Additional byte 0xFF is being sent on UARTE boot-up.

[Petar2635]

Hello,

I've been using nrfx UARTE driver on the nrf5 15.2.0 SDK on a nrf52832 and nrf52840 SoC and noticed that during the boot-up the first byte being sent is 0xFF.

Is there a reason for the UARTE to send that byte?

This picture above shows the first byte (0xFF) being sent at the boot-up and the sequence of bytes that are being sent properly.

Is this proper behaviour at the start? Tried different UARTE instance configurations but 0xFF byte was still being sent.

I can share the code if it is necessary but the 0xFF isn't sent explicitly.

Thanks in advance!

Petar.

[Edvin]

Hello Petar,

I believe that this is actually just a pulse that the UART receiver interprets as a start bit. It is the GPIO driver that sets the GPIO to low during startup.

Have you tried to step through apply_config() in nrfx_uart.c to see whether this is where the pulse comes from?

I see that it should set the pin before setting it as an output, but it may be that it is not possible to avoid this pulse due to HW reasons.

    if (p_config->pseltxd != NRF_UART_PSEL_DISCONNECTED)
    {
        nrf_gpio_pin_set(p_config->pseltxd);
        nrf_gpio_cfg_output(p_config->pseltxd);
    }

Best regards,

Edvin