TM4C123GH6PM: SPI communications and handling of FSS on the Tiva-C

Hello,

I initialize my spi peripheral on the tm4c123 as:

    // SSI0 initialization
    SysCtlPeripheralEnable(SYSCTL_PERIPH_SSI0);
    SysCtlPeripheralEnable(SYSCTL_PERIPH_GPIOA);

    GPIOPinConfigure(GPIO_PA2_SSI0CLK);
    GPIOPinConfigure(GPIO_PA3_SSI0FSS);
    GPIOPinConfigure(GPIO_PA4_SSI0RX);
    GPIOPinConfigure(GPIO_PA5_SSI0TX);
    GPIOPinTypeSSI(GPIO_PORTA_BASE, GPIO_PIN_5 | GPIO_PIN_4 | GPIO_PIN_3 | GPIO_PIN_2);
    SSIConfigSetExpClk(SSI0_BASE, SysCtlClockGet(), SSI_FRF_MOTO_MODE_0, SSI_MODE_MASTER, 4000000, 8);
    SSIEnable(SSI0_BASE);

And I was expecting FSS to be handled automatically. But however I failed getting read results from my spi sensor, untilI removed the GPIOPinConfigure(GPIO_PA3_SSI0FSS); statement and configured PA4 to be GPO port, handling the CS of the SPI myself.

Here are read and write register functions:

void writeReg(uint8_t addr, uint8_t data) {

    uint32_t dummy[1];

    addr = addr & 0x7F;

    GPIOPinWrite(GPIO_PORTA_BASE, GPIO_PIN_3, 0x00);

    SSIDataPut(SSI0_BASE, addr);
    SSIDataPut(SSI0_BASE, data);
    while(SSIBusy(SSI0_BASE));

    GPIOPinWrite(GPIO_PORTA_BASE, GPIO_PIN_3, GPIO_PIN_3);

    while (SSIDataGetNonBlocking(SSI0_BASE, &dummy[0]));

}

uint8_t readReg(uint8_t addr) {

    uint32_t dummy[1];
    uint32_t data;

    addr = addr | 0x80;

    GPIOPinWrite(GPIO_PORTA_BASE, GPIO_PIN_3, 0x00);

    SSIDataPut(SSI0_BASE, addr);
    SSIDataGet(SSI0_BASE, &dummy[0]);
    SSIDataPut(SSI0_BASE, 0x00);
    SSIDataGet(SSI0_BASE, &data);

    while(SSIBusy(SSI0_BASE));
    GPIOPinWrite(GPIO_PORTA_BASE, GPIO_PIN_3, GPIO_PIN_3);

    while (SSIDataGetNonBlocking(SSI0_BASE, &dummy[0]));

    return (uint8_t)(data & 0xFF);


}

Although they work now, under what circumstances we dont have to manage  CS of the SPI peripherals ourselves? Does the FSS drop low with each SPI get put statement? Notice that to read from a register, I willput the addr byte first, then get a byte and then pull up the CS line.

Best Regards,

C.A.