MCCIRM Freescale Semiconductor / Motorola, MCCIRM Datasheet - Page 68

• Manufacturer Part Number: MCCIRM
• Description: MCCIRM Multi-Channel Communications Interface Reference Manual
• Manufacturer: Freescale Semiconductor / Motorola

SCI Submodule
5.7.5 Queued Idle Characters
Reference Manual
68
If the transmitter is transmitting a character when software toggles SBK
on and off, exactly one break character is produced after the current
character is transmitted. If the transmitter is idle when SBK is toggled, it
is uncertain whether one or two break characters will be sent. When SBK
is set to 1, a break character is queued. When the transmit shift register
becomes available and synchronization requirements are met with
respect to the internal baud clock, the queued break character is
jammed into the shift register to be serially sent and, if the SBK bit is still
1, another break is queued. The transfer mechanism from the queue to
the shifter is internally synchronized to the baud clock; however, the
relationship of this clock to operating software is not normally known.
The instructions to write 1 and then write 0 to the SBK bit execute very
quickly relative to the normal baud-rate frequency, but there is still a
small chance that the baud-rate clock edge could occur between writing
the 1 and writing the 0 to SBK.
Idle characters are streams of 10 or 11 bits of logic 1 (depending on the
M bit in SCCR1). They are produced when the transmitter is enabled
from a disabled state (for instance, when TE is changed from 0 to 1).
When the transmitter is enabled, this idle character acts as a preamble.
The character-length period of logic 1 ensures that any receiver
connected to this transmitter will be resynchronized so that it can
properly recognize the leading edge of the start bit for the next character.
Software can queue an idle character into a serial data stream by
clearing and then setting TE. This queueing function is useful when
using the idle-line variation of receiver wakeup (WAKE = 0 in SCCR1).
In a multidrop SCI network, all receivers evaluate the first character or
characters of a message to decide whether or not this message is
important to this receiver. If not, receiver wakeup is invoked in the
receiver by writing a 1 to the RWU bit in SCCR1. A 1 in RWU causes the
receiver to ignore any other characters in the message, thus allowing the
MCU to perform more useful functions than responding to interrupts from
the SCI. The SCI receiver still monitors characters normally but does not
set status flags or generate interrupts. When idle-line wakeup is used,
the SCI receiver logic automatically clears RWU (waking up the receiver)
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SCI Submodule
Multichannel Communication Interface — Rev. 1.0
MOTOROLA