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Rev. 1.0, 07/01, page 184 of 372
14.3.1 Receive Shift Register (RSR)
RSR is a shift register that is used to receive serial data input from the RxD pin and convert it into
parallel data. When one frame of data has been received, it is transferred to RDR automatically.
RSR cannot be directly accessed by the CPU.
14.3.2 Receive Data Register (RDR)
RDR is an 8-bit register that stores received data. When the SCI has received one frame of serial
data, it transfers the received serial data from RSR to RDR, where it is stored. After this, RSR is
receive-enabled. As RSR and RDR function as a double buffer in this way, continuous receive
operations are possible. After confirming that the RDRF bit in SSR is set to 1, read RDR only
once. RDR cannot be written to by the CPU. RDR is initialized to H'00.
14.3.3 Transmit Shift Register (TSR)
TSR is a shift register that transmits serial data.
To perform serial data transmission, the SCI first
transfers transmit data from TDR to TSR automatically, then sends the data that starts from the
LSB to the TXD pin
.
TSR cannot be directly accessed by the CPU.
14.3.4 Transmit Data Register (TDR)
TDR is an 8-bit register that stores data for transmission. When the SCI detects that TSR is empty,
it transfers the transmit data written in TDR to TSR and starts transmission. The double-buffered
structure of TDR and TSR enables continuous serial transmission. If the next transmit data has
already been written to TDR during transmission of one-frame data, the SCI transfers the written
data to TSR to continue transmission. To achieve reliable serial transmission, write transmit data
to TDR only once after confirming that the TDRE bit in SSR is set to 1.