Every time the master sends a byte of data to the slave, the master always needs to wait for the slave to give a reply signal to confirm whether the slave has successfully received the data. The clock needed by the slave to reply to the master is still provided by the master. The reply occurs in the clock period immediately following the completion of 8 data bits transmission by the master. The low level 0 indicates the reply, and the low level 1 indicates the non reply,