Exit STOP mode using USART on STM32L496

Question

Posted on January 31, 2018 at 00:26I wish to place my Nucleo system into STOP1 mode and then have it wake up when a character is received on UART3. With the code below, I can successfully stop all the clocks and halt at the 'WFI' instruction. When a character is sent to the UART from my terminal, the interrupt routine runs and echoes the character back to me as expected, but the code remains stuck at the 'WFI' instruction (inside the call to HAL_PWREx_EnterSTOP1Mode). I have tried the code with and without the debugger and get the same results. How do I get the code to continue past the 'WFI'? My apologies for my use of the low level hardware instructions....I'm an old-school hardware guy so this is where I go when things aren't working!//**************************************************************************************__HAL_RCC_PWR_CLK_ENABLE();/* Ensure that MSI is wake-up system clock */__HAL_RCC_WAKEUPSTOP_CLK_CONFIG(RCC_STOP_WAKEUPCLOCK_MSI); USART3->CR1 &= ~0x00000001; // UE bit must be 0 to change WUS bits in CR3 USART3->CR3 |= 0x00700000; // set WFIE bit and WUS = 11 (RXNE interrupt) USART3->CR1 |= 0x00000002; // set the UESM bit to allow UART to wake up the MCU from stop mode USART3->CR1 |= 0x00000001; // re-enable UART3 SCB->SCR &= ~0x02; // clear the SLEEPONEXIT bit so the system will restart DBGMCU->CR &= ~0x02; // clear the DBG_STOP bit so debugger does not get clock signal HAL_PWREx_EnterSTOP1Mode(PWR_STOPENTRY_WFI); // System should now be stopped here waiting for UART interrupt to occur__HAL_RCC_PWR_CLK_ENABLE();  <-----I NEVER GET TO HERE!//*********************************************************************************************void USART3_IRQHandler(void){       if ((USART3->ISR & 0x20) == 0x20) // check for RXNE bit      {             * UART3_RX_Pointer = USART3->RDR; // read available byte which should clear the RXNE flag                UART3_RX_Pointer ++; // update pointer                while ((USART3->ISR & 0x80) != 0x80); // wait for TX data register empty (or ready).                 USART3->TDR = (uint8_t)USART3->RDR; //echo character to the COM port         }}//*************************************************************************************************#exit-stop #stop-mode #usart

Clonimus74 · Accepted Answer

Posted on February 19, 2018 at 11:56 I would suggest staring with the CubeMX example.  If your clock source to the LPUART is HSI (and it should) but your system clock on wakeup is MSI you might have to re-enable the HSI clock in the LPUART interrupt, see  https://community.st.com/0D50X00009XkW1lSAF  I don't see the clear of theWUF flag and the UESM flag which is recommended  When the debugger is connected you should disable systick interrupt and setDBG_STOP inDBGMCU->CR or you'll have unpredictable behavior.  See also  https://community.st.com/0D50X00009XkfHjSAJ

pauljohnston · Answer

Posted on February 19, 2018 at 19:39Thank you Clonimus74! I had the clearing of the UESM enable bit in the code immediately after the stop command (not shown above). By moving the bit clear into the ISR, all is now good!//*********************************************************************************************void USART3_IRQHandler(void){       if ((USART3->ISR & 0x20) == 0x20) // check for RXNE bit      {             * UART3_RX_Pointer = USART3->RDR; // read available byte which should clear the RXNE flag               USART3->CR1 &= ~0x00000002; // clear UESM bit (wakeup in STOP mode disabled)                UART3_RX_Pointer ++; // update pointer                while ((USART3->ISR & 0x80) != 0x80); // wait for TX data register empty (or ready).                 USART3->TDR = (uint8_t)USART3->RDR; //echo character to the COM port         }}

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