Why there are some pins cant be the ss pin of SPI

[Aku]

HI

I'm using SPI connecting to mutiple max31856(two now) on nrf52DK and SDK15.3

so I control my ss pin to pull low by myself 

chip select by changing spi_pin like some code below

void MAX31856_setThermocoupleType(max31856_thermocoupletype_t type ,uint32_t spi_pin)
{   
    nrf_gpio_pin_clear(spi_pin);
    MAX31856_write_reg(MAX31856_CR1_REG);
    uint8_t t = MAX31856_read_reg(0xFF);
    nrf_gpio_pin_set(spi_pin);

    t &= 0xF0; // mask off bottom 4 bits
    t |= (uint8_t)type & 0x0F;

    nrf_gpio_pin_clear(spi_pin);
    MAX31856_write_reg(MAX31856_CR1_REG|0x80);
    MAX31856_write_reg(t);
    nrf_gpio_pin_set(spi_pin);
}

my miso is 5, mosi is 7 , sck is 8

and ss1 is 6 , ss2 is 9 at first

then i found that ss1 works fine ,but ss2 doesnt

SO I TRY OTHER PINS, AND FOUND THAT pin 2,3,4,6,12,13,14,15,18,19,20,22,23,24,26,27,28,29,30,31 can work

BUT pin 9,10,11,13,17,21,25 cant work

MY QUESTION  is why that happened

thanks

[Nguyen Hoan Hoang]

Pin 9 is by default NFC.  To use it, you need to configure in the UICR register for that pin to act as GPIO.

    /* Configure NFCT pins as GPIOs if NFCT is not to be used in your code. If CONFIG_NFCT_PINS_AS_GPIOS is not defined,
       two GPIOs (see Product Specification to see which ones) will be reserved for NFC and will not be available as
       normal GPIOs. */
    #if defined (CONFIG_NFCT_PINS_AS_GPIOS)
        if ((NRF_UICR->NFCPINS & UICR_NFCPINS_PROTECT_Msk) == (UICR_NFCPINS_PROTECT_NFC << UICR_NFCPINS_PROTECT_Pos)){
            NRF_NVMC->CONFIG = NVMC_CONFIG_WEN_Wen << NVMC_CONFIG_WEN_Pos;
            while (NRF_NVMC->READY == NVMC_READY_READY_Busy){}
            NRF_UICR->NFCPINS &= ~UICR_NFCPINS_PROTECT_Msk;
            while (NRF_NVMC->READY == NVMC_READY_READY_Busy){}
            NRF_NVMC->CONFIG = NVMC_CONFIG_WEN_Ren << NVMC_CONFIG_WEN_Pos;
            while (NRF_NVMC->READY == NVMC_READY_READY_Busy){}
            NVIC_SystemReset();
        }
    #endif