定时器触发事件一般为程序内部触发，若外部触发事件，可以用系统消息触发，以按键触发为例。

在初始化时候，InitBoard()中，注册了按键回调函数

HalKeyConfig( OnboardKeyIntEnable, OnBoard_KeyCallback);

该函数中注册按键回调函数到pHalKeyProcessFunction，并启动定时器事件，ID为HAL层ID，事件为按键事件，调用HalKeyPoll(),启动按键轮询，等待按键触发。

halProcessKeyInterrupt()通过HAL_ISR_FUNCTION()注册到OSAL的中断机制中，此部分代码没有公开，可能是底层按键触发中断，然后调用halProcessKeyInterrupt()上报事件，HalKeyPoll()中调用初始化时候注册的按键回调函数(pHalKeyProcessFunction) ()进行处理;

在回调函数中，调用OnBoard_SendKeys()函数发送系统消息，消息结构如下

typedef struct

{

  void   *next;

  uint16 len;

  uint8  dest_id;

} osal_msg_hdr_t;

typedef struct

{

  uint8  event;

  uint8  status;

} osal_event_hdr_t;

typedef struct

{

  osal_event_hdr_t hdr;

  uint8             state;  

  uint8             keys; 

} keyChange_t;

回调函数OnBoard_SendKeys()原型如下：

 1 /*********************************************************************
 2 
 3  * @fn      OnBoard_SendKeys
 4 
 5  *
 6 
 7  * @brief   Send "Key Pressed" message to application.
 8 
 9  *
10 
11  * @param   keys  - keys that were pressed
12 
13  *          state - shifted
14 
15  *
16 
17  * @return  status
18 
19  *********************************************************************/
20 
21 uint8 OnBoard_SendKeys( uint8 keys, uint8 state )
22 
23 {
24 
25   keyChange_t *msgPtr;
26 
27   if ( registeredKeysTaskID != NO_TASK_ID )
28 
29   {
30 
31     // Send the address to the task
32 
33     msgPtr = (keyChange_t *)osal_msg_allocate( sizeof(keyChange_t) );
34 
35     if ( msgPtr )
36 
37     {
38 
39       msgPtr->hdr.event = KEY_CHANGE;
40 
41       msgPtr->state = state;
42 
43       msgPtr->keys = keys;
44 
45 
46       osal_msg_send( registeredKeysTaskID, (uint8 *)msgPtr );
47 
48     }
49 
50     return ( SUCCESS );
51 
52   }
53 
54   else
55 
56     return ( FAILURE );
57 
58 }

在OnBoard_SendKeys()函数中，调用osal_msg_allocate()函数申请一块内存,

 1 /*********************************************************************
 2 
 3  * @fn      osal_msg_allocate
 4 
 5  *
 6 
 7  * @brief
 8 
 9  *
10 
11  *    This function is called by a task to allocate a message buffer
12 
13  *    into which the task will encode the particular message it wishes
14 
15  *    to send.  This common buffer scheme is used to strictly limit the
16 
17  *    creation of message buffers within the system due to RAM size
18 
19  *    limitations on the microprocessor.   Note that all message buffers
20 
21  *    are a fixed size (at least initially).  The parameter len is kept
22 
23  *    in case a message pool with varying fixed message sizes is later
24 
25  *    created (for example, a pool of message buffers of size LARGE,
26 
27  *    MEDIUM and SMALL could be maintained and allocated based on request
28 
29  *    from the tasks).
30 
31  *
32 
33  *
34 
35  * @param   uint8 len  - wanted buffer length
36 
37  *
38 
39  *
40 
41  * @return  pointer to allocated buffer or NULL if allocation failed.
42 
43  */
44 
45 uint8 * osal_msg_allocate( uint16 len )
46 
47 {
48 
49   osal_msg_hdr_t *hdr;
50 
51  
52   if ( len == 0 )
53 
54     return ( NULL );
55 
56  
57 
58   hdr = (osal_msg_hdr_t *) osal_mem_alloc( (short)(len + sizeof( osal_msg_hdr_t )) );
59 
60   if ( hdr )
61 
62   {
63 
64     hdr->next = NULL;
65 
66     hdr->len = len;
67 
68     hdr->dest_id = TASK_NO_TASK;
69 
70     return ( (uint8 *) (hdr + 1) );
71 
72   }
73 
74   else
75 
76     return ( NULL );
77 
78 }

其内存布局如下：

    申请成功后，返回值为keyChange_t部分的首地址，因此在随后的消息检查，填充等操作中会有结构体指针减一的操作。

消息创建完成后，调用osal_msg_send()将消息发送出去，该函数调用osal_msg_enqueue_push()，将消息发送至OSAL消息链表，并调用osal_set_event( destination_task, SYS_EVENT_MSG );向目标任务发送一个系统消息事件，在主循环中调用目标任务的回调函数，进入系统消息处理分支接收并解析处理消息。