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14.5
Usage Notes
Note the following points when using the SCI.
SCTDR Writing and TDRE Flag: The TDRE bit in the serial status register (SCSSR) is a status
flag indicating loading of transmit data from SCTDR into SCTSR. The SCI sets TDRE to 1 when
it transfers data from SCTDR to SCTSR. Data can be written to SCTDR regardless of the TDRE
bit state. If new data is written in SCTDR when TDRE is 0, however, the old data stored in
SCTDR will be lost because the data has not yet been transferred to SCTSR. Before writing
transmit data to SCTDR, be sure to check that TDRE is set to 1.
Simultaneous Multiple Receive Errors: Table 14.14 indicates the state of SCSSR status flags
when multiple receive errors occur simultaneously. When an overrun error occurs, the SCRSR
contents cannot be transferred to SCRDR, so receive data is lost.
Table 14.14 SCSSR Status Flags and Transfer of Receive Data
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 1
X: Receive data is not transferred from SCRSR to SCRDR.
O: Receive data is transferred from SCRSR to SCRDR.
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 FER is set and the parity error flag (PER) may also be set. In the break state, the SCI
receiver continues to operate, so if the FER bit is cleared to 0, it will be set to 1 again.
Sending a Break Signal: The TxD pin I/O condition and level can be determined by means of the
SCP0DT bit in the port SC data register (SCPDR) and bits SCP0MD0 and SCP0MD1 in the port
SC control register (SCPCR). This feature can be used to send breaks. To send a break during
serial transmission, clear the SCP0DT bit to 0 (designating low level), then clear the TE bit to 0
(halting transmission). When the TE bit is cleared to 0, the transmitter is initialized regardless of
the current transmission state, and 0 is output from the TxD pin.
490
SCSSR Status Flags
RDRF
ORER
FER PER
1
1
0
0
0
1
0
0
0
1
1
1
1
1
0
0
0
1
1
1
Receive Data Transfer
SCRSR → SCRDR
0
X
0
O
1
O
0
X
1
X
1
O
1
X
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