Serial Communications Interface (SCI)
Receiver Wake-up Operation
MC68HC(7)05H12
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Rev. 1.0
General Release Specification
MOTOROLA
Serial Communications Interface (SCI)
131
L
G
R
addressed to remain in a dormant state for the remainder of the
unwanted message. This eliminates any further software overhead to
service the remaining characters of the unwanted message and thus
improves system performance.
The receiver is placed in wake-up mode by setting the receiver wake-up
bit (RWU) in the SCCR2 register. While RWU is set, all of the receiver
related status flags (RDRF, IDLE, OR, NF, and FE) are inhibited (cannot
become set). Note that the idle line detect function is inhibited while the
RWU bit is set. Although RWU may be cleared by a software write to
SCCR2, it would be unusual to do so. Normally RWU is set by software
and gets cleared automatically with hardware by one of the two methods
described below.
11.4.1 Idle Line Wake-up
In idle line wake-up mode, a dormant receiver wakes up as soon as the
RDI line becomes idle. Idle is defined as a continuous logic high level on
the RDI line for ten (or eleven) full bit times. Systems using this type of
wake-up must provide at least one character time of idle between
messages to wake up sleeping receivers, but must not allow any idle
time between characters within a message.
11.4.2 Address Mark Wake-up
In address mark wake-up, the most significant bit (MSB) in a character
is used to indicate that the character is an address (1) or a data (0)
character. Sleeping receivers will wake up whenever an address
character is received. Systems using this method for wake-up would set
the MSB of the first character of each message and leave it clear for all
other characters in the message. Idle periods may be present within
messages and no idle time is required between messages for this wake-
up method.