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Exit STOP mode using USART on STM32L496

Question asked by johnston.paul.001 on Jan 31, 2018
Latest reply on Feb 19, 2018 by johnston.paul.001

I 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!




/* Ensure that MSI is wake-up system clock */


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



// System should now be stopped here waiting for UART interrupt to occur





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