一、串口低功耗模式介绍
1.1 介绍
stm32可以通过串口来从stop模式唤醒，如果基于hal库，只要进行一些配置就能使用，这种通过串口唤醒的低功耗模式可以用在带电池的设备上，在需要设备工作的时候才将其唤醒处理事务，处理完后进入stop模式降低功耗。
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1.2 三种模式和相关的寄存器
1.2.1 寄存器
" f" h/ X' A# |) x4 o1.2.1.1 CR3[21:20] WUS[1:0]
功能：选择从低功耗模式中断唤醒的标志
配置说明：
! Y7 \4 Q; p  V# s' J2 j$ e00：地址匹配模式
01：保留
10：检测起始位
4 F6 w# w# D- O# l: V11：RXNE/RXFNE中断

1.2.1.2 CR3[22] WUFIE
功能：使能从低功耗模式唤醒
配置说明：
0：关闭中断
; q% T3 _& L$ c5 A1：使能WUF中断，即当ISR的WUF为1时产生一个中断

1.2.1.3 CR1[1] UESM
功能：使能串口的低功耗模式
配置说明：
% `: _, N. C  L: t0：串口不能唤醒低功耗模式
. L  p1 P, R/ u: b, C; e2 H' V9 Z1：串口能唤醒低功耗模式
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1.2.1.4 PWR_CR1[2:0] LPMS[2:0]
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- e/ \  F+ Y; J# k( U  B配置说明：
1 J* H2 E) S* f# }; n' H000：stop 0模式
001：stop1模式
) S' N/ u$ O- x1 |* v6 e8 U010：保留
% N! e9 ?# p0 t+ W4 c/ {: c011：stanby模式
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1.2.1.5 SCB_SCR[2] SLEEPDEEP
& R' q! {  B2 j功能：决定处理器在低功耗模式时选择睡眠模式还是深度睡眠模式，SCB寄存器的内容在m0+编程手册里。
7 E: w) {, |. P7 k配置说明：
0：睡眠模式
1：深度睡眠模式
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2 S2 W/ K. a6 I8 [地址匹配模式
检测起始位
- p  G: f7 m/ r( JRXNE/RXFNE中断模式

二、用法
( }5 ]8 B# \8 O2.1 地址匹配模式
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1. /* make sure that no LPUART transfer is on-going */
   
2.   while (__HAL_UART_GET_FLAG(&hlpuart1, USART_ISR_BUSY) == SET);
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3. 
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4.   /* make sure that LPUART is ready to receive
   
5.    * (test carried out again later in HAL_UARTEx_StopModeWakeUpSourceConfig) */
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6.   while (__HAL_UART_GET_FLAG(&hlpuart1, USART_ISR_REACK) == RESET);
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7. 
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8.   /* set the wake-up event:
   
9.    * specify address-to-match type.
   
10.    * The address is 0x29, meaning the character triggering the
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11.    * address match is 0xA9 */
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12.   WakeUpSelection.WakeUpEvent   = UART_WAKEUP_ON_ADDRESS;
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13.   WakeUpSelection.AddressLength = UART_ADDRESS_DETECT_7B;
    
14.   WakeUpSelection.Address       = 0x29;
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15.   if (HAL_UARTEx_StopModeWakeUpSourceConfig(&hlpuart1, WakeUpSelection) != HAL_OK)
    
16.   {
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17.     Error_Handler();
    
18.   }
    
19. 
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20.   /* Enable the LPUART Wake UP from stop mode Interrupt */
    
21.   __HAL_UART_ENABLE_IT(&hlpuart1, UART_IT_WUF);
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22. 
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23.   /* about to enter stop mode: switch off LED1 */
    
24.   BSP_LED_Off(LED1);
    
25.   /* enable MCU wake-up by LPUART */
    
26.   HAL_UARTEx_EnableStopMode(&hlpuart1);
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27.   /* enter STOP mode */
    
28.   HAL_PWR_EnterSTOPMode(PWR_LOWPOWERREGULATOR_ON, PWR_STOPENTRY_WFI);
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29. 
    
30.   /* ... STOP mode ... */
    
31. 
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32. 
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33.   /* at that point, MCU has been awoken: LED1 has been turned back on */
    
34.   SystemClock_Config_fromSTOP();
    
35. 
    
