STM32F103C8 LED Blinking Timer -- ARM Cortex-M3

Configured STM32F103C8 (Blue Pill) GPIO and TIM2 peripheral to blink an on-board LED at precise 2-second intervals using HAL library, demonstrating register-level timer prescaler and auto-reload calculations for embedded bare-metal programming.

Overview

Configured the STM32F103C8T6 "Blue Pill" microcontroller to blink its on-board LED (PC13) at precise 2-second intervals using the TIM2 general-purpose timer peripheral. The project demonstrates fundamental ARM Cortex-M3 bare-metal concepts including GPIO port configuration (output push-pull), RCC clock enable for GPIOC and TIM2, timer prescaler and auto-reload register calculations, and NVIC interrupt handling via HAL callbacks.

The timer is driven from the APB1 bus clock (72 MHz after PLL multiplication from the 8 MHz HSE crystal), with a prescaler of 7200 and auto-reload of 10000 yielding exactly 2.0 seconds between toggle events. Built and flashed using STM32CubeIDE with the HAL library framework, verified on hardware with an oscilloscope on PC13 confirming the 0.5 Hz square wave output.

Key Metrics

Metric	Value
Blink Interval	2.0 seconds
Timer	TIM2 peripheral
System Clock	72 MHz
Framework	HAL + bare-metal

Hardware Connection Table

Component	STM32 Pin	Connection	Notes
On-board LED	PC13	Active-low (built-in)	LED ON = Pin LOW
HSE Crystal	PD0 (OSC_IN)	8 MHz crystal	Pierce oscillator
HSE Crystal	PD1 (OSC_OUT)	8 MHz crystal	20 pF load caps
ST-Link SWDIO	PA13	ST-Link V2	Serial Wire Data
ST-Link SWCLK	PA14	ST-Link V2	Serial Wire Clock
ST-Link 3.3V	3.3V	ST-Link V2	Target power
ST-Link GND	GND	ST-Link V2	Common ground
NRST	NRST	ST-Link V2	Reset (optional)

Clock Tree

HSE Crystal (8 MHz)
        v
PLL (x9 multiplication)
        v
SYSCLK = 72 MHz -> AHB = 72 MHz
                    v
              APB1 = 36 MHz (/2)
                    v
         Timer Clock = 72 MHz (auto-doubled when APB1 prescaler != 1)
                    v
              TIM2 (PSC=7199, ARR=9999)
                    v
         1 Hz overflow -> LED toggle -> 2.0s blink period

Timer Calculation

Parameter	Value	Explanation
TIM2 Clock	72 MHz	APB1 timer auto-double
PSC (Prescaler)	7199	72 MHz / 7200 = 10 kHz tick
ARR (Auto-Reload)	9999	10 kHz / 10000 = 1 Hz
Toggle Period	2.0 s	Toggle on each overflow

Tech Stack

• MCU: STM32F103C8T6 (ARM Cortex-M3 @ 72 MHz)
• Toolchain: STM32CubeIDE
• Framework: HAL Library + bare-metal register access
• Debugger: ST-Link V2 (SWD)
• Language: C

Author

Mothi Charan Naik Desavath -- Embedded Systems Engineer