
27. Usage Notes
Rev.1.00
27.11 Serial Interfaces
27.11.1 Changing UiBRG Register (i = 0 to 6)
Set the UiBRG register after setting bits CLK1 and CLK0 in the UiC0 register. When bits CLK1 and CLK0 are
changed, the UiBRG register must be set again.
27.11.2 Clock Synchronous Serial I/O Mode
When bits INV03 and INV02 in the INVC0 register are set to 11b (forced cutoff of the three-phase output by an
“L” signal applied to the NMI pin), pins RTS2 and CLK2 are placed in high-impedance states by applying a
low-level (“L”) signal to the NMI pin.
27.11.2.1 Transmit Operation/Receive Operation
When the RTS function is used with an external clock, RTSi pin (i = 0 to 6) outputs “L”, which informs the
transmitter that the MCU is ready for a receive operation. The RTSi pin outputs high (“H”) when a receive
operation starts. Therefore, a transmit timing and receive timing can be synchronized by connecting the RTSi
pin to the CTSi pin of the transmitter. The RTS function is disabled when an internal clock is selected.
27.11.2.2 Transmit Operation
If an external clock is selected, the following conditions must be met while the external clock is held “H” when
the CKPOL bit in the UiC0 register (i = 0 to 6) is set to 0 (transmit data output at the falling edge and receive
data input at the rising edge of the serial clock), or while the external clock is held “L” when the CKPOL bit is
set to 1 (transmit data output at the rising edge and receive data input at the falling edge of the serial clock)
Set the TE bit in the UiC1 register to 1 (transmit operation enabled).
The TI bit in the UiC1 register is 0 (data in the UiTB register).
Apply an “L” signal to the CTSi pin if the CTS function is selected.
27.11.2.3 Receive Operation
In clock synchronous serial I/O mode, the source clock for the UARTi transmit register and the UARTi receive
register is generated by activating a transmitter. Set the UARTi-associated registers for a transmit operation
even if the MCU is used for receive operation only. Dummy data is output from the TXDi pin while receiving
if the TXDi pin is set in output mode.
When an internal clock is selected, the source clock for the UARTi transmit register and the UARTi receive
register is generated by setting the TE bit in the UiC1 register (i = 0 to 6) to 1 (transmit operation enabled) and
placing dummy data in the UiTB register.
When an external clock is selected, set the TE bit to 1 (transmit operation enabled), place dummy data in the
UiTB register, and input an external clock to the CLKi pin to generate the source clock.
If data is received continuously, an overrun error occurs when the RI bit in the UiC1 register is 1 (data in the
UiRB register) and the next data is received in the UARTi receive register. And then, the OER bit in the UiRB
register becomes 1 (overrun error occurred). At this time, the UiRB register is undefined. If an overrun error
occurs, set up both transmitter and receiver by programs to retransmit the last data. When an overrun error
occurs, the IR bit in the SiRIC register remains unchanged.
To receive data continuously, set dummy data in the low-order byte in the UiTB register per each receive
operation.
If an external clock is selected, the following conditions must be met while the external clock is held “H” when
the CKPOL bit is set to 0 (transmit data output at the falling edge and receive data input at the rising edge of the
serial clock), or while the external clock is held “L” when the CKPOL bit is set to 1 (transmit data output at the
rising edge and receive data input at the falling edge of the serial clock)
Set the RE bit in the UiC1 register to 1 (receive operation enabled).
Set the TE bit in the UiC1 register to 1 (transmit operation enabled).
The TI bit in the UiC1 register is 0 (data in the UiTB register).