HD64F2145 Datasheet, PDF (382/829 Pages) Renesas Technology Corp – Hitachi 16-Bit Single-Chip Microcomputer

15.3.1 Receive Shift Register (RSR)
RSR is a shift register used to receive serial data that converts 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.
15.3.2 Receive Data Register (RDR)
RDR is an 8-bit register that stores receive 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 can
receive the next data. Since RSR and RDR function as a double buffer in this way, continuous
receive operations can be performed. After confirming that the RDRF bit in SSR is set to 1, read
RDR for only once. RDR cannot be written to by the CPU. RDR is initialized to H’00.
15.3.3 Transmit Data Register (TDR)
TDR is an 8-bit register that stores transmit data. When the SCI detects that TSR is empty, it
transfers the transmit data written in TDR to TSR and starts transmission. The double-buffered
structures of TDR and TSR enables continuous serial transmission. If the next transmit data has
already been written to TDR when one frame of data is transmitted, 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 for only once after
confirming that the TDRE bit in SSR is set to 1. TDR is initialized to H’FF.
15.3.4 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.
Rev. 2.0, 08/02, page 342 of 788