[nRF51822] 8、基礎實驗代碼解析大全 · 實驗11 - PPI

 

前一篇分析了前十個基礎實驗的代碼，從這里開始分析后十個~

 

一、PPI原理：

PPI(Programmable Peripheral Interconnect)，中文翻譯為可編程外設互連。

在nRF51822 內部設置了PPI 方式，可以通過任務和事件讓不同外設之間進行互連，而不需要CPU 進行參與。

PPI 通過通道讓任務和事件連接在一起。PPI 通道由兩個端點組成：

• 任務端點：Task End-Point (TEP)。
• 事件端點：Event End-Point (EEP)。

所謂的互聯就是將任務端點寫入需要連接的任務寄存器地址，事件端點寫入需要連接事件寄存器地址，之后，使能該PPI 通道，即實現了任務和事件的互聯。

可以通過如下兩種方式使能和關閉PPI 通道：

• 1) 通過獨立設置CHEN，CHENSET 和CHENCLR 寄存器。
• 2) 通過PPI 通道組的使能和關閉任務。使用這種方式，在觸發任務之前，需要先配置好哪些PPI 通道屬於哪個組。

 

二、運行邏輯：

實驗中，用到了3 個定時器：Timer 0、Timer 1 和Timer 2。

1) Timer 0 配置為計數器，在主循環中每100ms 被觸發一次，並通過串口打印出計數值。
2) Timer 1 每個偶數秒（2、4、6、8……）產生一次比較匹配事件，該事件通過PPI通道0 和Timer 0 的STOP Task 互聯，互聯后通過該事件觸發Timer 0 的STOP Task。
3) Timer 2 每個奇數秒（1、3、5、7……）產生一次比較匹配事件，該事件通過PPI通道1 和Timer 0 的START Task 互聯，互聯后通過該事件觸發Timer 0 的START Task。

實驗原理框圖如圖1 所示：

 

三、核心代碼分析

系統運行后，在循環中Timer 0 計數器的計數值每100ms 增加一次，在偶數秒時，Timer2 產生比較匹配事件，通過PPI 觸發Timer 0 的STOP Task，Timer 0 停止計數。此時，盡管主循環中每隔100ms 觸發一次Timer 0 計數，但是由於Timer 0 已經停止，所以，計數值不會增加。每個奇數秒，Timer2 產生比較匹配事件，通過PPI 觸發Timer 0 的START Task，Timer 0 恢復計數。

main函數部分：

int main(void)
{
    timer0_init(); // Timer used to blink the LEDs.
    timer1_init(); // Timer to generate events on even number of seconds.
    timer2_init(); // Timer to generate events on odd number of seconds.
    ppi_init();    // PPI to redirect the event to timer start/stop tasks.

       串口初始化（略）

    // Enabling constant latency as indicated by PAN 11 "HFCLK: Base current with HFCLK 
    // running is too high" found at Product Anomaly document found at
    // https://www.nordicsemi.com/eng/Products/Bluetooth-R-low-energy/nRF51822/#Downloads
    //
    // @note This example does not go to low power mode therefore constant latency is not needed.
    //       However this setting will ensure correct behaviour when routing TIMER events through 
    //       PPI (shown in this example) and low power mode simultaneously.
    NRF_POWER->TASKS_CONSTLAT = 1;
    
    // Start clock.
    nrf_drv_timer_enable(&timer0);
    nrf_drv_timer_enable(&timer1);
    nrf_drv_timer_enable(&timer2);

    // Loop and increment the timer count value and capture value into LEDs. @note counter is only incremented between TASK_START and TASK_STOP.
    while (true)
    {

        printf("Current cout: %d\r\n", (int)nrf_drv_timer_capture(&timer0,NRF_TIMER_CC_CHANNEL0));

        /* increment the counter */
        nrf_drv_timer_increment(&timer0);

        nrf_delay_ms(100);
    }
}

 

定時器初始化部分：

// Timer even handler. Not used since timer is used only for PPI.
void timer_event_handler(nrf_timer_event_t event_type, void * p_context){}

/** @brief Function for Timer 0 initialization, which will be started and stopped by timer1 and timer2 using PPI.
*/
static void timer0_init(void)
{
    ret_code_t err_code = nrf_drv_timer_init(&timer0, NULL, timer_event_handler);
    APP_ERROR_CHECK(err_code);
}

