Getting UART to Print on STM32 Without HAL

What I was trying to do

Send “hello” over UART2 on an STM32F4 Discovery, no HAL, registers only.

What went wrong

Nothing on the serial monitor. Code compiled and ran fine.

How I debugged it

Logic analyzer on PA2 — the line was flat. So the pin was never actually driven by the UART peripheral.

The bug

I enabled the USART2 clock but forgot the GPIOA clock, and never set the alternate function. The pin stayed a plain input.

RCC->AHB1ENR |= RCC_AHB1ENR_GPIOAEN;   // this line was missing
GPIOA->MODER |= (2 << (2 * 2));        // PA2 -> alternate function
GPIOA->AFR[0] |= (7 << (4 * 2));       // AF7 = USART2

What I learned

On STM32, a peripheral needs two clocks enabled: the peripheral’s own and the GPIO port’s. Reading the datasheet’s clock tree earlier would have saved me two hours.