Peripheral Daughterboard — PoE + NFC Module ​

• Form Factor: 120 mm × 15–20 mm
• Mounting: mezzanine stack using board-to-board header connectors
• Compatible Host: Xerxes Pi carrier board

🔋 Power over Ethernet (PoE) Subsystem ​

• PoE Standard: IEEE 802.3af (15.4W) or 802.3at (30W)
• PoE Module: Integrated PoE PD controller
• DC Output: 5V regulated to power both the host motherboard and daughterboard
• Isolation: Galvanically isolated Ethernet transformer + TVS protection
• RJ45 Port: Routed from the Xerxes Pi
• Cooling: Small thermal pad + integrated passive heatsink

🛠️ Board-to-Board Interface ​

• Connector: Dual-row 0.5 mm or 0.8 mm pitch high-density mezzanine connector

• Signals:

  • 5V, 3.3V power
  • SPI, I2C, UART
  • Ethernet pass-through or GPIO-triggered power control

• Optional Features:

  • EEPROM for daughterboard ID/config
  • Onboard status LED indicators (PoE OK, NFC active)

🛠️ NFC Interface ​

The ST25DV64K is a dual-interface NFC tag IC from STMicroelectronics with both I²C and ISO/IEC 15693 (NFC-V) interfaces. It's designed for secure communication, configuration, and data transfer between a host system (e.g. MCU) and an NFC reader. It offers 64 Kbits (8 KB) of EEPROM memory and advanced features.

Here’s a breakdown of the different modes the ST25DV64K can function under:

🛠️ 1. RF Mode (ISO 15693 NFC-V / ISO 18000-3 Mode 1) ​

• The tag is powered by the electromagnetic field from an NFC reader.

• Functions as a passive NFC-V tag.

• Used for:

  • Wireless configuration
  • Inventory management
  • Data read/write via smartphones or industrial readers

• Fast transfer mode (FTM) to speed up I2C ↔ RF communication

• Multiple read/write blocks

• Password protection per memory zone

• Field detect pin (GPO) to signal RF activity to the host MCU

👨🏻‍💻 2. I²C Mode (Wired Interface) ​

• Communicates with a host microcontroller over a standard I²C bus.

• Can be accessed by the host system for reading/writing EEPROM or monitoring RF status.

• Standard I²C slave interface (up to 1 MHz)

• Dual-port access control:

  • Host MCU can block or allow RF access
  • RF can signal host via GPO pin

• Supports energy harvesting (if RF field is present, it can power external circuits with up to ~4.5 mA at 3.3V)

🔁 3. Dual Interface Mode ​

• Both the I²C host and the RF reader can access the memory.
• The EEPROM can be written or read from either interface.
• Arbitration logic prevents simultaneous conflicting accesses.

Modes inside dual interface: ​

• Mailbox / Fast Transfer Mode (FTM):

  • Dedicated buffer for quick bidirectional data exchange between RF and I²C
  • Acts like a "mailbox" where RF writes a message and host reads it (and vice versa)

• GPO (General Purpose Output):

  • Can signal various events like:

    • RF field detected
    • Data ready
    • Memory area accessed

  • Useful for waking up a microcontroller when an NFC read/write happens

🔐 4. Security Modes ​

• Up to 4 configurable memory areas (zones) with different access controls:

  • Read/write by I²C only, RF only, or both
  • Public, password-protected, or permanently locked

• Multiple 64-bit passwords for read/write access

• Supports encrypted communication via NIST-recommended algorithms (AES support depends on variant)

🔋 5. Energy Harvesting Mode ​

• When in an RF field, the ST25DV64K can harvest energy to power external low-power electronics.
• Provides up to 4.5 mA @ 3.3V through its VCC pin, configurable via EEPROM
• Useful in battery-less or power-optimized applications

Summary Table ​

🛠️ Mechanical Integration ​

• Designed to slot beside the mainboard inside a 1U chassis
• Edge cutout for NFC antenna to reach chassis front
• Designed as swappable module for field upgrades

💡 Example Applications in a Home Lab ​

• Zero-cable networking: Only a PoE switch needed for power + network
• Scriptable NFC provisioning: Use NFC cards or phones to push configurations or trigger remote scripts
• Rack management: NFC for identifying server nodes or unlocking admin access