/** @brief Function for Timer 1 initialization.
 *  @details Initializes Timer 1 peripheral, creates event and interrupt every 2 seconds,
 *           by configuring CC[0] to timer overflow value, we create events at even number of seconds
 *           for example, events are created at 2,4,6 ... seconds. This event can be used to stop Timer 0
 *           with Timer1->Event_Compare[0] triggering Timer 0 TASK_STOP through PPI.
*/
static void timer1_init(void)
{
    // Configure Timer 1 to overflow every 2 seconds. Check TIMER1 configuration for details
    // The overflow occurs every 0xFFFF/(SysClk/2^PRESCALER).
    // = 65535/31250 = 2.097 sec
    ret_code_t err_code = nrf_drv_timer_init(&timer1, NULL, timer_event_handler);
    APP_ERROR_CHECK(err_code);

    nrf_drv_timer_extended_compare(&timer1, NRF_TIMER_CC_CHANNEL0, 0xFFFFUL, NRF_TIMER_SHORT_COMPARE0_CLEAR_MASK, false);//比較模式，Timer 1 每個偶數秒（2、4、6、8……）產生一次比較匹配事件，該事件通過PPI通道0 和Timer 0 的STOP Task 互聯，互聯后通過該事件觸發Timer 0 的STOP Task。 27 }

/** @brief Function for Timer 2 initialization.
 *  @details Initializes Timer 2 peripheral, creates event and interrupt every 2 seconds
 *           by configuring CC[0] to half of timer overflow value. Events are created at odd number of seconds.
 *           For example, events are created at 1,3,5,... seconds. This event can be used to start Timer 0
 *           with Timer2->Event_Compare[0] triggering Timer 0 TASK_START through PPI.
*/
static void timer2_init(void)
{
    // Generate interrupt/event when half of time before the timer overflows has past, that is at 1,3,5,7... seconds from start.
    // Check TIMER1 configuration for details
    // now the overflow occurs every 0xFFFF/(SysClk/2^PRESCALER)
    // = 65535/31250 = 2.097 sec */
    ret_code_t err_code = nrf_drv_timer_init(&timer2, NULL, timer_event_handler);
    APP_ERROR_CHECK(err_code);

    nrf_drv_timer_extended_compare(&timer2, NRF_TIMER_CC_CHANNEL0, 0x7FFFUL, NRF_TIMER_SHORT_COMPARE0_CLEAR_MASK, false);//Timer 2 每個奇數秒（1、3、5、7……）產生一次比較匹配事件，該事件通過PPI通道1 和Timer 0 的START Task 互聯，互聯后通過該事件觸發Timer 0 的START Task。 45 }

 

PPI連接事件部分：

/** @brief Function for initializing the PPI peripheral.
*/
static void ppi_init(void)
{
    uint32_t err_code = NRF_SUCCESS;

    err_code = nrf_drv_ppi_init();
    APP_ERROR_CHECK(err_code);

    // Configure 1st available PPI channel to stop TIMER0 counter on TIMER1 COMPARE[0] match, which is every even number of seconds.
    err_code = nrf_drv_ppi_channel_alloc(&ppi_channel1);
    APP_ERROR_CHECK(err_code);
13 err_code = nrf_drv_ppi_channel_assign(ppi_channel1,//PPI連接事件 14 nrf_drv_timer_event_address_get(&timer1, NRF_TIMER_EVENT_COMPARE0), 15 nrf_drv_timer_task_address_get(&timer0, NRF_TIMER_TASK_STOP)); 16     APP_ERROR_CHECK(err_code);

    // Configure 2nd available PPI channel to start timer0 counter at TIMER2 COMPARE[0] match, which is every odd number of seconds.
    err_code = nrf_drv_ppi_channel_alloc(&ppi_channel2);
    APP_ERROR_CHECK(err_code);
21 err_code = nrf_drv_ppi_channel_assign(ppi_channel2, 22 nrf_drv_timer_event_address_get(&timer2, NRF_TIMER_EVENT_COMPARE0), 23 nrf_drv_timer_task_address_get(&timer0, NRF_TIMER_TASK_START)); 24     APP_ERROR_CHECK(err_code);

    // Enable both configured PPI channels
    err_code = nrf_drv_ppi_channel_enable(ppi_channel1);
    APP_ERROR_CHECK(err_code);
    err_code = nrf_drv_ppi_channel_enable(ppi_channel2);
    APP_ERROR_CHECK(err_code);
}

 

 

 

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