GORGON - SDI-12 RTD Multiplexer

A 7-channel RTD temperature measurement system with SDI-12 sensor interface for the Raspberry Pi Pico.

Overview

GORGON is an embedded firmware that implements a precision multichannel RTD (Resistance Temperature Detector) data acquisition system. It measures temperatures from up to 7 independent PT100 RTD sensors and exposes them via the industry-standard SDI-12 protocol, making it compatible with environmental data loggers and SCADA systems.

Key Features

• 7-channel RTD measurement with PT100 4-wire configuration
• SDI-12 protocol compliance (1200 baud) for sensor communication
• 24-bit precision ADC (AD7124-8) with ratiometric measurement
• Per-channel calibration with persistent NVM storage
• Production calibration mode with Python automation tools
• Internal temperature sensors (Pico + ADC) for diagnostics

Hardware

Components

Component	Part Number	Description
MCU	Raspberry Pi Pico (RP2040)	Dual-core ARM Cortex-M0+ @ 133MHz
ADC	AD7124-8	24-bit Sigma-Delta ADC, 8 differential channels
Mux	ADG708	8-channel CMOS analog multiplexer
RTD	PT100	100Ω @ 0°C, α = 0.00385 °C⁻¹
R_ref	5030Ω (nominal)	Precision reference resistor

Pin Configuration

Function	GPIO	Notes
SPI MISO	16	ADC data out
SPI CS	17	ADC chip select (active low)
SPI SCK	18	1 MHz clock
SPI MOSI	19	ADC data in
SDI-12 Data	10	Requires 3.3V↔5V level shifter
Mux Enable	20	Active high
Mux A0	21	Channel select bit 0
Mux A1	22	Channel select bit 1
Mux A2	23	Channel select bit 2
UART TX	0	Debug output (115200 baud)
UART RX	1	Debug input

Power Requirements

• USB: Programming and debug
• 12V external: Sensor operation (current limit: 130mA)

Building

Prerequisites

• Raspberry Pi Pico SDK (v2.2.0+)
• ARM GCC Toolchain
• CMake 3.13+

Build Steps

# Clone and enter directory
cd sdi12-analog-mux

# Create build directory
mkdir build && cd build

# Configure
cmake ..

# Build
make -j4

Output: build/sdi12-analog-mux.uf2

Flashing

1. Hold BOOTSEL button (or short BOOTSEL jumper) while connecting USB
2. Copy sdi12-analog-mux.uf2 to the mounted RPI-RP2 drive
3. Device reboots automatically

SDI-12 Interface

The device responds to standard SDI-12 commands at address 0.

Supported Commands

Command	Response	Description
0!	0	Acknowledge
0I!	013GEMSGORGO1.0 001	Device identification
0M1!	00011	Start RTD channel 1 measurement
0M2!	00011	Start RTD channel 2 measurement
...	...	...
0M7!	00011	Start RTD channel 7 measurement
0M8!	00011	Pico internal temperature
0M9!	00011	AD7124 internal temperature
0D0!	0+25.123	Retrieve measurement data

Identification String Format

013GEMSGORGO1.0 001
│││   │     │   │
││└───│─────│───│── SDI-12 version (1.3)
│└────│─────│───│── Vendor: GEMS
└─────│─────│───│── Address: 0
      └─────│───│── Model: GORGON (truncated)
            └───│── Version: 1.0
                └── Serial: 001

Example Session

# Request identification
0I!
0013GEMSGORGO1.0 001

# Measure channel 1
0M1!
00011          # Response: ready in 001 seconds, 1 value

# Wait for measurement, then retrieve
0D0!
0+23.456       # Temperature in °C

UART Commands

Connect via serial terminal at 115200 baud (8N1) for debug and configuration.

NVM Commands

Command	Description
nvm_dump	Display all NVM data
nvm_reset	Factory reset
nvm_set_sn <serial>	Set device serial number
nvm_set_date YYYY-MM-DD	Set manufacturing date
nvm_set_board <revision>	Set board revision
nvm_set_hw_ver <version>	Set hardware version
nvm_set_fw_ver <version>	Set firmware version
nvm_cal_rref <ch> <value>	Set reference resistor (Ω)
nvm_cal_offset <ch> <value>	Set offset calibration
nvm_cal_scale <ch> <value>	Set scale factor

Calibration Mode Commands

Command	Description
cal_mode_start	Enter calibration mode (disables SDI-12)
cal_mode_stop	Exit calibration mode
cal_set_serial <sn>	Set serial number
cal_set_hw_ver <ver>	Set hardware version
cal_set_fw_ver <ver>	Set firmware version
cal_set_date YYYY-MM-DD	Set manufacturing date
cal_measure <channel>	Measure RTD channel (1-7)

Calibration

Production calibration requires precision reference resistors and the Python calibration tool.

Quick Start

cd calibration
pip install pyserial
./calibrate.py /dev/ttyACM0 -s 001 -d 10 -i 10

See calibration/README.md for the complete calibration process.

Project Structure

sdi12-analog-mux/
├── CMakeLists.txt           # Build configuration
├── pico_sdk_import.cmake    # Pico SDK integration
├── sdi12-analog-mux.c       # Main application
├── sdi12.c / sdi12.h        # SDI-12 protocol implementation
├── sdi12_uart.pio           # PIO assembly for 1200 baud UART
├── ad1724.c / ad1724.h      # AD7124-8 ADC driver
├── adg708.c / adg708.h      # ADG708 multiplexer driver
├── nvm.c / nvm.h            # Non-volatile memory storage
├── calibration_mode.c/.h    # Production calibration mode
└── calibration/
    ├── calibrate.py         # Calibration automation script
    ├── README.md            # Calibration quick reference
    └── CALIBRATION_GUIDE.md # Detailed calibration guide

Technical Details

Measurement Principle

The system uses ratiometric 4-wire RTD measurement:

1. 50µA excitation current flows through RTD and reference resistor
2. ADC measures voltage ratio: V_rtd / V_ref
3. RTD resistance calculated: R_rtd = R_ref × (V_rtd / V_ref)
4. Temperature derived using Callendar-Van Dusen equation

NVM Data Structure

Calibration and manufacturing data are stored in flash with CRC32 protection:

• Magic number: 0x5344494D ("SDIM")
• Version: Structure version for forward compatibility
• Manufacturing: Serial, date, board revision, HW/FW versions
• Calibration: Per-channel R_ref, offset, scale; ADC offset/gain; temp coefficient

Measurement Optimization

• Dummy read: Charges input capacitors before actual measurement
• Settling time: 20ms delay after mux switching
• Timeout protection: 100 attempts × 10ms for ADC conversion

License

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