Question: Question about the serial port control register (SCON). Will the Receive Buffers Full (BBF) bit read 0 or 1 when the receive buffer register (RBR) is full but the receive shift register (RSR) is empty?

Answer: It should read 0. This bit is a flag that will get set when both of these registers are full. Basic operation is that a data word comes into the RSR and when all bits have come in, the data word is written into the RBR. Then the program should come and read the word out of RBR while another data word is coming into RSR. If the RSR fills up and the RBR has not been read (emptied) yet, then you don't want the RSR to overwrite the RBR, which would cause data loss. In this instance, the BBF bit gets set to indicate that both the RSR and RBR are full, and receive operation ceases.