Blue pill is the nickname given to the most popular, ultra-cheap, and compact STM32F103 board. This board can be uploaded with the Arduino bootloader so it is possible to be programmed in an Arduino environment. This board is clocked at 72 MHz. 

STM32F103C8T6 is a very powerful Microcontroller and with its 32-bit CPU based on ARM Cortex-M3 Architecture manufactured by STMicroelectronics., it can easily beat Arduino UNO in performance 4.5 times faster.

STM32F103C8T6 Specifications:

•  ARM 32-bit Cortex – M3 CPU core with a maximum frequency of 72 MHz.

• Memories: contains 64 Kbytes of Flash and 20 Kbytes of SRAM
• GPIO Pins – 32 with external interrupt capability
• Timers – 3 16-bit Timers, 1 16-bit PWM Timer
• PWM Pins – 15
• Analog – 10 channels of 12-bit ADC
• I2C – 2 I2C Peripherals
• USART – 3 USART Peripherals with hardware control
• SPI – 2 SPI Peripherals
• Other Peripherals – USB 2.0 Full Speed, CAN 2.0B

The Blue Pill Board Pinout:

 each pin of the STM32F103C8T6 MCU can have multiple functions. Also, note that some IO pins are 5V tolerant, which means that you can connect 5V compatible IO on those pins without any worry. The firmware can be installed either via USART1 with the permanently installed serial bootloader or via SWD with an ST-Link adapter.

The Boot1 jumper (= PB2) can be used for your own purposes during normal operation.

It is advisable to re-solder the frame of the sensitive USB socket. The board must not be supplied via USB and a 5 V power supply at the same time (short circuit). An additional 3.3 V supply is okay, however.

The USB pull-up resistor R10 and PA12 is often incorrectly equipped with 10 kΩ instead of the correct 1.5 kΩ so that the USB port works unreliably. To correct this, a 1.8 kΩ resistor can also be connected in parallel (from PA12 to 3.3 V).

If the clock quartz is used, the pins on PC14 and PC15 should be removed so that it vibrates stably.

How to get started with Blue Pill :