c - 在STM32上用C产生纳秒延迟

Question

我正在使用 STM32F2 Controller 和我正在连接一个 ST7036通过8位并行接口(interface)的LCD显示。

数据表说地址保持和设置时间之间应该有 20 纳秒的延迟。

如何在 C 中生成 20 纳秒的延迟？

Answer 1

使用下面的 stopwatch_delay(4) 来实现大约 24ns 的延迟。它使用 STM32 的 DWT_CYCCNT 寄存器，该寄存器专门用于计算实际时钟节拍，位于地址 0xE0001004。

要验证延迟精度(参见main)，您可以调用STOPWATCH_START，运行stopwatch_delay(ticks)，然后调用 STOPWATCH_STOP 并使用 CalcNanosecondsFromStopwatch(m_nStart, m_nStop) 进行验证。根据需要调整 ticks。

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 CLK_SPEED         168000000 // EXAMPLE for CortexM4, EDIT as needed

#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 nSystemCoreTicksPerMicrosec;

    // Convert (clk speed per sec) to (clk speed per microsec)
    nSystemCoreTicksPerMicrosec = CLK_SPEED / 1000000;

    // Elapsed ticks
    nDiffTicks = nStop - nStart;

    // Elapsed nanosec = 1000 * (ticks-elapsed / clock-ticks in a microsec)
    return 1000 * nDiffTicks / nSystemCoreTicksPerMicrosec;
} 

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

    // =============================================
    // Example: use a delay, and measure how long it took
    STOPWATCH_START;
    stopwatch_delay(168000); // 168k ticks is 1ms for 168MHz core
    STOPWATCH_STOP;

    timeDiff = CalcNanosecondsFromStopwatch(m_nStart, m_nStop);
    printf("My delay measured to be %d nanoseconds\n", timeDiff);

    // =============================================
    // Example: measure function duration in nanosec
    STOPWATCH_START;
    // run_my_function() => do something here
    STOPWATCH_STOP;

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