mc68hc11d0cfn2 Freescale Semiconductor, Inc, mc68hc11d0cfn2 Datasheet - Page 75

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

6.4 Receive Operation
6.5 Wakeup Feature
6.5.1 Idle-Line Wakeup
MC68HC711D3 — Rev. 2
MOTOROLA
During receive operations, the transmit sequence is reversed. The serial shift
register receives data and transfers it to a parallel receive data register (SCDR) as
a complete word. Refer to
character to be shifted in serially while another character is already in the SCDR.
An advanced data recovery scheme distinguishes valid data from noise in the
serial data stream. The data input is selectively sampled to detect receive data, and
a majority voting circuit determines the value and integrity of each bit.
The wakeup feature reduces SCI service overhead in multiple receiver systems.
Software for each receiver evaluates the first character of each message. The
receiver is placed in wakeup mode by writing a 1 to the RWU bit in the SCCR2
register. While RWU is 1, all of the receiver-related status flags (RDRF, IDLE, OR,
NF, and FE) are inhibited (cannot become set). Although RWU can be cleared by
a software write to SCCR2, to do so would be unusual. Normally, RWU is set by
software and is cleared automatically with hardware. Whenever a new message
begins, logic alerts the sleeping receivers to wake up and evaluate the initial
character of the new message.
Two methods of wakeup are available:
During idle line wakeup, a sleeping receiver awakens as soon as the RxD line
becomes idle. In the address mark wakeup, logic 1 in the most significant bit (MSB)
of a character wakes up all sleeping receivers.
To use the receiver wakeup method, establish a software addressing scheme to
allow the transmitting devices to direct a message to individual receivers or to
groups of receivers. This addressing scheme can take any form as long as all
transmitting and receiving devices are programmed to understand the same
scheme. Because the addressing information is usually the first frame(s) in a
message, receivers that are not part of the current task do not become burdened
with the entire set of addressing frames. All receivers are awake (RWU = 0) when
each message begins. As soon as a receiver determines that the message is not
intended for it, software sets the RWU bit (RWU = 1), which inhibits further flag
setting until the RxD line goes idle at the end of the message. As soon as an idle
line is detected by receiver logic, hardware automatically clears the RWU bit so that
the first frame of the next message can be received. This type of receiver wakeup
requires a minimum of one idle-line frame time between messages and no idle time
between frames in a message.
•
•
Freescale Semiconductor, Inc.
Idle line wakeup
Address mark wakeup
For More Information On This Product,
Serial Communications Interface (SCI)
Go to: www.freescale.com
Figure
6-2. This double-buffered operation allows a
Serial Communications Interface (SCI)
Receive Operation
Data Sheet
75