HD6433640 HITACHI [Hitachi Semiconductor], HD6433640 Datasheet - Page 351

• Manufacturer Part Number: HD6433640
• Description: H8/3644 Series Hardware Manual
• Manufacturer: HITACHI [Hitachi Semiconductor]

7. Relation between RDR reads and bit RDRF
Communication
line
RDRF
RDR
In a receive operation, 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 bit
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 illustrated in figure 10.27.
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 should be transferred to RAM, etc., and the RAM contents used. Also, ensure that
there is sufficient margin in an RDR read operation before reception of the next frame is
completed. To be precise in terms of timing, the RDR read should be completed before bit 7 is
transferred in synchronous mode, or before the STOP bit is transferred in asynchronous mode.
Figure 10.27 Relation between RDR Read Timing and Data
Frame 1
Data 1
RDR read
Frame 2
Data 2
Data 1
(A)
RDR read
Data 1 is read at point
Data 2 is read at point
(B)
Frame 3
Data 3
Data 3
(B)
(A)
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