H83834U Datasheet, PDF (310/497 Pages) Renesas Technology Corp – Single-Chip Microcomputer

2. Clock
The clock source is determined by bit COM in SMR and bits CKE1 and CKE0 in serial control
register 3 (SCR3). See table 10-4-9 for the settings. Either an internal clock source can be used to
run the built-in baud rate generator, or an external clock source can be input at pin SCK₃.
When an external clock source is input, it should have a frequency 16 times the desired bit rate.
When an internal clock source is used, SCK₃ is used as the clock output pin. The clock output has
the same frequency as the serial bit rate, and is synchronized as in figure 10-4-4 so that the rising
edge of the clock occurs in the center of each bit of transmit/receive data.
Clock
Serial
data
0 D0 D1 D2 D3 D4 D5 D6 D7 0/1 1 1
1 character (1 frame)
Figure 10-4-4 Phase Relation of Output Clock and Communication Data in Asynchronous
Mode (8-Bit Data, Parity Bit Added, and 2 Stop Bits)
3. Data transmit/receive operations
• SCI3 initialization
Before data is sent or received, bits TE and RE in serial control register 3 (SCR3) must be cleared
to 0, after which initialization can be performed using the procedure shown in figure 10-4-5.
Note:
When modifying the operation mode, transfer format or other settings, always be sure to clear bits
TE and RE first. When TE is cleared to 0, bit TDRE will be set to 1. Clearing RE does not clear
the status flags RDRF, PER, FER, or OER, or alter the contents of the receive data register (RDR).
When an external clock is used in asynchronous mode, do not stop the clock during operation,
including during initialization. When an external clock is used in synchronous mode, do not
supply the clock during initialization.
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