Address
Data
Last data
S
ta
rt
S
ta
rt
S
ta
rt
Address frame
Data frame
Fewer than
10 idle bits
Idle period
One block of frames
Blocks separated by 10 or more idle bits
Blocks of frames
Data format
(pins SCIRX,
SCITX)
Data format
expanded
S
to
p
St
o
p
St
op
Data frame
P
a
ri
ty
P
a
ri
ty
P
a
ri
ty
SCI Communication Formats
1330
SPNU503C – March 2018
Copyright © 2018, Texas Instruments Incorporated
Serial Communication Interface (SCI) Module
26.2.4.1 Idle-Line Multiprocessor Modes
In idle-line multiprocessor mode, a frame that is preceded by an idle period (10 or more idle bits) is an
address frame. A frame that is preceded by fewer than 10 idle bits is a data frame.
illustrates
the format of several blocks and frames with idle-line mode.
There are two ways to transmit an address frame using idle-line mode:
Method 1:
In software, deliberately leave an idle period between the transmission of the last data frame of
the previous block and the address frame of the new block.
Method 2:
Configure the SCI to automatically send an idle period between the last data frame of the
previous block and the address frame of the new block.
Although Method 1 is only accomplished by a delay loop in software, Method 2 can be implemented by
using the transmit buffer and the TXWAKE bit in the following manner:
Step 1 : Write a 1 to the TXWAKE bit.
Step 2 : Write a dummy data value to the SCITD register. This triggers the SCI to begin the idle period as
soon as the transmitter shift register is empty.
Step 3 : Wait for the SCI to clear the TXWAKE flag.
Step 4 : Write the address value to SCITD.
As indicated by Step 3, software should wait for the SCI to clear the TXWAKE bit. However, the SCI
clears the TXWAKE bit at the same time it sets TXRDY (that is, transfers data from SCITD into
SCITXSHF). Therefore, if the TX INT ENA bit is set, the transfer of data from SCITD to SCITXSHF causes
an interrupt to be generated at the same time that the SCI clears the TXWAKE bit. If this interrupt method
is used, software is not required to poll the TXWAKE bit waiting for the SCI to clear it.
When idle-line multiprocessor communications are used, software must ensure that the idle time exceeds
10 bit periods before addresses (using one of the methods mentioned above), and software must also
ensure that data frames are written to the transmitter quickly enough to be sent without a delay of 10 bit
periods between frames. Failure to comply with these conditions will result in data interpretation errors by
other devices receiving the transmission.
Figure 26-4. Idle-Line Multiprocessor Communication Format