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# STM32 Blue Pill (F103C8)

A cheap 53 x 23 mm STM32F103C8T6 board: 72 MHz Cortex-M3, 64 KB flash, 20 KB RAM, USB and 3.3 V I/O.

Source page: https://www.tinkered.ai/components/stm32-bluepill-f103c8
Datasheet: https://stm32-base.org/boards/STM32F103C8T6-Blue-Pill

## In short

The Blue Pill is a 53 x 23 mm board around ST's STM32F103C8T6, a 72 MHz Arm Cortex-M3 with 64 KB of flash and 20 KB of SRAM running from 2.0 V to 3.6 V (ST STM32F103x8 datasheet, pp. 1, 10; stm32-base.org). Power it from micro-USB, a 5V pin or a 3.3V pin, one at a time, and wire parts to its 3.3 V pins; most are 5 V tolerant in digital mode, but PA0 to PA7, PB0, PB1, PB5 and PC13 to PC15 are not (datasheet pin table, pp. 28 to 32). Program it with an ST-LINK on the SWD header, or over USART1 (PA9, PA10) with the BOOT0 jumper set to 1 (datasheet p. 15).

The Blue Pill is the best-known STM32 board: a small blue PCB, sold by many makers, built around ST's STM32F103C8T6. It has an 8 MHz crystal for the main clock, a 32.768 kHz crystal for the real-time clock, a micro-USB socket wired to the chip's USB pins (PA11, PA12), a 3.3 V regulator, a RESET button, BOOT0 and BOOT1 jumpers, a power LED, a user LED on PC13 and a 4-pin SWD header. Two 20-pin headers 0.6 in apart let it straddle a breadboard (stm32-base.org).

The STM32F103C8 is a medium-density Cortex-M3 at up to 72 MHz with 64 KB of flash and 20 KB of SRAM in a 48-pin LQFP package, with 37 GPIO and 10 ADC channels in that package (ST datasheet, pp. 1, 10). It brings two 12-bit 1 µs ADCs, three general-purpose timers plus an advanced-control timer, two I2C, three USARTs, two SPI, a CAN 2.0B controller and a full-speed USB 2.0 device (pp. 1, 10). There is no DAC.

It runs Arduino code through the STM32duino core, as well as ST's STM32Cube HAL, libopencm3 and others. Out of the box it has no USB bootloader: the chip's built-in bootloader reprograms flash over USART1 only (datasheet p. 15), so most people flash it with a cheap ST-LINK V2 on the SWD header. Many boards on the market carry clone chips (CS32F103, APM32F103 and others) rather than ST's, which mostly behave the same but can be refused by ST's tools.

## Pinout

| Pin | Function | Type |
| --- | --- | --- |
| GND | GND (G) | power in |
| GND_2 | GND_2 (G) | power in |
| 3V3 | 3V3 (3.3) · 3.3 V | power out |
| NRST | Digital · 3.3 V | input |
| PB11 | Digital, I2C2 SDA, PWM, UART USART3 RX · 3.3 V, 5 V tolerant | bidirectional |
| PB10 | Digital, I2C2 SCL, PWM, UART USART3 TX · 3.3 V, 5 V tolerant | bidirectional |
| PB1 | Digital, ADC12_IN9, PWM · 3.3 V | bidirectional |
| PB0 | Digital, ADC12_IN8, PWM · 3.3 V | bidirectional |
| PA7 | Digital, ADC12_IN7, SPI1 MOSI, PWM · 3.3 V | bidirectional |
| PA6 | Digital, ADC12_IN6, SPI1 MISO, PWM · 3.3 V | bidirectional |
| PA5 | Digital, ADC12_IN5, SPI1 SCK · 3.3 V | bidirectional |
| PA4 | Digital, ADC12_IN4, SPI1 NSS · 3.3 V | bidirectional |
| PA3 | Digital, ADC12_IN3, PWM, UART USART2 RX · 3.3 V | bidirectional |
| PA2 | Digital, ADC12_IN2, PWM, UART USART2 TX · 3.3 V | bidirectional |
| PA1 | Digital, ADC12_IN1, PWM, UART USART2 RTS · 3.3 V | bidirectional |
| PA0 | Digital, ADC12_IN0, PWM, UART USART2 CTS · 3.3 V | bidirectional |
| PC15 | Digital · 3.3 V | bidirectional |
| PC14 | Digital · 3.3 V | bidirectional |
| PC13 | Digital · 3.3 V | bidirectional |
| VBAT | VBAT (VB): the backup domain (RTC, BKP), a coin cell keeps it | power in |
| PB12 | Digital, SPI2 NSS · 3.3 V, 5 V tolerant | bidirectional |
| PB13 | Digital, SPI2 SCK, PWM, UART USART3 CTS · 3.3 V, 5 V tolerant | bidirectional |
| PB14 | Digital, SPI2 MISO, PWM, UART USART3 RTS · 3.3 V, 5 V tolerant | bidirectional |
| PB15 | Digital, SPI2 MOSI, PWM · 3.3 V, 5 V tolerant | bidirectional |
| PA8 | Digital, PWM · 3.3 V, 5 V tolerant | bidirectional |
| PA9 | Digital, PWM, UART USART1 TX · 3.3 V, 5 V tolerant | bidirectional |
| PA10 | Digital, PWM, UART USART1 RX · 3.3 V, 5 V tolerant | bidirectional |
| PA11 | Digital, CAN CAN RX, PWM, UART USART1 CTS, USB USB D- · 3.3 V, 5 V tolerant | bidirectional |
| PA12 | Digital, CAN CAN TX, UART USART1 RTS, USB USB D+ · 3.3 V, 5 V tolerant | bidirectional |
| PA15 | Digital, SPI1 NSS, PWM · 3.3 V, 5 V tolerant | bidirectional |
| PB3 | Digital, SPI1 SCK, PWM · 3.3 V, 5 V tolerant | bidirectional |
| PB4 | Digital, SPI1 MISO, PWM · 3.3 V, 5 V tolerant | bidirectional |
| PB5 | Digital, SPI1 MOSI, PWM · 3.3 V | bidirectional |
| PB6 | Digital, I2C1 SCL, PWM, UART USART1 TX · 3.3 V, 5 V tolerant | bidirectional |
| PB7 | Digital, I2C1 SDA, PWM, UART USART1 RX · 3.3 V, 5 V tolerant | bidirectional |
| PB8 | Digital, CAN CAN RX, I2C1 SCL, PWM · 3.3 V, 5 V tolerant | bidirectional |
| PB9 | Digital, CAN CAN TX, I2C1 SDA, PWM · 3.3 V, 5 V tolerant | bidirectional |
| 5V | 5V (5V) | power in |
| GND_3 | GND_3 (G) | power in |
| 3V3_2 | 3V3_2 (3.3) | power out |

