在STM32F411RCTx里，利用定时器2的通道1(对应引脚PA0)来测量一个外部脉冲信号的周期和频率，外部脉冲信号利用引脚PA6输入。假设被测信号的频率为500kHz，采用测频法测量信号的频率。

首先要配置定时器2的通道1为输入捕获模式，以便测量外部脉冲信号的周期。然后通过计算周期来获得频率。

以下是基本的代码实现：

#include "stm32f4xx.h"

void TIM2_IRQHandler(void)
{
    static uint32_t lastCapture = 0;
    static uint32_t currentCapture = 0;
    static uint32_t period = 0;
    static uint32_t frequency = 0;
    
    if (TIM_GetITStatus(TIM2, TIM_IT_CC1) != RESET)
    {
        TIM_ClearITPendingBit(TIM2, TIM_IT_CC1);
        
        lastCapture = currentCapture;
        currentCapture = TIM_GetCapture1(TIM2);
        period = currentCapture - lastCapture;
        frequency = (uint32_t)(SystemCoreClock / period);
    }
}

void TIM2_Configuration(void)
{
    TIM_ICInitTypeDef TIM_ICInitStructure;
    NVIC_InitTypeDef NVIC_InitStructure;
    
    RCC_APB1PeriphClockCmd(RCC_APB1Periph_TIM2, ENABLE);
    
    TIM_ICInitStructure.TIM_Channel = TIM_Channel_1;
    TIM_ICInitStructure.TIM_ICPolarity = TIM_ICPolarity_Rising;
    TIM_ICInitStructure.TIM_ICSelection = TIM_ICSelection_DirectTI;
    TIM_ICInitStructure.TIM_ICPrescaler = TIM_ICPSC_DIV1;
    TIM_ICInitStructure.TIM_ICFilter = 0x0F;
    TIM_ICInit(TIM2, &TIM_ICInitStructure);
    
    TIM_ITConfig(TIM2, TIM_IT_CC1, ENABLE);
    
    NVIC_InitStructure.NVIC_IRQChannel = TIM2_IRQn;
    NVIC_InitStructure.NVIC_IRQChannelPreemptionPriority = 0;
    NVIC_InitStructure.NVIC_IRQChannelSubPriority = 0;
    NVIC_InitStructure.NVIC_IRQChannelCmd = ENABLE;
    NVIC_Init(&NVIC_InitStructure);
    
    TIM_Cmd(TIM2, ENABLE);
}

void GPIO_Configuration(void)
{
    GPIO_InitTypeDef GPIO_InitStructure;
    
    RCC_AHB1PeriphClockCmd(RCC_AHB1Periph_GPIOA, ENABLE);
    
    GPIO_InitStructure.GPIO_Pin = GPIO_Pin_0;
    GPIO_InitStructure.GPIO_Mode = GPIO_Mode_AF;
    GPIO_InitStructure.GPIO_Speed = GPIO_Speed_100MHz;
    GPIO_InitStructure.GPIO_OType = GPIO_OType_PP;
    GPIO_InitStructure.GPIO_PuPd = GPIO_PuPd_UP;
    GPIO_Init(GPIOA, &GPIO_InitStructure);
    
    GPIO_InitStructure.GPIO_Pin = GPIO_Pin_6;
    GPIO_InitStructure.GPIO_Mode = GPIO_Mode_AF;
    GPIO_InitStructure.GPIO_Speed = GPIO_Speed_100MHz;
    GPIO_InitStructure.GPIO_OType = GPIO_OType_PP;
    GPIO_InitStructure.GPIO_PuPd = GPIO_PuPd_UP;
    GPIO_Init(GPIOA, &GPIO_InitStructure);
    
    GPIO_PinAFConfig(GPIOA, GPIO_PinSource0, GPIO_AF_TIM2);
    GPIO_PinAFConfig(GPIOA, GPIO_PinSource6, GPIO_AF_TIM2);
}

int main(void)
{
    TIM2_Configuration();
    GPIO_Configuration();
    
    while (1);
}

在上面的代码中，我们使用了定时器2的通道1来捕获PA6引脚的输入信号，计算出输入信号的周期和频率。其中，TIM_ICInitStructure结构体用来配置输入捕获模式的参数，NVIC_InitStructure结构体用来配置定时器2中断的优先级和使能定时器2中断。在main函数中，我们只需要调用TIM2_Configuration和GPIO_Configuration函数来配置定时器2和GPIO即可。

需要注意的是，这个代码只是一个基本的框架，还需要根据具体的要求进行修改。比如，如果被测信号的频率过高，需要使用更高分辨率的定时器，并且需要调整输入捕获模式的参数。