在STM32上生成C语言的纳秒延迟

Question

我正在使用STM32F2控制器，并通过8位并行接口与ST7036液晶显示器接口。

数据手册说，在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)来完成大约24 of的延迟。它使用STM32的DWT_CYCCNT寄存器，该寄存器专为计数实际时钟节拍而设计，位于地址0xE0001004。

要验证延迟准确性(参见main)，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);
}

Answer 2

我发现的Stm32f2的第一个规范假设时钟频率为120 MHz。这大约是每个时钟周期8 8ns。在连续的写或读/写操作之间需要大约三条单周期指令。在C中，a++;可能会这样做(如果a位于堆栈中)。

Answer 3

您应该查看芯片中可用的FSMC外设。虽然配置可能很复杂，特别是如果您没有插入它设计的内存部分，但您可能会发现并行接口设备很好地映射到其中一种内存接口模式。

这些类型的外部内存控制器必须有一堆可配置的时序选项，以支持各种不同的内存芯片，因此您将能够保证数据手册所需的时序。

能够做到这一点的好处是，您的LCD将看起来像任何旧的内存映射外围设备，抽象出较低级别的接口细节。