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15.5
Usage Notes
The following points should be noted when using the SCI.
SCTDR1 Writing and the TDRE Flag: The TDRE flag in SCSSR1 is a status flag that indicates
that transmit data has been transferred from SCTDR1 to SCTSR1. When the SCI transfers data
from SCTDR1 to SCTSR1, the TDRE flag is set to 1.
Data can be written to SCTDR1 regardless of the state of the TDRE flag. However, if new data is
written to SCTDR1 when the TDRE flag is cleared to 0, the data stored in SCTDR1 will be lost
since it has not yet been transferred to SCTSR1. It is therefore essential to check that the TDRE
flag is set to 1 before writing transmit data to SCTDR1.
Simultaneous Multiple Receive Errors: If a number of receive errors occur at the same time, the
state of the status flags in SCSSR1 is as shown in table 15.13. If there is an overrun error, data is
not transferred from SCRSR1 to SCRDR1, and the receive data is lost.
Table 15.13 SCSSR1 Status Flags and Transfer of Receive Data
Receive Errors
Overrun error
Framing error
Parity error
Overrun error + framing error
Overrun error + parity error
Framing error + parity error
Overrun error + framing error +
parity error
O: Receive data is transferred from SCRSR1 to SCRDR1.
X: Receive data is not transferred from SCRSR1 to SCRDR1.
Break Detection and Processing: Break signals can be detected by reading the RxD pin directly
when a framing error (FER) is detected. In the break state the input from the RxD pin consists of
all 0s, so the FER flag is set and the parity error flag (PER) may also be set. Note that the SCI
receiver continues to operate in the break state, so if the FER flag is cleared to 0 it will be set to 1
again.
Rev. 3.0, 04/02, page 628 of 1064
SCSSR1 Status Flags
RDRF
ORER
FER
1
1
0
0
0
1
0
0
0
1
1
1
1
1
0
0
0
1
1
1
1
Receive Data
Transfer
SCRSR1 SCRDR1
PER
0
X
0
O
1
O
0
X
1
X
1
O
1
X
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