
M68HC11
REFERENCE MANUAL
ASYNCHRONOUS SERIAL COMMUNICATIONS INTERFACE
MOTOROLA
9-13
This is the normal indication that a character has been received by the SCI. The NF
and FE status bits provide additional information about this normally received charac-
ter in the SCDR. If set, the OR flag would indicate that another character was serially
received and was ready to be transferred to the SCDR, but the previously received
character was not yet read. Software should check RDRF, OR, NF, and FE to tell if
there was any error in the reception of a character (RDRF should be set and the other
three flags should be clear). The RDRF flag is cleared by reading SCSR, followed by
a read of the SCDR. RDRF must be read as one during the read of SCSR, or the first
step of the clearing sequence is not satisfied. Since the NF and FE flags are set at the
same time as RDRF, the clearing sequence for RDRF will also clear NF and FE if they
were set for this received character.
IDLE — Idle-Line Detect
0 = The RxD line is either active now or has never been active since IDLE was last
cleared.
1 = The RxD line has become idle.
The idle condition is defined as at least a full character time of logic one on the RxD
line. A character time is 10 bit times if M = 0 or 11 bit times if M = 1. The IDLE flag is
cleared by reading SCSR, followed by a read of the SCDR. IDLE must be read as one
during the read of SCSR, or the first step of the clearing sequence is not satisfied.
Once IDLE has been cleared, it cannot be set again until the RxD line has been active
and then becomes idle again. The idle-line function is inhibited while the receiver
wake-up function is enabled (RWU = 1).
OR — Overrun Error
0 = No overrun error.
1 = Indicates that another character was serially received and was ready to be
transferred to the SCDR, but the previously received character was not yet
read.
Since the SCI receiver is double buffered, there is a full character time between recep-
tion of a character and when it must be read from the SCDR to avoid an OR caused
by a subsequent character. In an OR condition, the character that caused the OR is
lost, but the previously received character in the SCDR is not disturbed. The NF and
FE status bits are associated with the normally received character in the SCDR (never
the character that caused an OR). The OR flag is cleared by reading SCSR, followed
by a read of the SCDR. OR must be read as one during the read of SCSR, or the first
step of the clearing sequence is not satisfied.
NF — Noise Flag
0 = No noise detected during reception of the character in the SCDR.
1 = Data recovery logic detected noise during reception of the character in the
SCDR.
NF does not generate interrupt requests because it is always associated with the set-
ting of RDRF, which does cause interrupts. Even if the noise is detected early in the
reception of the character, NF is not set until RDRF is set at the end of reception of a
character. Perceived noise in any of the data bit times or the start or stop bit times will
cause NF to be set. During data bit times and the stop bit time, noise is indicated if the
three samples taken near the middle of the bit time do not unanimously agree. During