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[操作系统][nRF51822] 8、基础实验代码解析大全 实验11


 

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:

 1 int main(void) 2 { 3   timer0_init(); // Timer used to blink the LEDs. 4   timer1_init(); // Timer to generate events on even number of seconds. 5   timer2_init(); // Timer to generate events on odd number of seconds. 6   ppi_init();  // PPI to redirect the event to timer start/stop tasks. 7     串口初始化(略)28 29   // Enabling constant latency as indicated by PAN 11 "HFCLK: Base current with HFCLK 30   // running is too high" found at Product Anomaly document found at31   // https://www.nordicsemi.com/eng/Products/Bluetooth-R-low-energy/nRF51822/#Downloads32   //33   // @note This example does not go to low power mode therefore constant latency is not needed.34   //    However this setting will ensure correct behaviour when routing TIMER events through 35   //    PPI (shown in this example) and low power mode simultaneously.36   NRF_POWER->TASKS_CONSTLAT = 1;37   38   // Start clock.39   nrf_drv_timer_enable(&timer0);40   nrf_drv_timer_enable(&timer1);41   nrf_drv_timer_enable(&timer2);42 43   // Loop and increment the timer count value and capture value into LEDs. @note counter is only incremented between TASK_START and TASK_STOP.44   while (true)45   {46 47     printf("Current cout: %d\r\n", (int)nrf_drv_timer_capture(&timer0,NRF_TIMER_CC_CHANNEL0));48 49     /* increment the counter */50     nrf_drv_timer_increment(&timer0);51 52     nrf_delay_ms(100);53   }54 }

 

定时器初始化部分:

 1 // Timer even handler. Not used since timer is used only for PPI. 2 void timer_event_handler(nrf_timer_event_t event_type, void * p_context){} 3  4 /** @brief Function for Timer 0 initialization, which will be started and stopped by timer1 and timer2 using PPI. 5 */ 6 static void timer0_init(void) 7 { 8   ret_code_t err_code = nrf_drv_timer_init(&timer0, NULL, timer_event_handler); 9   APP_ERROR_CHECK(err_code);10 }11 12 /** @brief Function for Timer 1 initialization.13  * @details Initializes Timer 1 peripheral, creates event and interrupt every 2 seconds,14  *      by configuring CC[0] to timer overflow value, we create events at even number of seconds15  *      for example, events are created at 2,4,6 ... seconds. This event can be used to stop Timer 016  *      with Timer1->Event_Compare[0] triggering Timer 0 TASK_STOP through PPI.17 */18 static void timer1_init(void)19 {20   // Configure Timer 1 to overflow every 2 seconds. Check TIMER1 configuration for details21   // The overflow occurs every 0xFFFF/(SysClk/2^PRESCALER).22   // = 65535/31250 = 2.097 sec23   ret_code_t err_code = nrf_drv_timer_init(&timer1, NULL, timer_event_handler);24   APP_ERROR_CHECK(err_code);25 26   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。27 }28 29 /** @brief Function for Timer 2 initialization.30  * @details Initializes Timer 2 peripheral, creates event and interrupt every 2 seconds31  *      by configuring CC[0] to half of timer overflow value. Events are created at odd number of seconds.32  *      For example, events are created at 1,3,5,... seconds. This event can be used to start Timer 033  *      with Timer2->Event_Compare[0] triggering Timer 0 TASK_START through PPI.34 */35 static void timer2_init(void)36 {37   // Generate interrupt/event when half of time before the timer overflows has past, that is at 1,3,5,7... seconds from start.38   // Check TIMER1 configuration for details39   // now the overflow occurs every 0xFFFF/(SysClk/2^PRESCALER)40   // = 65535/31250 = 2.097 sec */41   ret_code_t err_code = nrf_drv_timer_init(&timer2, NULL, timer_event_handler);42   APP_ERROR_CHECK(err_code);43 44   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。45 }

 

PPI连接事件部分:

 1 /** @brief Function for initializing the PPI peripheral. 2 */ 3 static void ppi_init(void) 4 { 5   uint32_t err_code = NRF_SUCCESS; 6  7   err_code = nrf_drv_ppi_init(); 8   APP_ERROR_CHECK(err_code); 9 10   // Configure 1st available PPI channel to stop TIMER0 counter on TIMER1 COMPARE[0] match, which is every even number of seconds.11   err_code = nrf_drv_ppi_channel_alloc(&ppi_channel1);12   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));16   APP_ERROR_CHECK(err_code);17 18   // Configure 2nd available PPI channel to start timer0 counter at TIMER2 COMPARE[0] match, which is every odd number of seconds.19   err_code = nrf_drv_ppi_channel_alloc(&ppi_channel2);20   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);25 26   // Enable both configured PPI channels27   err_code = nrf_drv_ppi_channel_enable(ppi_channel1);28   APP_ERROR_CHECK(err_code);29   err_code = nrf_drv_ppi_channel_enable(ppi_channel2);30   APP_ERROR_CHECK(err_code);31 }

 

 

 

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