STM32 UART Driver — Three Abstraction Levels

Overview

Same project implemented in three different ways : Control an LED via Bluetooth (HC-05) using UART commands.

Objective : understand what happens under the hood at each level.

Hardware

• STM32F103 Blue Pill
• HC-05 Bluetooth Module
• 1 LED on PA5

Wiring

HC-05	STM32
TX	PA10 (USART1 RX)
RX	PA9 (USART1 TX)
GND	GND
VCC	5V

How it works

Send a command from your phone via Bluetooth terminal :

Command	Result	Response
led on	LED turns on	"LED ON"
led off	LED turns off	"LED OFF"
other	Nothing	"Error: unknown command"

Project structure

STM32_UART/
├── HAL/         → using STM32 HAL library
├── LL/          → using Low-Layer library
├── BM/          → bare metal (direct register access)
└── README.md

The three levels explained

Level	Description	What you write
HAL	High abstraction	Just call HAL functions
LL	Low abstraction	Check flags manually, no handle
BM	No abstraction	Direct USART1->SR and USART1->DR

Key difference in code

/* HAL */
HAL_UART_Transmit(&huart1, (uint8_t*)"hello", 5, 100);

/* LL */
while (!LL_USART_IsActiveFlag_TXE(USART1));
LL_USART_TransmitData8(USART1, 'h');

/* Bare Metal */
while (!(USART1->SR & (1U << 7)));
USART1->DR = 'h';

What I learned

• UART protocol — frame, baud rate, sampling
• HAL polling TX and RX
• LL driver written from scratch
• Bare metal register access (SR, DR)
• Command parsing in C
• Handling '\r' and '\n' edge cases
• Buffer overflow protection
• Why sprintf and strcpy are dangerous in firmware

Bluetooth app

Android : Serial Bluetooth Terminal Baud rate : 9600