why NRF_LOG_ENABLED = 1 in sdk_config.h is making P0.06 high ?

[AqibK]

Hello, 

I am using nRF52840 dev kit with sdk 17.0.2, s140 softdevice and dfu bootloader also implemented.

When I download the firmware to dev kit it works as intended, only problem is that pin P0.06 in automatically getting high, even thought there is nothing in my code which makes it high. After some debugging I fount that writing NRF_LOG_ENABLED = 1 is causing pin P0.06 to be high. However if I disable  nrf_log i.e make NRF_LOG_ENABLED = 0 then pin stays low.

On dev kit this pin is high and its not a big problem because I am not using pin P0.06 but my team has developed a custom board based on nrf52840 and on pin 0.06 we have attached a buzzer. With nrf_log enabled it keeps buzzer always high, even if we hard code it to low. For example, in main function after log_init() and before idle loop we are making pin0.06 low but it gets high again and stays high.

At this point we need nrf_log functionality in our program so I cannot disable it. On the other hand I am unable to find reason that why it is making pin0.06 high. Any help in this regard will be appreciated.

[Turbo J]

P0.6 is UART TX pin in the default board config (in pca10056.h). NRF_LOG_ENABLED will enable the UART to output its logs by default.

Fix is simple: 

#define NRF_LOG_BACKEND_UART_ENABLED 0
#define NRF_LOG_BACKEND_RTT_ENABLED 1

Then use the Jlink RTT viewer for Log messages via SWD connection.

I recommend useing a custom board config - define BOARD_CUSTOM in Makefile, use "custom_board.h" as board header file in your project.

[AqibK]

Oh I see. So with this fix it is working fine but if we want to use UART then what is solution? Should we not use P0.6 when using UART? 

Thank you for last suggestion, we are already doing it like that but I don't understand how it will fix this problem? Dont we have to change our custom board's hardware also? 

[Turbo J]

All you needed to do is change the TX_PIN_NUMBER definition to something that fits your hardware.

[AqibK]

Alright, I get it now. Thank you!

[Marte Myrvold]

Hi,

In addition to what Turbo J has already said, I recommend checking out nRF52840 pin assignments if you have not already, to see recommendations and limitations for pin configurations. For default settings for other pins on nRF52840 DK see nRF52840 DK > Connecter interface.

Best regards,

Marte

[AqibK]

Hi, 

I actually checked nRF52840 pin assignments and there P0.06 was just a normal GPIO but I didn't know it was being used as UART pin for nrf_log. I should have checked  nRF52840 DK > Connecter interface.

Thank you for the information, will definitely help in future.