author	Andrew Diller / AI
Status	Draft
Type	Informational
Created	2025-07-20
Version	1.0
Input	CO, OS (Discord)

Abstract

This document outlines a structured, packet-based communication protocol designed specifically for efficient command and response exchanges between the RP2350 Programmable I/O (PIO) subsystem and an ESP32 microcontroller within a FujiNet system. The protocol utilizes a clearly defined packet structure with little-endian byte ordering to optimize parsing efficiency on embedded hardware. Each packet includes synchronization bytes, command identifiers, sequence numbering for tracking, payload length indicators, and an optional CRC16 checksum to ensure data integrity. This design facilitates robust, error-resistant, and extensible communication suited for embedded device interactions.

Current FN Protocol Definitions:

• https://github.com/FujiNetWIFI/fujinet-firmware/blob/f7110461ed06554796e4b4855fc52e85ce39acf8/lib/fuji/fujiCmd.h

FujiNet RP2350–ESP32 Communication Protocol

Packet Structure

#pragma pack(push, 1)
typedef struct {
    uint8_t start_byte;       // 0xAA for packet synchronization
    uint8_t packet_type;      // Command = 0x01, Response = 0x02
    uint8_t command_id;       // Command identifier
    uint8_t sequence_num;     // Incremental packet sequence number
    uint16_t payload_length;  // Length of payload (little-endian)
    uint8_t payload[];        // Variable-length payload
    // uint16_t crc;          // CRC16 checksum follows payload
} FujiNet_packet;
#pragma pack(pop)

Explanation of Fields

• start_byte (0xAA): Packet synchronization.
• packet_type: Command (0x01), Response (0x02).
• command_id: Identifies command type.
• sequence_num: Matches commands and responses.
• payload_length: Length of payload (little-endian).
• payload: Data payload (little-endian).
• crc: CRC16 checksum (optional but recommended).

Packet Examples

Command Packet (status request, command_id = 0x10):

| start_byte     | AA |
| packet_type    | 01 |
| command_id     | 10 |
| sequence_num   | 05 |
| payload_length | 00 00 |
| crc            | CRC16 |

Response Packet (with 4-byte payload):

| start_byte     | AA |
| packet_type    | 02 |
| command_id     | 10 |
| sequence_num   | 05 |
| payload_length | 04 00 |
| payload        | [status data] |
| crc            | CRC16 |

Synchronization & Integrity

• Start byte (0xAA) for packet framing.
• CRC16 checksum recommended.

Implementation Steps

1. Define packet structure.
2. Implement serialization/deserialization.
3. CRC16 checksum validation.
4. Thorough synchronization tests.
5. Incremental command extensions.

Serialization Example in C

void serialize_packet(uint8_t *buffer, FujiNet_packet *pkt) {
    buffer[0] = pkt->start_byte;
    buffer[1] = pkt->packet_type;
    buffer[2] = pkt->command_id;
    buffer[3] = pkt->sequence_num;
    buffer[4] = pkt->payload_length & 0xFF;
    buffer[5] = (pkt->payload_length >> 8) & 0xFF;
    memcpy(&buffer[6], pkt->payload, pkt->payload_length);
    uint16_t crc = crc16_ccitt(buffer, 6 + pkt->payload_length);
    buffer[6 + pkt->payload_length] = crc & 0xFF;
    buffer[7 + pkt->payload_length] = (crc >> 8) & 0xFF;
}

Benefits

• Structured, efficient, and maintainable.
• Robust parsing and error checking.
• Extensible design.