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Functional Description
Serial Communications Interface (SCI), Rev. 4
Freescale Semiconductor
10-19
The maximum percent difference between the receiver count and the transmitter count of a fast
9-bit character with no errors is:
10.6.4.8 Receiver Wake Up
In order for the SCI to ignore transmissions intended only for other receivers in multiple- receiver
systems, the receiver can be placed into a standby state. Setting the Receiver Wake Up (RWU)
bit in the SCI Control Register (SCICR) places the receiver into a standby state while receiver
interrupts are disabled.
The transmitting device can address messages to selected receivers by including addressing
information in the initial frame or frames of each message.
The WAKE bit in the SCI Control Register (SCICR) determines how the SCI is brought out of
the standby state to process an incoming message. The WAKE bit enables either idle line wake
up or address mark wake up:
Idle Input Line Wake Up (WAKE = 0)—In this wake up method, an idle condition on the
RXD pin clears the RWU bit and wakes up the SCI. The initial frame or frames of every
message contain addressing information. All receivers evaluate the addressing information,
and receivers for which the message is addressed process the following frames. Any
receiver for which a message is not addressed can set its RWU bit and return to the standby
state. The RWU bit remains set and the receiver remains on standby until another preamble
appears on the RXD pin.
Idle line wake up requires messages be separated by at least one preamble and no message
contains preambles.
The preamble waking a receiver does not set the receiver Idle (IDLE) bit or the Receive Data
Register Full (RDRF) flag.
Address Mark Wake up (WAKE = 1)—In this wake up method, a Logic 1 in the Most
Significant Bit (MSB) position of a frame clears the RWU bit and wakes up the SCI. The
Logic 1 in the MSB position marks a frame as an address frame, containing the addressing
information. All receivers evaluate the addressing information as well as the receivers for
which the message is addressed in the following frames. Any receiver for which a message
is not addressed can set its RWU bit and return to the standby state. The RWU bit remains
set and the receiver remains on standby until another address frame appears on the RXD pin.
The Logic 1 MSB of an address frame clears the receiver’s RWU bit before the Stop bit is
received, setting the RDRF flag.
170
-----------------------
176
–
170
100
×
3.53%
=