MC68HC711D3CFB2 FREESCALE [Freescale Semiconductor, Inc], MC68HC711D3CFB2 Datasheet - Page 68

• Manufacturer Part Number: MC68HC711D3CFB2
• Description: Microcontrollers
• Manufacturer: FREESCALE [Freescale Semiconductor, Inc]

6.5 Wakeup Feature
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.
6.5.1 Idle-Line Wakeup
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.
6.5.2 Address-Mark Wakeup
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.
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Serial Communications Interface (SCI)
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Idle line wakeup
Address mark wakeup
MC68HC711D3 Data Sheet, Rev. 2.1
Freescale Semiconductor