MC68HC711KS2MFN4 Freescale Semiconductor, MC68HC711KS2MFN4 Datasheet - Page 156

• Manufacturer Part Number: MC68HC711KS2MFN4
• Description: 32K EPROM - SLOW MODE
• Manufacturer: Freescale Semiconductor

Serial Communications Interface (SCI)
7.6 Wakeup Feature
Technical Data
156
The wakeup feature reduces SCI service overhead in multiple receiver
systems. If a system generates address information at the beginning of
every message, each receiver can determine whether or not it is the
intended recipient of a message by evaluating the first character(s)
through software.
If the message is intended for a different receiver, the SCI can be placed
in a sleep mode so that the rest of the message will not generate
requests for service. It does this by setting the RWU (receiver wakeup)
bit in SCI control register 2 (SCCR2), which inhibits all receiver-related
status flags (RDRF, IDLE, OR, NF, FE, PF, and RAF). A new message
clears the receiver’s RWU bit, enabling it to evaluate the new address
information. Although RWU can be cleared by a software write to
SCCR2, this is rarely done because hardware clears RWU
automatically.
Two methods of wakeup are available:
•
•
Idle line wakeup — A sleeping receiver wakes up as soon as the
RxD line becomes idle (for example, in a logic 1 state for at least
one frame time). A system using this type of wakeup must provide
at least one character time of idle between messages to wake up
sleeping receivers and must not allow any idle time between
characters within a message.
Address mark wakeup — Uses the most significant bit (MSB) to
distinguish address characters (MSB = 1) from data characters
(MSB = 0). A sleeping receiver wakes up whenever it receives an
address character. Unlike the idle line method, address mark
wakeup allows idle periods within messages and does not require
idle time between messages. However, message processing is
less efficient because the start bit of each character must be
evaluated.
Serial Communications Interface (SCI)
M68HC11K Family
MOTOROLA