Serial Module
MOTOROLA
MC68307 USER’S MANUAL
8-19
8.4.1.3 STATUS REGISTER (USR). The USR indicates the status of the characters in the
FIFO and the status of the transmitter and receiver.
RB—Received Break
1 = An all-zero character of the programmed length has been received without a stop
bit. The RB bit is only valid when the RxRDY bit is set. Only a single FIFO position
is occupied when a break is received. Further entries to the FIFO are inhibited until
RxD returns to the high state for at least one-half bit time, which is equal to two suc-
cessive edges of the internal or external 1
× clock or 16 successive edges of the
external 16
× clock.
The received break circuit detects breaks that originate in the middle of a received
character. However, if a break begins in the middle of a character, it must persist
until the end of the next detected character time.
0 = No break has been received.
FE—Framing Error
1 = A stop bit was not detected when the corresponding data character in the FIFO was
received. The stop-bit check is made in the middle of the first stop-bit position. The
bit is valid only when the RxRDY bit is set.
0 = No framing error has occurred.
PE—Parity Error
1 = When the with parity or force parity mode is programmed in the UMR1, the corre-
sponding character in the FIFO was received with incorrect parity. When the mul-
tidrop mode is programmed, this bit stores the received A/D bit. This bit is valid only
when the RxRDY bit is set.
0 = No parity error has occurred.
OE—Overrun Error
1 = One or more characters in the received data stream have been lost. This bit is set
upon receipt of a new character when the FIFO is full and a character is already in
the shift register waiting for an empty FIFO position. When this occurs, the charac-
ter in the receiver shift register and its break detect, framing error status, and parity
error, if any, are lost. This bit is cleared by the reset error status command in the
UCR.
0 = No overrun has occurred.
USR
MBASE + $103
7
6
5
4
3
2
1
0
RB
FE
PE
OE
TxEMP
TxRDY
FFULL
RxRDY
RESET:
0
Read Only
Supervisor or User