[nRF51822] 8、基础实验代码解析大全 · 实验11 - PPI

前一篇分析了前十个基础实验的代码，从这里开始分析后十个~

一、PPI原理：

PPI(Programmable Peripheral Interconnect)，中文翻译为可编程外设互连。

在nRF51822 内部设置了PPI 方式，可以通过任务和事件让不同外设之间进行互连，而不需要CPU 进行参与。

PPI 通过通道让任务和事件连接在一起。PPI 通道由两个端点组成：

• 任务端点：Task End-Point (TEP)。
• 事件端点：Event End-Point (EEP)。

所谓的互联就是将任务端点写入需要连接的任务寄存器地址，事件端点写入需要连接事件寄存器地址，之后，使能该PPI 通道，即实现了任务和事件的互联。

可以通过如下两种方式使能和关闭PPI 通道：

• 1) 通过独立设置CHEN，CHENSET 和CHENCLR 寄存器。
• 2) 通过PPI 通道组的使能和关闭任务。使用这种方式，在触发任务之前，需要先配置好哪些PPI 通道属于哪个组。

二、运行逻辑：

实验中，用到了3 个定时器：Timer 0、Timer 1 和Timer 2。

1) Timer 0 配置为计数器，在主循环中每100ms 被触发一次，并通过串口打印出计数值。
2) Timer 1 每个偶数秒（2、4、6、8……）产生一次比较匹配事件，该事件通过PPI通道0 和Timer 0 的STOP Task 互联，互联后通过该事件触发Timer 0 的STOP Task。
3) Timer 2 每个奇数秒（1、3、5、7……）产生一次比较匹配事件，该事件通过PPI通道1 和Timer 0 的START Task 互联，互联后通过该事件触发Timer 0 的START Task。

实验原理框图如图1 所示：

三、核心代码分析

系统运行后，在循环中Timer 0 计数器的计数值每100ms 增加一次，在偶数秒时，Timer2 产生比较匹配事件，通过PPI 触发Timer 0 的STOP Task，Timer 0 停止计数。此时，尽管主循环中每隔100ms 触发一次Timer 0 计数，但是由于Timer 0 已经停止，所以，计数值不会增加。每个奇数秒，Timer2 产生比较匹配事件，通过PPI 触发Timer 0 的START Task，Timer 0 恢复计数。

main函数部分：

 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 = ;

     // 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();
     }
 }

定时器初始化部分：

 // 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);//比较模式，Timer 1 每个偶数秒（2、4、6、8……）产生一次比较匹配事件，该事件通过PPI通道0 和Timer 0 的STOP Task 互联，互联后通过该事件触发Timer 0 的STOP Task。
 }

 /** @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 每个奇数秒（1、3、5、7……）产生一次比较匹配事件，该事件通过PPI通道1 和Timer 0 的START Task 互联，互联后通过该事件触发Timer 0 的START Task。
 }

PPI连接事件部分：

 /** @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连接事件
14                                           nrf_drv_timer_event_address_get(&timer1, NRF_TIMER_EVENT_COMPARE0),
15                                           nrf_drv_timer_task_address_get(&timer0, NRF_TIMER_TASK_STOP));
     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));
     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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