STM32F103C8T6 Blue Pill

Fig. 1. STM32F103C8T6 Blue Pill, 2x20 2.54 mm, 53 x 22 mm. Hover or tap a signal to light up every pin that carries it.

• The analog pins (PA0 to PA7, PB0, PB1) and PC13 to PC15 are NOT 5 V tolerant. Most of PA8 to PA15 and PB3 to PB15 are, but check the FT column in the datasheet for the exact pin before you rely on it.
• PC13 drives the on-board LED and sinks it, so the LED is on when the pin is low. PC13, PC14 and PC15 are limited to 3 mA and cannot be used as fast outputs.
• USB rarely works out of the box: the D+ pull-up should be 1.5k and most boards ship 10k or 4.7k. Replace R10, or bodge a 1.8k from PA12 to 3.3 V.
• PA13 and PA14 are SWDIO and SWCLK on the 4-pin debug header, not on the side headers. That header is the reason to pick this board: real breakpoints over a 3 dollar ST-Link clone.
• PA15, PB3 and PB4 default to JTAG. They need the JTAG remap disabled in software before they work as ordinary GPIO.
• BOOT0 high at reset runs the built-in serial bootloader on PA9/PA10. BOOT0 low runs your code. Move the jumper back after flashing or the next power cycle sits in the bootloader.
• The 5V header pin is wired straight to USB VBUS with no protection. Do not feed it from a bench supply with USB also plugged in.
• Plenty of boards in circulation carry a CKS or CS32 die rather than a real ST part. They mostly work, but SWD connection and USB behave differently, and the 128 KB flash on a C8-marked chip is unofficial.

• Blink the on-board LED on PC13

  CMSIS, no HAL

  #include "stm32f1xx.h"
  
  /* 1. Clock the port. Nothing else works until this line runs. */
  RCC->APB2ENR |= RCC_APB2ENR_IOPCEN;
  
  /* 2. Configure PC13. Pins 8-15 live in CRH, 4 bits each,
        so PC13 sits at bit (13 - 8) * 4 = 20.
        MODE = 10 (output, 2 MHz), CNF = 00 (push-pull). */
  GPIOC->CRH &= ~(0xFu << 20);
  GPIOC->CRH |=  (0x2u << 20);
  
  /* 3. BSRR writes without a read-modify-write, so it is
        interrupt-safe. The LED is active low on this board. */
  GPIOC->BSRR = GPIO_BSRR_BR13;   /* LED on  */
  GPIOC->BSRR = GPIO_BSRR_BS13;   /* LED off */

  The peripheral clock enable in RCC is the step everyone misses. Without it the GPIO registers read and write as zero and the pin simply never moves, with no error anywhere.

Table 1. Pin assignments.