Problem setting & clearing specific pins

[noob_with_ucontrollers]

Trying to run the SPI Master application on a custom nrf52382 based device (custom PCB) to communicate with an ADC chip that is taking analog values from a Multiplexer. Not using any softdevice, using SDK 15.1, NRF52382, environment is uVision Keil 5.0, and I am programming the custom board through an NRF Development kit. 

Problem is, there are some pins that I cannot seem to set or clear (i.e. they are tied high or low permanently no matter what code or application I run). I observed that P0.21 is tied HIGH and the code I run does not change that (the pin stays at 3.3V and never gets back to floating even if left unprogrammed). Similarly, P0.6 is tied HIGH and P0.9 & P0.10 are tied LOW no matter what application is running on the board. If I leave these pins unprogrammed they don't have floating value but stay at respective HIGH and LOW states. 

The GPIO pin assignments page on Nordic Infocenter (http://infocenter.nordicsemi.com/index.jsp?topic=%2Fcom.nordic.infocenter.sdk5.v12.2.0%2Fgroup__nrf__gpio.html&cp=4_0_1_6_6_7)  says these pins can be used as general purpose I/O. P0.21 especially should be able to be configured as general output pin if I haven't configured it as Reset. 

The following is my code. Any guidance as to where I am going wrong will be helpful. Is there a problem with the PCB? Thank you so much. 

 #define MYPIN  6

nrf_gpio_cfg_output(MYPIN);

nrf_gpio_pin_clear (MYPIN);

[RK]

As pointed out weekly here, P0.09 and P0.10 are NFC by default so you have to turn that off before you can use them as GPIOs. The other two pins are fine for use as GPIOs which makes me wonder if you have assigned them to SPI or UART or one of the other peripherals which means they no-longer function as GPIOs

[noob_with_ucontrollers]

Thanks a lot RK. I think this is the problem because when I redefine the UART TX pin to be P0.5 in the SDK.h , I can set and clear pin 6 at will. 

However, I still could not figure out how to disable the NFC functionality of P0.9 and P0.10 and the reset functionality of P0.21. Would you have any recommendation for that?  

[RK]

CONFIG_NFCT_PINS_AS_GPIOS, look into system_nrf52.c for other pre-processor options you can set including the reset one. 

[noob_with_ucontrollers]

I include a line of code " #define CONFIG_NFCT_PINS_AS_GPIOS" in my main project file. And I write a line "CFLAGS += -DCONFIG_NFCT_PINS_AS_GPIOS" in the makefile of the project to disable the use of P0.9, and 10 as nfct as suggested in devzone.nordicsemi.com/.../how-to-enable-p0-09-and-p0-10-as-gpio-pins-on-the-nrf52-instead-of-nfc-pins

As for the reset pin 21. I am trying to change the pin number from 21 to sth else in the system_nrf52.c file.

But doing this gives me this error. 
I think I am doing sth wrong or missing sth out. 

[Ashish]

I include a line of code " #define CONFIG_NFCT_PINS_AS_GPIOS" in my main project file. 

This isn't required. Setting the FLAG in makefile should do it.

As for the reset pin 21. I am trying to change the pin number from 21 to sth else in the system_nrf52.c file.

To do this, did you create a new file, named system_nrf52_1.c?