how to check SPI signals

Question

I am working with ATmega32-A micro controller. I am trying to read ADC (AD7798) values. I am setting ADC registers as shown below. Images also attached.

char spi(char data)
{
    //Start transmision
    SPDR = data;
    //Wait for transmision complete
    while(!(SPSR & 0x80));
    return SPDR;
} 

void setupAd(){
    ADC_CS=0; 
    SPCR = 0x5D;
    spi(0x10);   //set up communication register for configuration reg (16bit).
    spi(0x07);    // sending 07 as MSB     
    spi(0x10);    //sending 10 as LSB 
    ADC_CS=1; 
}

I am trying to read this configuration setup using this

void ReadAdConfReg(void)
{              
    SPCR = 0x5D;
    ADC_CS=0;
    spi(0x50);
    adcConfig = spi(0xFF)<<8;
    adcConfig |= spi(0xFF);   
    ADC_CS=1;
}

printing like this

ReadAdConfReg();
printf("configreg:%d",adcConfig);

But my problem setup of that configuration registor was not working. I have checked with oscilloscope any mistakes in clock polarity and chip select. I have attached images as shown below.

Please help me why i am not able to set configuration register. clock polarities and chip select every thing ok according to ATmega32-A data sheet and AD7798 datasheet. please help me what mistake i am doing. or If I want to check my configuration register setup on oscilloscope What i have to do.

Answer 1

I have changed the functions to read 8 bit and 16 bit registers then it worked.

unsigned int Bit_16(unsigned char cmd, unsigned int value) 
{
    unsigned int data; 
    SPCR = 0x5D;        // mode #3 F_CPU/16 
    ADC_CS = 0;                 // enable 
    spi(cmd);                   //Place instruction in communication register 
    data = spi(value >> 8);     // read hi-byte 
    data = (data << 8) | spi(value);    // and lo-byte. 
    ADC_CS = 1;                 // disable 
    return data; 
} 

for write

AD7798_16(0x08, 0x000A);

Answer 2

You need to read the ADC datasheet more closely.

For each communication with the ADC you need to send a value for the communication register. Per the datasheet, CR7 must be low or no other bits are clocked into the communication register.

So for the first byte you send it must have the MSB cleared.

Bit 6 is whether you want a read or a write. Bits 5-3 are what register you want to operate on. Bit 2 is whether you want to continuously read Bits 1-0 should be 0's.

Please look over the datasheet's "Digital Interface" section, as well as the "On-Chip Registers" section.