STM32F103 Blue Pill

What is the STM32F103 Blue Pill?

The STM32F103 Blue Pill is a tiny, very cheap development board built around the STM32F103C8T6 — a genuine 32-bit ARM Cortex-M3 microcontroller. For a couple of dollars you get far more speed, memory and peripherals than an 8-bit Arduino Uno.

It gets its nickname from the small blue PCB. It is the low-cost gateway into the STM32 world, and it works with the Arduino IDE (via the STM32duino core), STM32CubeIDE or PlatformIO.

A map of the Blue Pill: the STM32F103C8 Cortex-M3, micro-USB, an 8 MHz and a 32.768 kHz crystal, the BOOT0/BOOT1 jumpers, reset, the PWR and PC13 LEDs, and the 4-pin SWD header.

The Brain: STM32F103C8

At the centre sits the STM32F103C8T6 — a 32-bit ARM Cortex-M3 core clocked up to 72 MHz (multiplied by an internal PLL from the 8 MHz crystal), with hardware for USB, CAN and a rich set of timers and serial buses.

Inside the STM32F103C8: a Cortex-M3 core + 64 KB flash / 20 KB SRAM + 37 GPIO, two 12-bit ADCs, timers, and hardware USB & CAN.

• Flash (64 KB) — stores your program. (The C8 is officially 64 KB; many boards actually have 128 KB of usable flash.)
• SRAM (20 KB) — working memory for variables — ten times the Uno's 2 KB.
• No EEPROM — there is no dedicated EEPROM; libraries emulate it in flash instead.
• 72 MHz Cortex-M3 — a 32-bit core, so maths and data handling are dramatically faster than an 8-bit AVR.

Full STM32F103 Blue Pill Pinout

Here is the complete pinout. Unlike an Arduino, STM32 pins are named by port and number — PA0…PA15, PB0…PB15 and PC13…PC15 — and each has several alternate functions.

The full Blue Pill pinout — the two 20-pin rows with every port pin and its alternate functions. PA13/PA14 are SWDIO/SWCLK on the SWD header; PB2 is BOOT1.

Prefer a simpler view? This colour-coded map groups the pins by function:

Quick overview — GPIO on port A/C along the top, power and port B below. Note everything is 3.3 V logic.

Every Pin Explained

Let's go through each group of pins. On the STM32 a single pin often has many roles — you pick one in software.

GPIO pins (PA / PB / PC)

37 general-purpose pins that read or write HIGH (3.3 V) or LOW (0 V) with pinMode(), digitalWrite() and digitalRead(). Each handles about 20 mA (25 mA max). They have configurable built-in pull-ups and pull-downs.

Analog input pins (10 channels)

Two 12-bit ADCs share 10 external channels, each reading 0–3.3 V as 0–4095:

• PA0–PA7 (ADC channels 0–7) and PB0, PB1 (channels 8, 9).
• Read them with analogRead(PA0); call analogReadResolution(12) to get the full 12-bit range.
• Warning: the ADC pins are not 5 V-tolerant — keep them at 3.3 V max.

The buses (and their pins)

Power & programming pins

5V (USB input / output), 3.3V (regulated out), GND, VBAT (backup domain), RESET, plus the 4-pin SWD header (3V3, SWDIO = PA13, SWCLK = PA14, GND) for the ST-Link. There is no barrel jack.

3.3 V Logic — and the 5 V Trap

The single biggest difference from an Uno: the Blue Pill runs at 3.3 V. Most digital pins are 5 V-tolerant as inputs, but the analog pins are not — getting this wrong can damage the chip.

3.3 V sensors wire straight through. For 5 V modules use a divider or level shifter — and never put 5 V on an analog pin (PA0–PA7, PB0/PB1).

• 5 V-tolerant (FT) pins — most PA/PB/PC digital pins can safely read a 5 V input.
• NOT 5 V-tolerant — the analog pins PA0–PA7, PB0 and PB1, plus a few others.
• Outputs always swing 0–3.3 V. A 3.3 V HIGH is still read as HIGH by most 5 V chips, so many links only need shifting in one direction.

Powering the Blue Pill

Power it from micro-USB (5 V) or feed 5 V into the 5V pin. An on-board AMS1117 regulator makes the 3.3 V the chip and every GPIO run on.

Micro-USB or the 5V pin → the AMS1117 LDO → the 3.3 V rail for the STM32 and all GPIO. You can also feed a regulated 3.3 V straight into the 3.3V pin.

Digital I/O

Output a HIGH/LOW to drive an LED or relay, or read a HIGH/LOW input from a button or sensor — all at 3.3 V.

Output drives an LED (via a 220 Ω resistor); an input reads a button using the built-in INPUT_PULLUP — no external resistor needed.

// digital output + input (STM32, 3.3 V)
void setup() {
  pinMode(PB9, OUTPUT);          // LED pin
  pinMode(PB5, INPUT_PULLUP);    // button to GND
}
void loop() {
  digitalWrite(PB9, !digitalRead(PB5));   // LED follows the button
}

PWM Output (~)

PWM fakes an analog voltage by switching a pin on and off fast. The STM32F103 has powerful timers giving up to 15 PWM channels across many PA/PB pins.

Higher duty → higher average voltage (of 3.3 V). Use analogWrite(pin, 0–255) on any timer pin such as PA8.

