SPI Clock Problem

Hello,

I want to use an external ADC (AD7885) with the SPI interface utilizing TIVA C Series Connected Launchpad TM4C1294XL. I need an external ADC due to the high project demands.

The external ADC should be clocked with 48MHz. Furthermore, the received data should be stored without CPU intervention (DMA).

I implemented the following code:

int main(void)
{
   uint32_t  valueX[1];

  SysCtlClockFreqSet((SYSCTL_OSC_INT | SYSCTL_USE_PLL | SYSCTL_CFG_VCO_480), 120000000);

  SysCtlPeripheralEnable(SYSCTL_PERIPH_SSI2);
  SysCtlPeripheralEnable(SYSCTL_PERIPH_GPIOD);

  GPIOPinConfigure(GPIO_PD0_SSI2XDAT1);
  GPIOPinConfigure(GPIO_PD1_SSI2XDAT0);
  GPIOPinConfigure(GPIO_PD2_SSI2FSS);
  GPIOPinConfigure(GPIO_PD3_SSI2CLK);

  GPIOPinTypeSSI(GPIO_PORTD_BASE, GPIO_PIN_0 | GPIO_PIN_1 | GPIO_PIN_2 | GPIO_PIN_3);

  SSIClockSourceSet(SSI2_BASE, SSI_CLOCK_SYSTEM);

  SSIConfigSetExpClk(SSI2_BASE, systemClock, SSI_FRF_MOTO_MODE_2, SSI_MODE_MASTER, 40000000, 16);

  SSIEnable(SSI2_BASE);

  while(1)
  {
   SSIDataPut(SSI2_BASE, 0);
   SSIDataGet(SSI2_BASE, &valueX[0]);
  }

}

Unfortunately it was not possible to configure the demanded 48MHz clock signal. When I set the Bit rate to 48,000,000, with a system clock frequency of 120 MHz, the clock output (SSI2CLK) amounts 58 MHz. Furthermore the duty cycle is not 50% (see picture).

Microcontroller: Master

D0 = SPI Clock
D2 = Chip select
D4 = RX
D7 = TX

Why is the time, in which the CS pin is high, so long?

I found the following equation in the reference manual:

SSInclk = Sysclock / (CPSDVSR * (1 + SCR))

When I insert the minimum possible parameter I get the following result:

SSInclk = 120x10^6 (2 * (1 + 1)) = 30 MHz

However the duty cycle at 30MHz is also not 50%. I want to receive the data continuously but without CPU intervention, is it possible to connect the DMA to the SPI? I only want to read data from the external ADC.

Is it possible to increase the clock signal of the SPI to 48 MHz?

Thank you in advance for all the help!