15. GPIO Input · Pull-up/Pull-down

What you'll learn

• 1Set GPIO to input mode and read pin state via IDR
• 2Explain floating-input danger and the need for pull-up/pull-down
• 3Select the internal pull-up/pull-down via the ODR bit on F1
• 4Implement active-low/high button logic
• 5Turn the LED on/off from button input

Introduction

Buttons·switches are embedded basics. A floating input drifts between 0 and 1 with noise, so the button may read pressed when it isn't. Pull-up (idle 1) or pull-down (idle 0) fixes the default level.

Key concepts

1) Input-mode 4 bits and pull selection

GPIOA->CRL |= (0x8u << 0);   /* PA0 = pull-up/down input */
GPIOA->ODR |= (1u << 0);     /* ODR=1 → pull-up, ODR=0 → pull-down */

2) Reading (IDR) and active logic

if ((GPIOA->IDR & (1u << 0)) == 0u) { /* PA0 = 0 (pressed) */ }
/* pull-up + button to GND → idle=1, pressed=0 (active-low) */

IDR (Input Data Register) is read-only. Output is via ODR/BSRR/BRR.

Core example

#include "stm32f10x.h"
#define BTN 0u   /* PA0 */
#define LED 13u  /* PC13 */
int main(void) {
    RCC->APB2ENR |= RCC_APB2ENR_IOPAEN | RCC_APB2ENR_IOPCEN;
    GPIOA->CRL &= ~(0xFu << (BTN*4u));
    GPIOA->CRL |=  (0x8u << (BTN*4u));   /* input pull-up/down */
    GPIOA->ODR |=  (1u << BTN);          /* pull-up */
    GPIOC->CRH &= ~(0xFu << ((LED-8u)*4u));
    GPIOC->CRH |=  (0x2u << ((LED-8u)*4u));
    while (1) {
        if ((GPIOA->IDR & (1u << BTN)) == 0u) GPIOC->BRR  = (1u << LED); /* pressed ON */
        else                                  GPIOC->BSRR = (1u << LED); /* OFF */
    }
}

Logic check: IDR=0x0001→released(OFF), IDR=0x0000→PRESSED(ON). In the simulator, mimic with `PORTA &= ~0x0001`(pressed)/`PORTA |= 0x0001`(released).

Common mistakes

Q. The value wobbles when nothing is pressed.

A. Pull-up/down isn't enabled — it's floating. Use CNF=10 (0x8) and pick pull-up(1)/pull-down(0) via ODR.

Q. I set CNF=10 but no pull-up engages.

A. On F1 the ODR bit selects pull-up/down. In input mode ODR=1 is pull-up, 0 is pull-down. Don't skip the ODR setting.

Q. Does writing IDR change the pin?

A. IDR is read-only. Output via ODR/BSRR/BRR. IDR only reads the current input state.

Summary

• Input is MODE=00 with CNF picking the type (0x8 = pull-up/down input)
• Enable pull-up/down to avoid floating; F1 selects it via ODR
• Read pin state via read-only IDR
• Pull-up + button to GND = pressed is 0 (active-low)
• Bounce handling is in debouncing (lesson 21)

Exercises

1. Switch PA0 to pull-down (ODR=0) with the button to VCC and write the logic
2. Compare mirroring (LED on while held) vs edge-detect toggle
3. Explain the precedence trap in masking ((IDR & m) == 0)