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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