HD6473847RHV Datasheet, PDF (334/546 Pages) Renesas Technology Corp – Renesas 8-Bit Single-Chip Microcomputer H8 Family / H8/300L Super Low Power Series

Section 10 Serial Communication Interface 3 (SCI3)
10.7.6 Relation between Writing to TDR and Bit TDRE
Bit TDRE in the serial status register (SSR) is a status flag that indicates that data for serial
transmission has not been prepared in TDR. When data is written to TDR, bit TDRE is cleared to
0 automatically. When the SCI3 transfers data from TDR to TSR, bit TDRE is set to 1.
Data can be written to TDR irrespective of the state of bit TDRE, but if new data is written to
TDR while bit TDRE is cleared to 0, the data previously stored in TDR will be lost if it has not yet
been transferred to TSR. Accordingly, to ensure that serial transmission is performed dependably,
you should first check that bit TDRE is set to 1, then write the transmit data to TDR only once (not
two or more times).
10.7.7 Relation between RDR Reading and bit RDRF
In a receive operation, the SCI3 continually checks the RDRF flag. If bit RDRF is cleared to 0
when reception of one frame ends, normal data reception is completed. If bit RDRF is set to 1, this
indicates that an overrun error has occurred.
When the contents of RDR are read, bit RDRF is cleared to 0 automatically. Therefore, if RDR is
read more than once, the second and subsequent read operations will be performed while bit
RDRF is cleared to 0. Note that, when an RDR read is performed while bit RDRF is cleared to 0,
if the read operation coincides with completion of reception of a frame, the next frame of data may
be read. This is shown in figure 10.17.
Communication line
Frame 1
Data 1
Frame 2
Data 2
Frame 3
Data 3
RDRF
RDR
Data 1
Data 2
RDR read
(A) (B)
RDR read
Data 1 is read at point (A)
Data 2 is read at point (B)
Figure 10.17 Relation between RDR Read Timing and Data
In this case, only a single RDR read operation (not two or more) should be performed after first
checking that bit RDRF is set to 1. If two or more reads are performed, the data read the first time
Rev. 7.00 Mar. 08, 2010 Page 302 of 510
REJ09B0024-0700