在STM32上生成C中的纳秒延迟

Question

我正在使用STM32F2控制器,我通过8位并行接口与ST7036 LCD显示器连接.

数据表说地址保持和建立时间之间应该有20纳秒的延迟.

如何在C中产生20纳秒的延迟？

Answer 1

使用stopwatch_delay(ticks)来完成延迟.它使用STM32的DWT_CYCCNT寄存器,该寄存器专门用于计算实际时钟周期,位于地址0xE0001004.

要验证延迟准确性(请参阅参考资料main),您可以调用STOPWATCH_START,运行stopwatch_delay(ticks),然后调用STOPWATCH_STOP并验证CalcNanosecondsFromStopwatch(m_nStart, m_nStop).ticks根据需要调整.

uint32_t m_nStart;               //DEBUG Stopwatch start cycle counter value
uint32_t m_nStop;                //DEBUG Stopwatch stop cycle counter value

#define DEMCR_TRCENA    0x01000000

/* Core Debug registers */
#define DEMCR           (*((volatile uint32_t *)0xE000EDFC))
#define DWT_CTRL        (*(volatile uint32_t *)0xe0001000)
#define CYCCNTENA       (1<<0)
#define DWT_CYCCNT      ((volatile uint32_t *)0xE0001004)
#define CPU_CYCLES      *DWT_CYCCNT

#define STOPWATCH_START { m_nStart = *((volatile unsigned int *)0xE0001004);}
#define STOPWATCH_STOP  { m_nStop = *((volatile unsigned int *)0xE0001004);}


static inline void stopwatch_reset(void)
{
    /* Enable DWT */
    DEMCR |= DEMCR_TRCENA; 
    *DWT_CYCCNT = 0;             
    /* Enable CPU cycle counter */
    DWT_CTRL |= CYCCNTENA;
}

static inline uint32_t stopwatch_getticks()
{
    return CPU_CYCLES;
}

static inline void stopwatch_delay(uint32_t ticks)
{
    uint32_t end_ticks = ticks + stopwatch_getticks();
    while(1)
    {
            if (stopwatch_getticks() >= end_ticks)
                    break;
    }
}

uint32_t CalcNanosecondsFromStopwatch(uint32_t nStart, uint32_t nStop)
{
    uint32_t nDiffTicks;
    uint32_t nClkTicksPerMicrosec;

    nDiffTicks = nStop - nStart;
    nDiffTicks *= 1000;                               // Scale diff by 1000.
    nClkTicksPerMicrosec = SystemCoreClock / 1000000; // Convert (clkTicks/sec) to (clkTicks/microsec), SystemCoreClock = 168000000

    return nDiffTicks / nClkTicksPerMicrosec;         // nanosec = (ticks * 1000) / (clkTicks/microsec)
} 

void main(void)
{
    int timeDiff = 0;
    stopwatch_reset();

    STOPWATCH_START;
    run_my_function();
    STOPWATCH_STOP;

    timeDiff = CalcNanosecondsFromStopwatch(m_nStart, m_nStop);
    printf("My function took %d nanoseconds\n", timeDiff);
}

Answer 2

我发现Stm32f2的第一个规范假定时钟频率为120 MHz.每个时钟周期约为8ns.在连续写或读/写操作之间需要大约三个单周期指令.在C中,a++;可能会这样做(如果a位于堆栈中).