Hi Shubham,
All of the tosc timing is based off the inverse of the fosc clock of the ADS1218, which is the master clock for the ADC. This is determined by the crystal used for the input or external clock provided to the XIN pin. The SPI serial clock must be at least 1/4 of the master clock frequency for proper operation.
The ADS1218 will start converting after power up as long as the PDWN, RESET and DSYNC pins are logic high.
Best regards,
Bob B