25.2.3.2.1 Normal Clock Stretching

Following an ACK if the R/W bit is set (a read request), the slave hardware will clear CKP. This allows the slave time to update SSPxBUF with data to transfer to the master. If the Stretch Enable (SEN) bit is set, the slave hardware will always stretch the clock after the ACK sequence. Once the slave is ready; CKP is set by software and communication resumes.