tempLoggerNG

A four-channel PT-100 temperature logging device built around a Texas Instruments ADS1248 precision delta-sigma ADC and an ATmega328p microcontroller with V-USB (software-only USB, no dedicated USB controller chip). The device connects directly to a PC over USB and is controlled by a small command-line tool that triggers measurements and logs resistance/temperature readings to a file.

https://emsyfs.blogspot.com/2017/07/pt-100-temperature-logger.html

Features

• 4 independent PT-100 RTD input channels, read via a single ADS1248 24-bit ADC using its internal multiplexer and a programmable current source (IDAC) to excite each sensor.
• Full 4-wire measurement on all 4 channels, eliminating lead resistance errors for accurate readings even with long sensor cables.
• On-board diode temperature sensor, used to compensate for temperature drift of the measurement electronics themselves.
• Linear temperature-compensation coefficients (a, b) stored in the ATmega's EEPROM, so each device can be individually calibrated and keeps its calibration across power cycles.
• USB connectivity without a dedicated USB interface chip, using the V-USB software USB implementation on the ATmega328p.
• A command-line host tool (templogger) that:
  • reads out the 4 channels at a configurable sample rate,
  • converts raw resistance to temperature using the Callendar-Van Dusen equation for PT-100 sensors,
  • logs readings (time, diode voltage, and temperature or resistance per channel) to a plain-text data file,
  • optionally applies/updates the temperature-compensation calibration,
  • can read/write individual ADS1248 configuration registers for debugging, and
  • includes a USB round-trip echo self-test.
• Full KiCad hardware design (schematic, PCB layout, custom footprints, Specctra autorouter export, and a rendered schematic PDF) for building the device yourself.

Repository layout

firmware/     ATmega328p firmware (C), including the V-USB driver and
              the ADS1248 driver
commandline/  Host-side command-line tool ("templogger") using libusb
hardware/     KiCad schematic/PCB project, footprints, and a PDF of
              the schematic

Hardware

The design files live in hardware/ as a KiCad project (tempLoggerNG.pro, tempLoggerNG.sch, tempLoggerNG.kicad_pcb). A rendered PDF of the schematic is included as tempLoggerNG-schematic.pdf if you just want to look at the circuit without opening KiCad.

Note: This project was originally designed in an older version of KiCad (legacy schematic/library format). Opening it in a modern KiCad release will likely trigger the automatic "symbol rescue" feature, which copies any symbols it can't resolve against current libraries into local rescue files. This is expected and does not modify the original schematic; if KiCad has already run through this once in your checkout, you may need to clean up before committing anything.

Firmware

The firmware lives in firmware/ and is built with avr-gcc for the ATmega328p.

Requirements

• avr-gcc / avr-libc (AVR toolchain)
• avrdude
• An AVR ISP programmer, such as a USBasp (the Makefile defaults to avrdude -c usbasp -p m328p — edit firmware/Makefile if you use a different programmer)

Build and flash

cd firmware
make          # builds main.hex
make flash    # flashes main.hex to the ATmega328p via avrdude/usbasp

The ATmega328p is clocked at 12 MHz (F_CPU, set in main.c and the Makefile) and must be fused accordingly (external crystal, no clock-divide) before flashing application code — the fuse bytes are not set by this Makefile and must be programmed separately with avrdude, matching whatever oscillator is fitted on your board.

Other useful targets:

make disasm   # disassemble main.bin with avr-objdump
make clean    # remove build artifacts

The firmware never leave debug output enabled in a flashed device — the comment in the Makefile is explicit that debug builds distort the timing the USB stack depends on.

Command-line tool

The host tool lives in commandline/ and talks to the device over USB using libusb (the classic 0.1 API, via libusb-config).

Requirements

• gcc
• libusb (0.1, providing libusb-config)

Build

cd commandline
make

This produces a templogger binary. The device identifies itself over USB using obdev's free shared VID/PID (0x16C0/0x05DC) with manufacturer string www.obdev.at and product string Temp-Logger, so no special udev rule beyond generic USB access permissions is normally required — just make sure your user can access the USB device (e.g. via a udev rule granting access for that VID/PID, or by running as root).

Usage

usage: ./templogger [ch1] [ch2] [ch3] [ch4] [all] [options]

Specify which channel(s) to log with one or more of ch1, ch2, ch3, ch4, or all.

Options:

 -v          : verbose
 -q          : quiet
 -o filename : name of data file. default is "temperature.dat"
 -N n        : stop measurement after n samples
 -t t        : stop measurement after t seconds
 -d delta    : take a sample every delta seconds. Default is 0.5, min is 0.15
 -r          : read registers (for debugging purposes)
 -h          : output this help

Example — log channels 1 and 2 to run1.dat, once per second, for 60 samples:

./templogger ch1 ch2 -o run1.dat -d 1 -N 60

Calibration procedure

Each device's on-board diode-based temperature compensation uses two coefficients, a and b, such that:

R_comp = a - b * V_diode
R      = R_measured - R_comp

To determine and store them for a given device:

1. Connect a fixed, known resistor (e.g. 100 Ω) in place of a sensor.
2. Disable temperature compensation with -x and log resistance directly with -R.
3. Heat up the device itself (not the resistor) and record data while it cools down.
4. Plot V_diode against R_measured - 100 Ω and fit a straight line to obtain a and b.
5. Write the coefficients to the device's EEPROM:

   ./templogger -c <a> <b>

Once written, the coefficients are read back from EEPROM automatically and applied during normal logging unless -x is given.

Self-test

./templogger test

Sends 1000 random values to the device and checks that they are echoed back correctly, as a basic USB communication sanity check.

License

The firmware and command-line tool (firmware/, commandline/) are licensed under the GNU General Public License v2 (see LICENSE). The bundled V-USB driver (firmware/usbdrv/) is dual-licensed under GPLv2 or a commercial license from Objective Development — see firmware/usbdrv/License.txt and firmware/usbdrv/CommercialLicense.txt for details, and firmware/usbdrv/USB-ID-FAQ.txt / USB-IDs-for-free.txt regarding use of the shared VID/PID.

The hardware design (hardware/) is licensed under the CERN Open Hardware Licence Version 2 - Strongly Reciprocal (CERN-OHL-S v2), see LICENSE-HARDWARE.