Hi Nagaraj,
On power up, the device is held in reset for 2^16 fclk cycles. This delay allows for the analog portion of the ADS1248 to complete the power up and all circuitry to settle. If using the internal oscillator this equates to about 16ms. You cannot communicate to the ADS1248 before this time has completed. Also, the START pin must be high before sending the RESET command. Only a very limited set of commands will function with the START pin low.
Best regards,
Bob B