对我启发较大的是 App_Link_Thread_Entry 线程，这个线程会定期检测网络物理连接。

接下来是 App_Main_Thread_Entry 线程，这个线程主要是完成联网的准备工作。还有就是对 App_Link_Thread_Entry 线程数据的初始化。

1. 
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2. /* Exported macro ------------------------------------------------------------*/
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3. /* USER CODE BEGIN EM */
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4. #define PRINT_IP_ADDRESS(addr)    do { \
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5.              printf("STM32 %s: %lu.%lu.%lu.%lu \n", #addr, \
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6.              (addr >> 24) & 0xff,                          \
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7.              (addr >> 16) & 0xff,                          \
   
8.              (addr >> 8) & 0xff,                           \
   
9.              (addr & 0xff));                               \
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10.              } while(0)

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主要的工作线程是 App_SNTP_Thread_Entry，这个线程完成整个过程。

1. /**
   
2. * @brief  SNTP thread entry.
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3. * @param thread_input: ULONG user argument used by the thread entry
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4. * @retval none
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5. */
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6. /* Define the client thread.  */
   
7. static void App_SNTP_Thread_Entry(ULONG info)
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8. {
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9.   UINT ret;
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10.   RtcHandle.Instance = RTC;
    
11.   ULONG  seconds, fraction;
    
12.   ULONG  events = 0;
    
13.   UINT   server_status;
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14.   NXD_ADDRESS sntp_server_ip;
    
15.   NX_PARAMETER_NOT_USED(info);
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16.   sntp_server_ip.nxd_ip_version = 4;
    
17.   /* Create a DNS client */
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18.   ret = dns_create(&DnsClient);
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19.   if (ret != NX_SUCCESS)
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20.   {
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21.     Error_Handler();
    
22.   }
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23.   /* Look up SNTP Server address. */
    
24.   ret = nx_dns_host_by_name_get(&DnsClient, (UCHAR *)SNTP_SERVER_NAME,
    
25.                                 &sntp_server_ip.nxd_ip_address.v4, NX_APP_DEFAULT_TIMEOUT);
    
26.     /* Check for error. */
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27.   if (ret != NX_SUCCESS)
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28.   {
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29.     Error_Handler();
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30.   }
    
31.    /* Create the SNTP Client */
    
32.   ret =  nx_sntp_client_create(&SntpClient, &NetXDuoEthIpInstance, iface_index, &NxAppPool, NULL, kiss_of_death_handler, NULL);
    
33.   /* Check for error. */
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34.   if (ret != NX_SUCCESS)
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35.   {
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36.     Error_Handler();
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37.   }
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38.   /* Setup time update callback function. */
    
39.    nx_sntp_client_set_time_update_notify(&SntpClient, time_update_callback);
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40.   /* Use the IPv4 service to set up the Client and set the IPv4 SNTP server. */
    
41.   ret = nx_sntp_client_initialize_unicast(&SntpClient, sntp_server_ip.nxd_ip_address.v4);
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42.   if (ret != NX_SUCCESS)
    
43.   {
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44.     Error_Handler();
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45.   }
    
46.   /* Run whichever service the client is configured for. */
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47.   ret = nx_sntp_client_run_unicast(&SntpClient);
    
48.   if (ret != NX_SUCCESS)
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49.   {
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50.     Error_Handler();
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51.   }
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52.   else
    
53.   {
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54.     PRINT_CNX_SUCC();
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55.   }
    
56.   /* Wait for a server update event. */
    
57.   tx_event_flags_get(&SntpFlags, SNTP_UPDATE_EVENT, TX_OR_CLEAR, &events, PERIODIC_CHECK_INTERVAL);
    
58.   if (events == SNTP_UPDATE_EVENT)
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59.   {
    
60.     /* Check for valid SNTP server status. */
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61.     ret = nx_sntp_client_receiving_updates(&SntpClient, &server_status);
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62.     if ((ret != NX_SUCCESS) || (server_status == NX_FALSE))
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63.     {
    
64.       /* We do not have a valid update. */
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65.       Error_Handler();
    
66.     }
    
67.     /* We have a valid update.  Get the SNTP Client time.  */
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68.     ret = nx_sntp_client_get_local_time_extended(&SntpClient, &seconds, &fraction, NX_NULL, 0);
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69.     ret = nx_sntp_client_utility_display_date_time(&SntpClient,buffer,64);
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70.     if (ret != NX_SUCCESS)
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71.     {
    
72.       printf("Internal error with getting local time 0x%x\n", ret);
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73.       Error_Handler();
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74.     }
    
75.     else
    
76.     {
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77.       printf("\nSNTP update :\n");
    
78.       printf("%s\n\n",buffer);
    
79.     }
    
80.   }
    
81.   else
    
82.   {
    
83.     Error_Handler();
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84.   }
    
85.   /* Set Current time from SNTP TO RTC */
    
86.   rtc_time_update(&SntpClient);
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87.   /* We can stop the SNTP service if for example we think the SNTP server has stopped sending updates */
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88.   ret = nx_sntp_client_stop(&SntpClient);
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89.   if (ret != NX_SUCCESS)
    
90.   {
    
91.     Error_Handler();
    
92.   }
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93.   /* When done with the SNTP Client, we delete it */
    
94.   ret = nx_sntp_client_delete(&SntpClient);
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95.   if (ret != NX_SUCCESS)
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96.   {
    
97.     Error_Handler();
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98.   }
    
99.   /* Toggling LED after a success Time update */
    
100.   while(1)
     
101.   {
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102.     /* Display RTC time each second  */
     
103.     display_rtc_time(&RtcHandle);
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104.     HAL_GPIO_TogglePin(LED1_GPIO_Port, LED1_Pin);
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105.     /* Delay for 1s */
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106.     tx_thread_sleep(100);
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107.   }
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108. }

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这个程序目的是完成 SNTP 获取，设置到操作系统参数和 RTC 设置中，就可以不断地获取时间了。

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可以看到时间显示出来了。但这里需要注意一个“坑”，就是发现时间好像和计算机显示的时间对不上，因为这个时间与UTC时间在不同时区，随即找到以下程序，

1. /* EPOCH_TIME_DIFF is equivalent to 70 years in sec
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2. calculated with www.epochconverter.com/date-difference
   
3. This constant is used to delete difference between :
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4. Epoch converter (referenced to 1970) and SNTP (referenced to 1900) */
   
5. time_t timestamp = client_ptr->nx_sntp_current_server_time_message.receive_time.seconds - EPOCH_TIME_DIFF;

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将程序修改成：

1. /* EPOCH_TIME_DIFF is equivalent to 70 years in sec
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2. calculated with www.epochconverter.com/date-difference
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3. This constant is used to delete difference between :
   
4. Epoch converter (referenced to 1970) and SNTP (referenced to 1900) */
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5. time_t timestamp = client_ptr->nx_sntp_current_server_time_message.receive_time.seconds - EPOCH_TIME_DIFF + 28800U;

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这样转换成 UTC 0800 时区时间。

后记：这个程序经长达3小时测试，未发现任何中断现象。反复的ping单片机地址，TTL 值一直很稳定，相比 lwIP 程序改善很多。

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来源：EEWORLD论坛网友 bigbat 版权归原作者所有

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