STM32F411与NRF24L01+ SPI通信同步异常问题排查

我要实现NRF24L01+与STM32F411（STM32F411-Disco开发板）之间的通信。此前做过SPI主从架构通信，当时ChipSelect引脚始终保持低电平，但NRF24通信要求每次发送数据都切换CS引脚。

目前SPI无法正常工作，推测是SPI时钟启动与CS引脚存在不同步问题。若在CS引脚拉高前添加少量延迟，通信即可正常进行，逻辑分析仪截图可佐证该现象：

• 截图1：通信正常，CS拉高前存在延迟
• 截图2：延迟时间更长的情况
• 截图3：无延迟时的异常情况

#include <stdint.h>
#include "stm32f4xx.h"


    void SPI1_INIT(void)
    {
        RCC->APB2ENR |= RCC_APB2ENR_SPI1EN; // Enable SPI1 Clock

        SPI1->CR1 |= SPI_CR1_BR_Msk;
        SPI1->CR1 |= SPI_CR1_MSTR;          //Bit 2 MSTR: Master selection
        SPI1->CR1 |= SPI_CR1_SSM | SPI_CR1_SSI;         
        SPI1->CR1 |= SPI_CR1_SPE;           //Bit 6 SPE: SPI enable
    }

    void SPI1_GPIO_INIT(void) // PA4 - SPI1_NSS | PA5-SPI1_SCK | PA6-SPI1_MISO| PA7-SPI1_MOSI
    {
        RCC->AHB1ENR |= RCC_AHB1ENR_GPIOAEN; //Enable GPIOA Clock

        GPIOA->MODER |= GPIO_MODER_MODER4_0;  //PA4 GPIO OUT mde
        GPIOA->MODER |= GPIO_MODER_MODER5_1;  //Enable Alternate function mode for GPIOA PA5
        GPIOA->MODER |= GPIO_MODER_MODER6_1;  //Enable Alternate function mode for GPIOA PA6
        GPIOA->MODER |= GPIO_MODER_MODER7_1;  //Enable Alternate function mode for GPIOA PA7

        GPIOA->AFR[0] |= 0x5UL<<GPIO_AFRL_AFSEL5_Pos;  //GPIO alternate function set 0101 AF5 SPI PA5
        GPIOA->AFR[0] |= 0x5UL<<GPIO_AFRL_AFSEL6_Pos;  //GPIO alternate function set 0101 AF5 SPI PA6
        GPIOA->AFR[0] |= 0x5UL<<GPIO_AFRL_AFSEL7_Pos;  //GPIO alternate function set 0101 AF5 SPI PA7
    }

    void spi_send(uint8_t data)
    {
        while(!(SPI1->SR & SPI_SR_TXE)); // Wait for TX buffer is emptly
        SPI1->DR = data;                  // Send data to TX buffer
    }

    uint8_t spi_receive(void)
    {
        while(!(SPI1->SR & SPI_SR_RXNE));
        return SPI1->DR;
    }


    int main(void)
    {

        SPI1_GPIO_INIT();
        SPI1_INIT();

        for(;;){

            GPIOA->BSRR = GPIO_BSRR_BR4;

            while(!(SPI1->SR & SPI_SR_TXE)); // Wait for TX buffer empty
            SPI1->DR = 0xFF;                  // Send data to TX buffer

            while(!(SPI1->SR & SPI_SR_TXE));
            while ((SPI1->SR & SPI_SR_BSY));

            //for(uint32_t i = 0; i<10; i++);

            GPIOA->BSRR = GPIO_BSRR_BS4;
        }
    }

更新 #

我通过调整SPI时钟频率解决了问题。原本时钟为62.5Khz，提升至4MHz后通信恢复正常，暂不清楚低时钟速率下故障的原因。

内容的提问来源于stack exchange，提问作者Ulfbeth