STM32F401 Black Pill

What is the STM32F401 Black Pill?

The STM32F401 Black Pill is a tiny, low-cost development board built around the STM32F401CCU6 — a 32-bit ARM Cortex-M4 with a hardware floating-point unit (FPU). It is the modern answer to the Blue Pill: faster, more memory, a reliable USB-C port, and a factory USB bootloader so you can flash it with no programmer.

Made popular by WeAct, it drops straight into a breadboard and works with the Arduino IDE (STM32duino core), STM32CubeIDE or PlatformIO.

A map of the Black Pill: the STM32F401CCU6 Cortex-M4, USB-C, a 25 MHz and a 32.768 kHz crystal, the NRST and BOOT0 buttons, the PWR and PC13 LEDs, and the 4-pin SWD header.

The Brain: STM32F401CCU6

At the centre sits the STM32F401CCU6 — a 32-bit ARM Cortex-M4 clocked to 84 MHz (multiplied by an internal PLL from the 25 MHz crystal). The Cortex-M4 adds a hardware FPU and DSP instructions, so floating-point maths runs far faster than on the Blue Pill's M3.

Inside the STM32F401CCU6: a Cortex-M4 core with FPU + 256 KB flash / 64 KB SRAM + 30+ GPIO, a 12-bit ADC, timers, and native USB OTG.

• Flash (256 KB) — four times the Blue Pill; plenty of room for big sketches.
• SRAM (64 KB) — working memory for variables and buffers.
• No EEPROM — there is no dedicated EEPROM; libraries emulate it in flash.
• Cortex-M4 + FPU — hardware floating point and DSP, great for audio, sensors and control maths.

Full STM32F401 Black Pill Pinout

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

The full Black 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. 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)

30+ 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), with configurable built-in pull-ups and pull-downs.

Analog input pins (10 channels)

One 12-bit ADC with 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) for the full 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), NRST, plus the 4-pin SWD header (3V3, SWDIO = PA13, SWCLK = PA14, GND). There is no barrel jack.

3.3 V Logic — and the 5 V Trap

Like all STM32 boards, the Black 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.

Powering the Black Pill

Power it from USB-C (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.

USB-C 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 STM32F401 has powerful timers giving many PWM channels across the 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 Black Pill's 12-bit ADC is far finer than an 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(115200);
  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 STM32F401 is generous with buses: three UARTs, three SPI and three I2C — plus native USB OTG. (It has no CAN, unlike the Blue Pill.)

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

How Code Gets onto the Black Pill

The Black Pill's best trick: a built-in USB DFU bootloader, so you can flash it straight over USB-C with no ST-Link.

Write → compile → enter DFU mode (BOOT0 + NRST) → flash over USB-C → the code runs from flash. No ST-Link required.

The BOOT0 Button & DFU Mode

Where the Blue Pill uses jumpers, the Black Pill has a BOOT0 button. Use it with NRST to drop into USB DFU mode.

Hold BOOT0, tap NRST, release — the board appears as a USB DFU device. BOOT0 = 0 (normal) runs your program from flash.

After flashing, just press NRST on its own (without BOOT0) to run your program. You only hold BOOT0 when you want to upload.

Black Pill vs Blue Pill vs Arduino Uno

Should you get a Black Pill or a Blue Pill? The Black Pill is faster and easier to flash; the Blue Pill keeps CAN and the ST-Link workflow.

The Black Pill wins on speed (Cortex-M4 + FPU), memory, USB-C and DFU flashing; the Blue Pill keeps CAN. Both dwarf the 8-bit Uno.

Choose the Black Pill for the most power, the FPU, USB-C and no-programmer flashing; choose the Blue Pill if you need CAN or already use ST-Link; choose the Uno/Nano for 5 V shields and the biggest beginner ecosystem. For a 32-bit ARM board that 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. Fast floating-point maths (the FPU at work)

void setup() { Serial.begin(115200); }
void loop() {
  float sum = 0;
  for (int i = 1; i <= 1000; i++) sum += sqrtf(i) * sinf(i);  // FPU-accelerated
  Serial.println(sum, 4);
  delay(500);
}

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 Black 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 STM32F401 Black Pill at a glance.

Key numbers for the Black 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 STM32F401 Black Pill.

Conclusion & Key Takeaways

The STM32F401 Black Pill is the Blue Pill done better — faster Cortex-M4 with an FPU, more memory, USB-C, and flashing with no programmer.

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