Tmp117

STM32F103C8 BluePill USB Temperature Monitor

This project is a USB-connected temperature measurement setup for the STM32F103C8T6 BluePill board.

It combines:

• a PT1000 analog sensor read by the STM32 ADC,
• a TMP117 digital sensor connected over I2C,
• direct USB CDC communication from the BluePill to the PC,
• and a desktop Python viewer for live plotting, software filtering, and CSV logging.

The board connects to the PC directly over USB and enumerates as a virtual COM port. No separate USB-UART adapter is required.

Project Overview

The firmware runs on an STM32F103C8-based BluePill board and streams temperature data over USB CDC.

Current default behavior:

• PT1000 is sampled through the ADC at 1 kHz
• the PT1000 ADC path is smoothed in firmware with a fixed-point LPF at about 10 Hz
• PT1000 temperature is transmitted over USB at 20 Hz
• TMP117 is still polled in firmware at 20 Hz
• TMP117 is not filtered in firmware
• TMP117 is not sent by default, but the code already supports switching to TMP117-only or both-sensors output

On the PC side, the included viewer can:

• connect to the USB virtual COM port,
• display live values,
• plot incoming data,
• apply a software LPF in the GUI,
• show or hide raw and filtered traces independently,
• and log data to CSV.

Hardware

Target MCU board

• STM32F103C8T6 BluePill

Measurement Ranges

Reference specifications used here:

• PT106052 / SA10200542 PT1000 listing
• TI TMP117 product page

PT1000 channel

The PT1000 element used for this project is PT106052 / SA10200542 style: Pt1000, class B 0.12 %, 3850 ppm/C, with a specified operating range of approximately -50 C to +500 C.

This is the intended practical measurement range for the PT1000 channel.

The firmware uses a simple linear PT1000 approximation:

R(T) = R0 * (1 + 0.00385 * T)

So the conversion is intentionally simple and integer-friendly for STM32F103. For better absolute accuracy across the full -50...500 C span, the next improvement would be a calibrated table or Callendar-Van Dusen conversion.

With the current divider value, the PT1000 range maps well into the 12-bit ADC input range:

The ADC itself has much wider mathematical headroom, but the useful documented PT1000 range should be treated as -50 C to +500 C unless the sensor, mounting, wiring, and calibration are validated outside that range.

TMP117 channel

The TMP117 channel is intended for the TMP117 operating range: -55 C to +150 C.

In the current firmware, TMP117 is polled at 20 Hz, but the default USB stream sends PT1000 only. TMP117 output can be enabled in firmware with USB_STREAM_MODE_TMP117 or USB_STREAM_MODE_BOTH.

PT1000 connection

The PT1000 is connected as a simple divider from 3.3 V:

• top resistor: PT1000
• bottom resistor: 1.65 kOhm
• divider midpoint: PA0 / ADC1_IN0

Divider:

3.3V -> PT1000 -> PA0 -> 1.65k -> GND

The 1.65 kOhm resistor was chosen to place the expected -50...500 C PT1000 resistance range comfortably inside the ADC range while keeping useful sensitivity over the whole span. It is not a common single resistor value, so it can be made from two 3.3 kOhm resistors in parallel:

3.3k || 3.3k = 1.65k

The divider resistor is used directly by the firmware conversion. In main.c, the value is defined as PT1000_BOTTOM_RESISTOR_MOHM. It is stored in milliohms; the current nominal value is 1650000, meaning 1650.000 Ohm.

If better absolute accuracy is needed, measure the actual parallel resistance and put that value into PT1000_BOTTOM_RESISTOR_MOHM in milliohms.

If the PT1000 and the 1.65 kOhm resistor are physically swapped during assembly, the firmware can be switched without rewiring. Set PT1000_DIVIDER_PT1000_ON_TOP to 0 in main.c. Default is 1, meaning 3.3V -> PT1000 -> PA0 -> 1.65k -> GND.

Swapping the divider mostly reverses the ADC slope. Sensitivity and self-heating are nearly the same because the same two resistors are still in series. With the correct define, measurements remain valid; with the wrong define, the temperature will be badly wrong.

TMP117 connection

• PB6 -> I2C1_SCL
• PB7 -> I2C1_SDA
• default I2C address: 0x48

Additional pins:

• PB8 -> TMP117 ALERT input with external pull-up
• PB9 -> TMP117 ADD0 output, held low with external pull-down

USB connection

The firmware uses the STM32 USB CDC device interface and appears on the PC as a virtual COM port.