## Specifications

| Spec | Value | Source |
| --- | --- | --- |
| Microcontroller | STM32F103C8T6, Arm Cortex-M3, up to 72 MHz | ST STM32F103x8 datasheet (Doc ID 13587 Rev 14), p. 1 |
| Memory | 64 KB flash, 20 KB SRAM | ST STM32F103x8 datasheet, Table 2, p. 10 |
| Chip supply voltage | 2.0 V to 3.6 V | ST STM32F103x8 datasheet, p. 1 |
| GPIO and ADC (LQFP48) | 37 GPIO; 2 x 12-bit ADC, 10 channels | ST STM32F103x8 datasheet, Table 2, p. 10 |
| I/O current | ±8 mA normal, ±20 mA with relaxed levels; 25 mA absolute max per pin, 150 mA total | ST STM32F103x8 datasheet, pp. 38, 65 |
| PC13 to PC15 | Sink 3 mA at most, 2 MHz max, never source current | ST STM32F103x8 datasheet, pin table note 5, p. 32 |
| Board regulator | TX6211B, 3.6 V to 5.5 V in, 3.3 V at 300 mA out | stm32-base.org, Blue Pill |
| Board clocks | 8 MHz HSE crystal, 32.768 kHz LSE crystal | stm32-base.org, Blue Pill |
| User LED | PC13, wired as a sink (lit LOW) | stm32-base.org, Blue Pill |
| Board size | 23 x 53 mm | stm32-base.org, Blue Pill |
| Logic level | 3.3 V | Tinkered component registry |
| Clock speed | 72 MHz | Tinkered component registry |
| Flash | 64 KB | Tinkered component registry |
| RAM | 20 KB | Tinkered component registry |
| GPIO pins | 32 | Tinkered component registry |
| ADC channels | 10 | Tinkered component registry |
| ADC resolution | 12 bits | Tinkered component registry |
| PWM channels | 15 | Tinkered component registry |
| Dimensions | 53 × 22.6 × 19.6 mm | Tinkered component registry |
| Weight | 4 g | Tinkered component registry |

## Circuit notes

- Logic is 3.3 V. Most pins are 5 V tolerant (FT) as digital inputs, but PA0 to PA7, PB0, PB1, PB5 and PC13 to PC15 are not, and no pin is 5 V tolerant in analog mode (ST datasheet pin table, pp. 28 to 32). Put 5 V signals on an FT pin or divide them down.
- Power from one source only. stm32-base warns that the 5V pins connect straight to the USB connector's 5 V with no protection, so never feed an external 5 V while USB is plugged in.
- The on-board regulator is rated 3.3 V at 300 mA on the reference board (stm32-base.org); the MCU takes its share, so power motors, servos and LED strips from their own supply with a common ground.
- PC13 to PC15 sit behind the backup power switch: they can sink only 3 mA, should not switch faster than 2 MHz and must never source current (datasheet p. 32). PC14 and PC15 carry the 32.768 kHz crystal.
- The micro-USB D+ line (PA12) needs a 1.5 kΩ pull-up to 3.3 V for USB to work (datasheet p. 74). Many boards ship with 10 kΩ or 4.7 kΩ instead (stm32-base.org); if a PC will not enumerate the board, that resistor is the usual cause.
- Keep each pin at 8 mA for clean logic levels and the whole chip within 150 mA (datasheet pp. 38, 65). Drive relays, motors and bright LEDs through a transistor.

