Synchronous Transmit During Sleep

To transmit during Sleep, all the following conditions must be met before entering Sleep mode:

Upon entering Sleep mode, the device will be ready to accept clocks on the TXx/CKx pin and transmit data on the RXx/DTx pin. When the data word in the TSR register has been completely clocked out by the external device, the pending byte in the TXxREG will transfer to the TSR and the TXxIF flag will be set. Thereby, waking the processor from Sleep. At this point, the TXxREG is available to accept another character for transmission. Writing TXxREG will clear the TXxIF flag.

Upon waking from Sleep, the instruction following the SLEEP instruction will be executed. If the Global Interrupt Enable (GIE) bit is also set then the Interrupt Service Routine (ISR) will be called.