Project-specific USB notes:

• PB5 is used for the external USB reconnect/reset wiring
• D+ has an external 1.5 kOhm pull-up to 3.3 V
• the firmware generates a reconnect pulse during startup so the BluePill re-enumerates cleanly over USB

Firmware

Firmware source is in:

• TMP117_stm32f103c8

Main application file:

• main.c

Firmware data path

PT1000 path

• sampled by ADC1 on PA0
• ADC timing tuned for slower, quieter sampling
• sampled at 1 kHz
• filtered in firmware with fixed-point arithmetic
• converted to temperature in degrees Celsius
• sent over USB at 20 Hz

TMP117 path

• read over I2C1
• polled at 20 Hz
• kept in firmware for reference / comparison
• not filtered in firmware
• not streamed by default

Default USB output

Current default build sends only PT1000:

PT1000=24.875 C

With line ending on the wire:

PT1000=24.875 C\r\n

Other firmware output modes

The firmware can also send:

• TMP117 only
• both PT1000 and TMP117 in one line

These modes are selected in main.c by the USB_STREAM_MODE define.

Typical options in the code:

• USB_STREAM_MODE_PT1000
• USB_STREAM_MODE_TMP117
• USB_STREAM_MODE_BOTH

Example combined line:

PT1000=24.875 C TMP117=24.937 C

Desktop Viewer

Viewer file:

• temperature_viewer.pyw

The viewer is intended for direct use with the BluePill USB CDC connection.

Viewer features

• automatic COM port refresh
• connect/disconnect control
• compact COM names such as COM3
• live readout for PT1000 and TMP117
• plot view for one sensor or both sensors
• separate Show raw and Show filtered checkboxes
• software LPF modes:
  • Off
  • EMA
  • SMA
• adjustable visible X-axis window
• Follow latest mode
• Auto Y
• Show all
• Reset view
• Clear plot
• Help button with built-in usage notes
• CSV logging

Important note about filtering

There are two different filtering layers in this project:

1. Firmware PT1000 LPF

   • applied on the MCU to the ADC path
   • fixed-point
   • currently about 10 Hz

2. Viewer software LPF

   • applied inside the PC application
   • tunable in the UI
   • available for display, comparison, and CSV logging

TMP117 currently has no firmware LPF. In the viewer, TMP117 can still be filtered in software for visualization.

Viewer input formats

The viewer accepts the current labeled firmware stream, for example:

PT1000=24.875 C

or:

PT1000=24.875 C TMP117=24.937 C

It also keeps compatibility with older single-value lines through the Single line selector in the UI.

CSV logging

The viewer can save CSV with these columns:

timestamp_s,pt1000_raw_c,pt1000_filtered_c,tmp117_raw_c,tmp117_filtered_c

Python Requirements

The viewer uses:

• Python 3
• PyQt6
• pyqtgraph
• pyserial

Example install command:

pip install PyQt6 pyqtgraph pyserial

Run the viewer with:

py temperature_viewer.pyw

On Windows, you can also launch the .pyw file directly.

Building the Firmware

This firmware project is based on an STM32CubeIDE-generated STM32F103 project.

Main project file:

• TMP117_stm32f103c8.ioc

Recommended build flow

1. Open the project in STM32CubeIDE
2. Open the .ioc if needed
3. Let CubeIDE regenerate project files if required
4. Build the project for the STM32F103C8T6 BluePill
5. Flash it with your usual ST-Link workflow

Command-line build

If the local Debug build directory already exists, the project can also be built with:

make -C TMP117_stm32f103c8/Debug all

Note:

• Debug/ is gitignored
• in a fresh clone, it is normal to open the project in CubeIDE first so the build files are present locally

Repository Layout

• TMP117_stm32f103c8 - STM32 firmware project
• temperature_viewer.pyw - desktop GUI viewer
• TMP117_PCB - KiCad PCB project

Inside the PCB folder:

• KiCad source project is in tmp117
• bom contains BOM-related output
• manuf contains manufacturing files

Practical Usage

Typical workflow:

1. Flash the firmware to the BluePill
2. Connect the board to the PC via USB
3. Wait for the USB CDC virtual COM port to appear
4. Start temperature_viewer.pyw
5. Select the COM port
6. Click Connect
7. View live temperature, tune software LPF, and log CSV if needed

Notes

• The repository ignores generated build outputs, Python cache files, KiCad local history, and KiCad backup archives.
• The current default USB stream is PT1000-only, which keeps the USB output simple for the viewer and logging workflow.
• TMP117 acquisition is still active in firmware, so enabling dual-sensor output later is straightforward.

PT1000 Wiring Quick Reference

Use this connection for the PT1000 divider:

BluePill 3V3 -> PT1000 -> PA0 / ADC1_IN0 -> 1.65k -> GND

This is the default firmware setting:

#define PT1000_DIVIDER_PT1000_ON_TOP 1U

If the parts are swapped:

BluePill 3V3 -> 1.65k -> PA0 / ADC1_IN0 -> PT1000 -> GND

use this firmware setting:

#define PT1000_DIVIDER_PT1000_ON_TOP 0U

The bottom resistor may be either:

single 1.65k resistor

or:

two 3.3k resistors in parallel

Keep the ADC node connected only to PA0, the PT1000, and the bottom resistor. Do not connect the divider to 5 V; the firmware assumes 3.3 V ADC reference behavior.