Receive Data Register (Rdr); Transmit Data Register (Tdr); Transmit Shift Register (Tsr) - Hitachi H8S/2215 Series Hardware Manual

13.3.2

Receive Data Register (RDR)

RDR is an 8-bit register that stores received data. When the SCI has received one byte 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.
13.3.3

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 serial transmission, the SCI transfers the written data to TSR
to continue transmission. Although TDR can be read from or written to by the CPU at all times, to
achieve reliable serial transmission, write transmit data to TDR only once after confirming that the
TDRE bit in SSR is set to 1.
13.3.4

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, then sends the data to the TxD pin. TSR cannot be
directly accessed by the CPU.
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