// fade an LED on a PWM/timer pin (PA8)
void setup() { pinMode(PA8, OUTPUT); }
void loop() {
  for (int v = 0; v <= 255; v++) { analogWrite(PA8, v); delay(5); }
  for (int v = 255; v >= 0; v--) { analogWrite(PA8, v); delay(5); }
}

Analog Input (ADC)

The Blue Pill's 12-bit ADC is far finer than the Uno's 10-bit one. Each reading turns 0–3.3 V into 0–4095 — about 0.8 mV per step.

A potentiometer on PA0 → the ADC reads 0–4095. Convert to volts with v * 3.3 / 4095.0. Keep analog pins at 3.3 V max.

// read a 12-bit analog input on PA0
void setup() {
  Serial.begin(9600);
  analogReadResolution(12);        // full 0..4095 range
}
void loop() {
  int raw = analogRead(PA0);         // 0..4095
  float volts = raw * 3.3 / 4095.0;
  Serial.println(volts);
  delay(200);
}

UART, SPI & I2C

The STM32F103 is generous with buses: three UARTs, two SPI and two I2C — plus native USB and CAN that an Uno simply doesn't have.

USART1 on PA9/PA10, SPI1 on PA5/PA6/PA7, I2C1 on PB6/PB7 — with a second copy of each bus, plus USB and CAN.

How Code Gets onto the Blue Pill

Unlike an Arduino, the Blue Pill usually needs a small ST-Link V2 programmer on its 4-pin SWD header. You can also upload over serial using the BOOT jumper.

Write → compile → ST-Link V2 over SWD → the code runs from flash. Or set BOOT0 = 1 and upload over USART1 (PA9/PA10) with a USB–TTL adapter.

The BOOT0 / BOOT1 Jumpers

The two little yellow jumpers decide where the chip starts running when it powers up or resets.

BOOT0 = 0 boots from flash and runs your program (normal). BOOT0 = 1 boots the built-in serial bootloader for uploading. Both back to 0 to run again.

For everyday work with an ST-Link, leave both jumpers at 0. Only move BOOT0 to 1 when you want to upload over serial without a programmer — then set it back and reset.

Blue Pill vs Arduino Uno vs Nano

Why choose a Blue Pill over an Arduino Uno? Speed, memory and peripherals — for a similar price. The trade-off is 3.3 V wiring and needing an ST-Link.

The Blue Pill is a 32-bit, 72 MHz machine with far more RAM, a 12-bit ADC and extra buses — a big step up from the 8-bit Uno and Nano.

Choose the Blue Pill when you need real 32-bit speed, more memory, a fine ADC or USB/CAN; choose the Uno/Nano for 5 V shields, simple USB uploading and the biggest beginner ecosystem. For a 32-bit ARM board that still uploads over USB like an Arduino, see the Arduino Due.

Code Examples

The everyday building blocks, written for the STM32duino core.

1. Blink the on-board PC13 LED (active-low)

void setup() { pinMode(PC13, OUTPUT); }
void loop() {
  digitalWrite(PC13, LOW);  delay(1000);   // LED on
  digitalWrite(PC13, HIGH); delay(1000);   // LED off
}

2. Print to the Serial Monitor over USB

void setup() {
  Serial.begin(115200);           // USB CDC serial
}
void loop() { Serial.println(millis()); delay(1000); }

3. Talk to a module on USART1 (PA9 / PA10)

// Serial1 is USART1 on PA9 (TX) / PA10 (RX)
void setup() {
  Serial.begin(115200);      // USB to PC
  Serial1.begin(9600);       // module on PA9/PA10
}
void loop() {
  if (Serial1.available()) Serial.write(Serial1.read());
}

Beginner Mini-Projects

Four small builds that show off the Blue Pill's strengths.

Project 1 wiring — blink the active-low PC13 LED (works with no parts) or add your own on PB9.

void setup() { pinMode(PC13, OUTPUT); }
void loop() {
  digitalWrite(PC13, LOW);  delay(500);
  digitalWrite(PC13, HIGH); delay(500);
}

void setup() {
  pinMode(PC13, OUTPUT);
  pinMode(PB5, INPUT_PULLUP);
}
void loop() {
  if (digitalRead(PB5) == LOW) digitalWrite(PC13, LOW);   // pressed = LED on
  else                          digitalWrite(PC13, HIGH);
}

void setup() {
  Serial.begin(115200);
  analogReadResolution(12);
}
void loop() {
  int raw = analogRead(PA0);
  Serial.println(raw * 3.3 / 4095.0, 3);   // volts
  delay(100);
}

#include <Wire.h>
void setup() {
  Serial.begin(115200);
  Wire.begin();                 // I2C1: SCL=PB6, SDA=PB7
}
void loop() {
  for (byte a = 1; a < 127; a++) {
    Wire.beginTransmission(a);
    if (Wire.endTransmission() == 0) { Serial.print("found 0x"); Serial.println(a, HEX); }
  }
  delay(2000);
}

Specifications

The STM32F103 Blue Pill at a glance.

Key numbers for the Blue Pill — MCU, core, clock, memory, GPIO, ADC, buses and programming.

Key Terms — Glossary

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Frequently Asked Questions

Quick answers to the questions people ask most about the STM32F103 Blue Pill.

Conclusion & Key Takeaways

The STM32F103 Blue Pill is the cheapest way into 32-bit ARM — huge power for the price, as long as you respect its 3.3 V world.

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