Question: Does HRDY only available at the second byte transfer? How does C54x guarantee the first byte transfer when host runs at a very high speed (faster than SAM mode)?

Answer: The HRDY is available in both low and high byte transfers. HRDY will go low when HDS_ goes high at the end of each byte. This allows the host to sample HRDY and extend the next access. HRDY will go high again after the internal access or synchronization is complete or when HCS_ goes high.