STM32 F4 max GPIO toggle frequency

Question

I just started using the STM32 F401VC Discovery Evaluation board and I'm trying to get the maximum GPIO toggling frequency without assambler. So my tought process is following:

The code I was planning to use is:

#include "stm32f4xx_gpio.h"
#include "stm32f4xx_rcc.h"
#include "stm32f4xx.h"
#include "stm32f4xx_spi.h"

void initializeGreenLed(void){
    GPIO_InitTypeDef GPIO_InitStructure;

    RCC_AHB1PeriphClockCmd(RCC_AHB1Periph_GPIOD, ENABLE);
    GPIO_InitStructure.GPIO_Pin = GPIO_Pin_12;
    GPIO_InitStructure.GPIO_Mode = GPIO_Mode_OUT;
    GPIO_InitStructure.GPIO_Speed = GPIO_Speed_50MHz; 
    GPIO_InitStructure.GPIO_OType = GPIO_OType_PP;
    GPIO_InitStructure.GPIO_PuPd = GPIO_PuPd_NOPULL;
    GPIO_Init(GPIOD,&GPIO_InitStructure);
}


int main(void)
{
    //RCC_CFGR;
    initializeGreenLed();
    //initialiseSysTick();
    GPIO_SetBits(GPIOD,GPIO_Pin_12);
    while(1)
    {
        GPIO_ToggleBits(GPIOD,GPIO_Pin_12);
    }
}

The max pin toggle frequency I get is around 230 KHz. Since the MCU can be run at 84 MHz, I have a feeling, that it's possible to increase the max toggle frequency beyond 230 KHz.

So I tought that I need to change/modify the clock source for RCC_AHB1Periph_GPIOD in RCC_AHB1PeriphClockCmd(RCC_AHB1Periph_GPIOD, ENABLE);. For that I used the STM32 F4 reference manual. So my plan was to use PLLCLK as SYSCLK. The PLLCLK would be sourced from 8MHz HSE.

Browsing through YouTube, found a nice example:

#include "stm32f4xx_gpio.h"
#include "stm32f4xx_rcc.h"
#include "stm32f4xx.h"
#include "stm32f4xx_flash.h"

void initializeGreenLed(void){
    GPIO_InitTypeDef GPIO_InitStructure;

    RCC_AHB1PeriphClockCmd(RCC_AHB1Periph_GPIOD, ENABLE);
    GPIO_InitStructure.GPIO_Pin = GPIO_Pin_12;
    GPIO_InitStructure.GPIO_Mode = GPIO_Mode_OUT;
    GPIO_InitStructure.GPIO_Speed = GPIO_Speed_100MHz; 
    GPIO_InitStructure.GPIO_OType = GPIO_OType_PP;
    GPIO_InitStructure.GPIO_PuPd = GPIO_PuPd_NOPULL;
    GPIO_Init(GPIOD,&GPIO_InitStructure);
}


int main() {



//---------------------------down is clc stuff---------------------------

    RCC_DeInit();

    ErrorStatus Errsts;
    RCC_HSEConfig(RCC_HSE_ON);
    Errsts = RCC_WaitForHSEStartUp();

    if (Errsts == SUCCESS) {
        //external clc is ok
        RCC_PLLConfig(RCC_PLLSource_HSI, 8, 336, 2, 7);
        RCC_PLLCmd(ENABLE);
        RCC_GetFlagStatus(RCC_FLAG_PLLRDY == RESET);

        FLASH_SetLatency(FLASH_Latency_5);

        RCC_HCLKConfig(RCC_SYSCLK_Div1);
        RCC_PCLK1Config(RCC_HCLK_Div4);
        RCC_PCLK2Config(RCC_HCLK_Div2);

        RCC_SYSCLKConfig(RCC_SYSCLKSource_HSE);

        initializeGreenLed();
        while(1){
            GPIO_SetBits(GPIOD,GPIO_Pin_12);
                }


        }
    else {
        //External clc no ready

        while(1);
    }
}

But it doesn't work, as in it just stays in one state.

So I have two questions:

1. Why isn't my code working?
2. If my abovementioned strategy is incorrect, how to increase the toggle frequency beyond 230 KHz?

Answer 1

In your second code, you have the line GPIO_SetBits(GPIOD,GPIO_Pin_12);, do you actually reset the pin at some point ?

It is faster to set the pin through the register rather than using the library function, as there is plenty of overhead.

Check this tutorial about GPIO.

To Set PC8

GPIOC->ODR |= 0x00000100; //( 0b00000000000000000000000100000000)

To clear pin PC8 independent of all other pins on GPIOC (RMW) you could use:

GPIOC->ODR &= ~(0x00000100); //( 0b00000000000000000000000100000000)

So you can try

while (1)
{
     GPIOC->ODR |=  0x00000100; 
     // maybe some delay needed here
     GPIOC->ODR &=  ~(0x00000100);
}

Of course adjusting to the proper pin / register you need.

Also the pin toggle timing is limited by the hardware.

You might toggle the pin so fast in software side, that the hardware might not have time to follow and thus stay stuck in one state.

It also depends what is connected to that pin, long wire, capacitance, impedance and inductance will slow down the toggle timing as it will be limited by the current the IC is able to draw from that pin, and the internal hardware switching latency.

Hardware latency are defined on the datasheet on page 95, for this example it cannot exceed 50-100Mhz depending the conditions.