
ML66517 Family User’s Manual
Chapter 12
Serial Port Functions
12 – 27
12.6 SIO1 and SIO6 Operations
12.6.1 Transmit Operation
UART mode
Figure 12-10 shows the timing diagram of operation during UART transmission.
The clock pulse from the baud rate generator (timer 3 or timer 4) or from an external input is divided by 16
to generate the transmit shift clock.
If an external clock is to be used with the UART mode, input the clock to the receive clock I/O pin (RXCn)
for SIOn. The externally input clock is processed as shown in figure 12-11, and is input to the 1/n dividing
counter as the baud rate clock.
In synchronization with the transmit shift clock that has been generated, the transmission circuit controls
transmission of the transmit data.
The SnBUF write signal (a signal that is output when an instruction to write to SnBUF is executed, for
example “STB A, SnBUF”) acts as a trigger to start transmission.
One CPU clock after the write signal is generated, transmit data in SnBUF is set in the transmit shift
register. At this time, synchronized to the signal indicating the beginning of an instruction (M1S1), a
transmit buffer empty signal is generated.
After the transmit data is set (after the fall of the data transfer signal to the transmit shift register),
synchronized to the falling edge of the next transmit shift clock, the start bit is output from the transmit data
output pin (TXDn). Thereafter, as specified by STnCON, the transmit data (LSB first), parity bit, and
finally the stop bit are output to complete the transmission of one frame.
At this time, if the next transmit data has not been written to SnBUF, a transmit complete signal is
generated in synchronization with M1S1, and the transmission is completed.
Because generation of the transmit shift clock is always unrelated to writes to SnBUF, from the time when
transmit data is written to SnBUF until the start bit is output, there is a delay of a maximum of 16 baud rate
clocks.
Because each of SIO1 and SIO6 has SnBUF and the transmit shift register which are designed in a duplex
construction, during a transmission it is possible to write the next transmit data to SnBUF. If SnBUF is
written to during a transmission, after the current one frame transmission is completed, the next transmit
data will be automatically set in the transmit shift register, and the data transmission will continue. After
one frame of data is transmit, if the next data to be transmit has been written to SnBUF, the transmit
complete signal will not be generated.
Figure 12-14 shows the timing diagram of operation during continuous transmission.