mc68hc11d0cfn2 Freescale Semiconductor, Inc, mc68hc11d0cfn2 Datasheet - Page 77

• Manufacturer Part Number: mc68hc11d0cfn2
• Description: M68hc11 Microcontrollers
• Manufacturer: Freescale Semiconductor, Inc

6.5.2 Address-Mark Wakeup
6.6 SCI Error Detection
MC68HC711D3 — Rev. 2
MOTOROLA
The serial characters in this type of wakeup consist of seven (eight if M = 1)
information bits and an MSB, which indicates an address character (when set
to 1 — mark). The first character of each message is an addressing character
(MSB = 1). All receivers in the system evaluate this character to determine if the
remainder of the message is directed toward this particular receiver. As soon as a
receiver determines that a message is not intended for it, the receiver activates the
RWU function by using a software write to set the RWU bit. Because setting RWU
inhibits receiver-related flags, there is no further software overhead for the rest of
this message. When the next message begins, its first character has its MSB set,
which automatically clears the RWU bit and enables normal character reception.
The first character whose MSB is set is also the first character to be received after
wakeup because RWU gets cleared before the stop bit for that frame is serially
received. This type of wakeup allows messages to include gaps of idle time, unlike
the idle-line method, but there is a loss of efficiency because of the extra bit time
for each character (address bit) required for all characters.
Three error conditions can occur during generation of SCI system interrupts:
Three bits (OR, NF, and FE) in the serial communications status register (SCSR)
indicate if one of these error conditions exists. The overrun error (OR) bit is set
when the next byte is ready to be transferred from the receive shift register to the
SCDR and the SCDR is already full (RDRF bit is set). When an overrun error
occurs, the data that caused the overrun is lost and the data that was already in
SCDR is not disturbed. The OR is cleared when the SCSR is read (with OR set),
followed by a read of the SCDR.
The noise flag (NF) bit is set if there is noise on any of the received bits, including
the start and stop bits. The NF bit is not set until the RDRF flag is set. The NF bit
is cleared when the SCSR is read (with FE equal to 1) followed by a read of the
SCDR.
When no stop bit is detected in the received data character, the framing error (FE)
bit is set. FE is set at the same time as the RDRF. If the byte received causes both
framing and overrun errors, the processor only recognizes the overrun error. The
framing error flag inhibits further transfer of data into the SCDR until it is cleared.
The FE bit is cleared when the SCSR is read (with FE equal to 1) followed by a read
of the SCDR.
•
•
•
Freescale Semiconductor, Inc.
Serial communications data register (SCDR) overrun
Received bit noise
Framing
For More Information On This Product,
Serial Communications Interface (SCI)
Go to: www.freescale.com
Serial Communications Interface (SCI)
SCI Error Detection
Data Sheet
77