Hi Shubham,
I think you are missing what is happening in the communication. Please look at the timing diagram of the datasheet. You must communicate in 8-bit (byte) transfers and not 16 or 32-bit words. All commands are structured as one byte in length followed by data. The ADS1218 must be able to decode the command (read register, write register, reset, calibration, etc.) before the device knows which operation is being performed. The delay is the time between the command and the data. Also, you cannot use the SS function as it relates to SSI module as total transaction length can vary, and the communcation to the ADS1218 for the total transaction length requires that CS remain low throughout the entire transaction. So if you use CS on the ADS1218, then you must use a GPIO function of the processor to control CS of the ADS1218 and not the SS function of the SSI module.
Best regards,
Bob B