## Common mistakes

- **Expecting it to upload over USB like an Arduino.** The stock chip has no USB bootloader; its ROM bootloader works over USART1 (PA9, PA10) only (ST datasheet p. 15). Use an ST-LINK on the SWD header, or a USB-serial adapter with BOOT0 set to 1, or flash a USB bootloader first.
- **Driving 5 V into a non-FT pin.** PA0 to PA7, PB0, PB1, PB5 and PC13 to PC15 are not 5 V tolerant (datasheet pp. 28 to 32). These are also the ADC pins, so a 5 V analog sensor needs a divider.
- **Using PC13 as a current source.** PC13 to PC15 can sink only 3 mA and must not source current (datasheet p. 32). The on-board LED on PC13 is lit by driving the pin LOW.
- **USB not recognised.** Many Blue Pills fit a 10 kΩ or 4.7 kΩ pull-up on D+ instead of the 1.5 kΩ that USB needs (stm32-base.org; datasheet p. 74). Replacing it with 1.5 kΩ fixes enumeration on most PCs.
- **Leaving BOOT0 at 1.** With BOOT0 at 1 the chip starts its serial bootloader instead of your program after every reset. Move the jumper back to 0 once flashing is done.

## Questions

### Is the STM32 Blue Pill 5 V tolerant?

Partly. Pins marked FT tolerate 5 V as digital inputs, but PA0 to PA7, PB0, PB1, PB5 and PC13 to PC15 do not, and no pin does in analog mode (ST STM32F103x8 datasheet, pin table pp. 28 to 32). The chip itself runs from 2.0 V to 3.6 V (p. 1).

### Why won't my Blue Pill upload?

With an ST-LINK, check the four SWD wires (3.3, SWDIO, SWCLK, GND) and try holding RESET while the upload starts. Over serial, set BOOT0 to 1, press RESET and use PA9 (TX) and PA10 (RX) (ST datasheet p. 15). If ST's software refuses the chip ID, the board may carry a clone chip, which some tools need configured separately.

### Blue Pill vs Black Pill: which should I buy?

WeAct's STM32F411 Black Pill has a 100 MHz Cortex-M4 with FPU, 512 KB of flash, 128 KB of SRAM and USB-C, and its ROM bootloader flashes over USB DFU (ST STM32F411xC/xE datasheet, pp. 1, 12, 20). The Blue Pill's F103 has 72 MHz, 64 KB and 20 KB (ST STM32F103x8 datasheet, p. 10) and needs an ST-LINK or serial adapter. Choose the F411 Black Pill for new projects and the Blue Pill for existing F103 code.

### Does the Blue Pill have 64 KB or 128 KB of flash?

ST specifies the STM32F103C8 with 64 KB of flash; 128 KB is the STM32F103CB (ST datasheet, Table 2, p. 10). Many C8 chips accept writes past 64 KB, which is why PlatformIO offers a bluepill_f103c8_128k option, but only 64 KB is guaranteed and clone chips may not have more.

### Which pin is the Blue Pill's built-in LED?

PC13, wired as a sink so it lights when the pin is LOW (stm32-base.org). PC13 can sink only 3 mA (ST datasheet p. 32), which is enough for the on-board LED but not for driving other loads.

### How much current can the Blue Pill's 3.3V pin supply?

The reference board's TX6211B regulator is rated 300 mA at 3.3 V (stm32-base.org), shared with the MCU and its LEDs. Sensors and small modules are fine; motors, servos and Wi-Fi modules need their own supply.

## Related parts

- [STM32 Black Pill (F103C8)](https://www.tinkered.ai/components/stm32-blackpill-f103c8.md): same chip, another board
- [STM32 Black Pill (F411CE)](https://www.tinkered.ai/components/stm32-blackpill-f411ce.md): same job: an STM32 board
- [STM32 Nucleo-F446RE](https://www.tinkered.ai/components/stm32-nucleo-f446re.md): same job: an STM32 board
- [STM32 Nucleo-F401RE](https://www.tinkered.ai/components/stm32-nucleo-f401re.md): same job: an STM32 board
- [STM32 Nucleo-L476RG](https://www.tinkered.ai/components/stm32-nucleo-l476rg.md): same job: an STM32 board

## Try it

Tinkered simulates the STM32 Blue Pill (F103C8) in the browser: wire it on a 3D breadboard, run your real code on the board, and measure it. Free to start at https://www.tinkered.ai.