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An STM32F401 can read an LM35’s analog output with its ADC, convert the sampled voltage to temperature, and transmit formatted readings over USART/UART. The exact ADC pin, serial pins, reference voltage, and board setup depend on the specific STM32F401 package and development board, so verify those before wiring or configuring firmware.
How the temperature-monitoring system works
The LM35 is an external analog temperature sensor, not a UART or other digital-output sensor. Its output voltage varies with temperature. The STM32F401 samples that voltage through an ADC-capable input, firmware converts the ADC result to voltage and then temperature, and a USART/UART connection sends the result to a host such as a computer.
- Sense: Connect the selected LM35 variant’s output to a compatible STM32F401 ADC input, following the sensor’s datasheet for supply, pinout, and electrical limits. The sensor and MCU need a suitable shared ground reference.
- Convert: Use the ADC reading, configured resolution, and effective ADC reference to calculate input voltage. Apply the transfer relation specified for the exact LM35 model to convert that voltage into temperature.
- Report: Format the calculated reading in firmware and transmit it through a configured USART/UART interface. A host-side serial interface is needed to view the data.
Choose pins and board before wiring
ST documents the STM32F401 family with a 12-bit ADC and USART interfaces. That establishes the MCU family’s relevant capabilities, not a universal pin assignment: ADC channels and USART alternate-function pins vary with the exact MCU suffix, package, and board routing. Consult the datasheet and reference manual for the selected part, along with the development board schematic or pinout, before choosing pins. ST’s STM32F401 documentation page includes family resources and lists the NUCLEO-F401RE as an example board; that is not a recommendation that every setup use it.
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Convert ADC readings without assuming the reference
For an N-bit ADC, the idealized count-to-voltage calculation is approximately Vin = ADC count × Vref / (2N − 1). For a 12-bit conversion, the full-scale count is 4095. The value of Vref must reflect the reference actually used by the selected setup; do not assume it from the microcontroller name or substitute the MCU’s internal reference specification without confirming the ADC configuration.
Then use the transfer behavior in the datasheet for the exact LM35 variant to convert voltage to degrees. Do not infer project-level accuracy from a nominal conversion formula. The final result depends on sensor variation, ADC behavior, reference accuracy, wiring and noise, and any calibration. TI’s LM35 product information lists typical accuracy of ±¼°C at room temperature and ±¾°C across −55°C to 150°C; those are product-page typical-accuracy claims, not guaranteed accuracy for a complete STM32 project.
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Configure ADC and serial communication
ADC setup
Set up the ADC channel corresponding to the chosen analog pin and configure its conversion sequence and sampling time for the signal and source impedance. ST’s reference manual and datasheet contain the applicable ADC guidance; the exact external-input requirements depend on the circuit and selected device. The F401 reference manual also discusses sampling-time requirements for internal channels, but those instructions should not be confused with a completed configuration for an external LM35 input.
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Keep the ADC resolution and reference used in firmware consistent with the voltage calculation. If readings appear unstable or implausible, first confirm the selected channel and pin mapping, reference assumption, ground connection, and sampling configuration before changing the temperature equation.
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USART/UART setup
Choose a USART instance and its alternate-function pins that are available on the exact package and accessible on the board. Configure matching serial settings at both ends, including baud rate and frame format; these are implementation choices, not values established by the component-family capability alone. If the computer does not receive readable output, check pin routing, host interface, and whether the two ends use the same serial settings.
Keep the MCU’s internal temperature sensor separate
The STM32F401 also has an on-die temperature-sensor channel, but it is distinct from the external LM35. STMicroelectronics says in RM0368 Rev 6 that “The internal temperature sensor is more suited for applications that detect temperature variations instead of absolute temperatures.” That caution applies to the MCU’s internal sensor, not to the LM35.
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ST’s DS9716 Rev 11 (2019) gives the internal sensor an average slope of 2.5 mV/°C and a typical output voltage of 0.76 V at 25°C. The same datasheet specifies the internal reference voltage as 1.18 V minimum, 1.21 V typical, and 1.24 V maximum under its stated conditions. These are MCU specifications, not LM35 characteristics and not evidence of which reference a particular board uses for its ADC.
What this setup does—and does not—establish
The architecture provides a straightforward path from an analog sensor reading to serial telemetry, but a component pairing alone does not establish a working pinout, firmware implementation, sampling interval, baud rate, or measured accuracy. Those details must be determined for the exact board and circuit. Treat any displayed temperature as a calculated reading whose accuracy depends on the sensor, ADC reference and behavior, electrical setup, and calibration.
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