36.   /* Wake Up on 7-bit address detection successful */
    
37.   HAL_UARTEx_DisableStopMode(&hlpuart1);

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2.2 起始位模式
2 j) |( u! K5 T1 R串口上产生一个起始位可以唤醒。
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1. /* make sure that no LPUART transfer is on-going */
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2.   while (__HAL_UART_GET_FLAG(&hlpuart1, USART_ISR_BUSY) == SET);
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3. 
   
4.   /* make sure that LPUART is ready to receive
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5.    * (test carried out again later in HAL_UARTEx_StopModeWakeUpSourceConfig) */
   
6.   while (__HAL_UART_GET_FLAG(&hlpuart1, USART_ISR_REACK) == RESET);
   
7. 
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8.   /* set the wake-up event:
   
9.    * specify wake-up on start-bit detection */
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10.   WakeUpSelection.WakeUpEvent = UART_WAKEUP_ON_STARTBIT;
    
11.   if (HAL_UARTEx_StopModeWakeUpSourceConfig(&hlpuart1, WakeUpSelection) != HAL_OK)
    
12.   {
    
13.     Error_Handler();
    
14.   }
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15. 
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16.   /* Enable the LPUART Wake UP from STOP mode Interrupt */
    
17.   __HAL_UART_ENABLE_IT(&hlpuart1, UART_IT_WUF);
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18. 
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19.   /* about to enter STOP mode: switch off LED1 */
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20.   BSP_LED_Off(LED1);
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21.   /* enable MCU wake-up by LPUART */
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22.   HAL_UARTEx_EnableStopMode(&hlpuart1);
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23.   /* enter STOP mode */
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24.   HAL_PWR_EnterSTOPMode(PWR_LOWPOWERREGULATOR_ON, PWR_STOPENTRY_WFI);
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25. 
    
26.   /* ... STOP mode ... */
    
27. 
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28. 
    
29.   /* at that point, MCU has been awoken: LED1 has been turned back on */
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30.   SystemClock_Config_fromSTOP();
    
31. 
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32.   /* Wake Up on start bit detection successful */
    
33.   HAL_UARTEx_DisableStopMode(&hlpuart1);

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2.3 RXNE/RXFNE中断模式
接收到一个字符可以唤醒，接收的字节是完整的。
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1. /* make sure that no LPUART transfer is on-going */
   
2.   while (__HAL_UART_GET_FLAG(&hlpuart1, USART_ISR_BUSY) == SET);
   
3.   /* make sure that LPUART is ready to receive
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4.    * (test carried out again later in HAL_UARTEx_StopModeWakeUpSourceConfig) */
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5.   while (__HAL_UART_GET_FLAG(&hlpuart1, USART_ISR_REACK) == RESET);
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6. 
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7.   /* set the wake-up event:
   
8.    * specify wake-up on RXNE flag */
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9.   WakeUpSelection.WakeUpEvent = UART_WAKEUP_ON_READDATA_NONEMPTY;
   
10.   if (HAL_UARTEx_StopModeWakeUpSourceConfig(&hlpuart1, WakeUpSelection) != HAL_OK)
    
11.   {
    
12.     Error_Handler();
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13.   }
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14. 
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15.   /* Enable the UART Wake UP from STOP mode Interrupt */
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16.   __HAL_UART_ENABLE_IT(&hlpuart1, UART_IT_WUF);
    
17. 
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18.   /* about to enter STOP mode: switch off LED1 */
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19.   BSP_LED_Off(LED1);
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20.   /* enable MCU wake-up by LPUART */
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21.   HAL_UARTEx_EnableStopMode(&hlpuart1);
    
22.   /* enter STOP mode */
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23.   HAL_PWR_EnterSTOPMode(PWR_LOWPOWERREGULATOR_ON, PWR_STOPENTRY_WFI);
    
24. 
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25.   /* ... STOP mode ... */
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26. 
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27. 
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28.   /* at that point, MCU has been awoken: LED1 has been turned back on */
    
29.   SystemClock_Config_fromSTOP();
    
30. 
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31.   /* Wake Up based on RXNE flag successful */
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32.   HAL_UARTEx_DisableStopMode(&hlpuart1);

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三、注意事项
如果使用地址匹配模式，需要关闭RXNE的中断。
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1. 
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2. __HAL_UART_DISABLE_IT(&huart1,UART_IT_RXNE);

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