Electronic – What could be the reasons for a stepper motor stuttering with an A4988 driver

I am using an A4988 Stepper Motor Driver, which is controlled with an STM32F767ZI on a Nucleo 144 board. The stepper motor takes 12 V with a maximum of 350 mA.

When powered, the motor simply flickers and stutters, but moves at a negligible speed.

Here is a circuit diagram of the setup, with voltage readings taking from a multimeter:

The potentiometer has been set correctly.

The same results occur even with two other A4988 drivers.

For reference, here is the code (though I don't believe this is a software issue):

main.c

#include "./headers/stm32f767xx.h"
#include <stdint.h>

int main(void)
{
    initMotor(0); // initialise the motor
    initLed(7); // initialise the led
    unsigned long a = 0;
    while (1)
    {
        if (a == 50000)
        {
            toggleLed(7); // this LED flashes a little quicker than twice per second 
            stepMotor(0); // output a pulse to the driver to step the motor, attached to PA2
            a = 0;
        }
        a++;
    }
}

./drivers/led.c

#include "../headers/stm32f767xx.h"

void initLed(int pin)
{
    RCC->AHB1ENR |= RCC_AHB1ENR_GPIOBEN; // enable the GPIOB clock
    GPIOB->MODER |= (0x1 << (pin * 2)); // set to output
    GPIOB->OTYPER = 0x00; // push-pull mode
    GPIOB->ODR = 0x00; // set output register to 0 across all pins
}

void toggleLed(int pin)
{
    GPIOB->ODR ^= (0x1 << pin); // toggle the pin
}

./drivers/motor.c

#include "../headers/stm32f767xx.h"

void initMotor(int step_pin)
{
    RCC->AHB1ENR |= RCC_AHB1ENR_GPIOAEN; // enable the GPIOA clock
    GPIOA->MODER |= (0x1 << (step_pin * 2)); // set to output
    GPIOA->OTYPER = 0x00; // push-pull mode
    GPIOA->PUPDR |= (0x2 << (step_pin * 2)); // pull down the pin specified
    GPIOA->ODR = 0x00; // set output register to 0 across all pins
}

void stepMotor(int step_pin)
{
    GPIOA->ODR |= (1 << step_pin); // output to the pin specified
    GPIOA->ODR &= ~(1 << step_pin); // reset the output back to 0
}

With this code, I was expecting the motor to take steady and even steps, rather than the backwards-and-forwards stuttering it does.

I would appreciate any suggestions for the direction I should take from here, or any suggestions as to what the issue could be.

After some discussion, I updated my code a little to slow it down, here it is:

(only included the updated files)

main.c

#include "./headers/stm32f767xx.h"
#include <stdint.h>

int main(void)
{
    initMotor(0);
    initLed(0);
    uint32_t a = 0;
    while (1)
    {
        if (a >= 150000)
        {
            toggleLed(0);
            stepMotor(0);
            a = 0;
        }
        a++; 
    }
}

./drivers/motor.c

#include "../headers/stm32f767xx.h"

void initMotor(int step_pin)
{
    RCC->AHB1ENR |= RCC_AHB1ENR_GPIOGEN;
    GPIOG->MODER &= ~(0b11 << (step_pin * 2));
    GPIOG->MODER |= (0b01 << (step_pin * 2));
    GPIOG->OTYPER &= ~(0b1 << step_pin);
    GPIOG->PUPDR |= (0b10 << (step_pin * 2));
    GPIOG->ODR &= ~(0b1 << step_pin);
}

void stepMotor(int step_pin)
{
    GPIOG->ODR ^= (0b1 << step_pin);
}