UART Rx issue in MCU power 1.8V

Question

Hi

I am now working on a project which I use ble_app_uart_c as a reference.

SDK 16 and used softdevice s140.

UART works normally at 3.3V.

But if I lower the power by 1.8V, it can be UART TX but not RX.

The 1.8V TX signal is working well from the hardware, but the signal won't work if you just connect the RX Pin.

If I connect the RX Pin and send the TX signal, I can see the signal shaking a little.

So I added the following code to the main beginning.

    if ((NRF_UICR->REGOUT0 & UICR_REGOUT0_VOUT_Msk) != UICR_REGOUT0_VOUT_1V8) 
    {
        NRF_NVMC->CONFIG = NVMC_CONFIG_WEN_Wen << NVMC_CONFIG_WEN_Pos;
        while (NRF_NVMC->READY == NVMC_READY_READY_Busy){}
        NRF_UICR->REGOUT0 = (NRF_UICR->REGOUT0 & ~((uint32_t)UICR_REGOUT0_VOUT_Msk)) |
                            (UICR_REGOUT0_VOUT_1V8 << UICR_REGOUT0_VOUT_Pos);
        NRF_NVMC->CONFIG = NVMC_CONFIG_WEN_Ren << NVMC_CONFIG_WEN_Pos;
        while (NRF_NVMC->READY == NVMC_READY_READY_Busy){}
        NVIC_SystemReset();
    }

But it didn't work. Is there a UART setting part?

I need your help.

Jørgen Holmefjord · Accepted Answer

Hi, 
 Which voltage is the device you are interfacing with over UART running at? If the voltage level difference gets outside the input-high/input-low voltage levels of the GPIO peripheral , the communication may not work correctly without a level converter. This is also the case for the device on the other end of the communication. You should check the datasheet of that device as well, to make sure you are within the specifications. 
 Best regards, Jørgen

UART Rx issue in MCU power 1.8V

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