Clock
Either an internal clock generated by the on-chip baud rate generator or an external clock input at
the SCK pin can be selected as the SCI’s serial clock, according to the setting of the C/$ bit in
SCSMR1 and the CKE1 and CKE0 bits in SCSCR1. For details of SCI clock source selection, see
table 15.9.
When an external clock is input at the SCK pin, the clock frequency should be 16 times the bit rate
used.
When the SCI is operated on an internal clock, the clock can be output from the SCK pin. The
frequency of the clock output in this case is equal to the bit rate, and the phase is such that the
rising edge of the clock is at the center of each transmit data bit, as shown in figure 15.6.
0
D0 D1 D2
D3 D4 D5 D6 D7 0/1
One frame
1
1
Figure 15.6 Relation between Output Clock and Transfer Data Phase
(Asynchronous Mode)
Data Transfer Operations
SCI Initialization (Asynchronous Mode): Before transmitting and receiving data, it is necessary
to clear the TE and RE bits in SCSCR1 to 0, then initialize the SCI as described below.
When the operating mode, transfer format, etc., is changed, the TE and RE bits must be cleared to
0 before making the change using the following procedure. When the TE bit is cleared to 0, the
TDRE flag is set to 1 and SCTSR1 is initialized. Note that clearing the RE bit to 0 does not change
the contents of the RDRF, PER, FER, and ORER flags, or the contents of SCRDR1.
When an external clock is used the clock should not be stopped during operation, including
initialization, since operation will be unreliable in this case.
Figure 15.7 shows a sample SCI initialization flowchart.
Rev. 6.0, 07/02, page 625 of 986
RENESAS [ RENESAS TECHNOLOGY CORP ]
