IoT-PostBox v1.x

Complete technical documentation for IoT-PostBox v1.x series (ESP32-S2 based). This is the current generation with advanced features including dual connectivity, load sharing power management, and native USB programming.

Hardware Overview

Key Features

Core Platform:

• MCU: ESP32-S2 with up to 16MB Flash + 8MB PSRAM
• Native USB: Direct programming and debugging via USB-C connector
• Dual Connectivity: WiFi (2.4GHz) + LoRaWAN (868/915MHz RFM95W)
• Wake-up Method: GPIO interrupt-based from deep sleep (vs hardware reset in v0.x)

Power Management:

• Load Sharing Circuit: Automatically selects USB or battery power
• Dual LDO Design: Main (3V3_LDO) + Peripheral (3V3_LDO2) power domains
• Enhanced Monitoring: Dedicated ADC sensing for VBAT and VBUS voltages
• Smart Charging: Configurable charging current with multiple IC support

Connectivity & I/O:

• External Antennas: uFL connectors for WiFi and LoRaWAN (selectable via jumpers)
• Switch Sensors: Two GPIO interrupt-capable inputs (SW1, SW2)
• Expansion Ports: J6 (EXT1) and J7 (EXT2) with I2C, SPI, UART
• LED Support: Dedicated port for RGB LED strips (J3)
• Debug Interface: JTAG connector for advanced debugging

Power Management Architecture

Load Sharing System

The IoT-PostBox v1.x series features an advanced power management system that intelligently manages power sources:

USB 5V ──┬── Charger IC ──── Li-Po Battery (3.7V)
         │
         └── Load Sharing Circuit ──┬── Main LDO (3V3_LDO) ──── MCU & Core Systems
                                    │
                                    └── Peripheral LDO (3V3_LDO2) ──── LED Strip & EXT Ports

Key Benefits:

• Seamless Power Source Selection: USB power when connected, battery when disconnected
• Continuous Operation: Device remains powered during charging cycle
• Dual Power Domains: Core systems and peripherals on separate LDO rails (700mA each)
• LDO Protection: Current overload, output short circuit, and overheating protection
• Battery Protection: Overcharge, overdischarge, and reverse polarity protection via charger IC

Battery Charging Configuration

The charging system supports multiple charger ICs with configurable current using the RPROG resistor:

Supported Charger ICs:

• MCP73831/2: Max 500mA charging current
• TP4054: Max 800mA charging current
• TP4065: Max 600mA charging current

RPROG Resistor Selection:

• MCP73831/2 (max 500mA) or TP4054 (max 800mA):

$$I_\text{charge} (\mathrm{mA})= \frac{1000~(\mathrm{V})}{R~(\mathrm{k}\Omega)}$$

• TP4065 (max 600mA):

$$I_\text{charge} (\mathrm{mA})= \frac{1100~(\mathrm{V})}{R~(\mathrm{k}\Omega)}$$

Charging Status Indicators:

• VBAT_STAT: Tri-state (MCP73831) or Open-drain (MCP73832, TP4054, TP4065) monitored using ESP32-S2 GPIO3/ADC1_CH2 pin.
• D2 (Orange LED): Charging in progress
• D3 (Green LED): Battery fully charged
• Both LEDs Off: No USB connected (device may be powered by battery)

Programming & Initial Setup

Hardware Setup:

1. Power Connection: Connect USB-C cable to J4
2. Boot Mode: Hold BOOT button while connecting USB for programming mode
3. Driver Installation: Install ESP32-S2 USB drivers if needed

Firmware Upload:

# Using PlatformIO
pio run -e IoT-PostBox_v1 -t upload

# Using Arduino IDE
# Select "ESP32S2 Dev Module" board
# Select "USB CDC" for USB communication

Initial Configuration:

1. WiFi Setup: Configure network credentials via web dashboard
2. MQTT Settings: Set broker address and authentication
3. LoRaWAN Setup: Configure DevEUI, AppEUI, AppKey (if using LoRaWAN)

Interactive Bill of Materials

Technical Documentation

3D Model

Detailed Pinout Reference

Switch SW1 & SW2 Sensor Inputs

Typical Connection:

• Mechanical switches, hall sensors, or reed switches
• Pull-down resistors included on PCB
• Wake-up capable from deep sleep via GPIO interrupt

EXT1 Expansion Port (J6):

EXT2 Expansion Port (J7):

LED & Power Output Connectors

LED Strip Port (J3):

Power Output Port (J5):