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22.5
Usage Notes
The following points should be noted when using the SCI.
• Relation between Writes to TDR1 and the TDRE Flag
The TDRE flag in SSR1 is a status flag that indicates that transmit data has been transferred
from TDR1 to TSR1. When the SCI transfers data from TDR1 to TSR1, the TDRE flag is set
to 1.
Data can be written to TDR1 regardless of the state of the TDRE flag. However, if new data is
written to TDR1 when the TDRE flag is cleared to 0, the data stored in TDR1 will be lost since
it has not yet been transferred to TSR1. It is therefore essential to check that the TDRE flag is
set to 1 before writing transmit data to TDR1.
• Operation when Multiple Receive Errors Occur Simultaneously
If a number of receive errors occur at the same time, the state of the status flags in SSR1 is as
shown in table 22.13. If there is an overrun error, data is not transferred from RSR1 to RDR1,
and the receive data is lost.
Table 22.13 State of SSR1 Status Flags and Transfer of Receive Data
SSR1 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
Notes:
: Receive data is transferred from RSR1 to RDR1.
X: Receive data is not transferred from RSR1 to RDR1.
Receive Data Transfer
PER
RSR1 to RDR1
0
X
0
1
0
X
1
X
1
1
X
Receive Error Status
Overrun error
Framing error
Parity error
Overrun error + framing error
Overrun error + parity error
Framing error + parity error
Overrun error + framing error +
parity error
Rev. 1.0, 02/00, page 451 of 1141
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