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ML66525 Family User’s Manual
Chapter 11
Serial Port Functions
11 - 27
11.6 SIO6, SIO1 Operation
11.6.1 Transmit Operation
UART mode
Figure 11-10 shows the timing diagram of operation during UART transmission.
The clock pulse from the baud rate generator (timer 3 for SIO6 and timer 4 for SIO1) 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 (RXC1, 6) for SIO1, 6.
The externally input clock is processed as shown in figure 11-11,
and is input to the 1/n dividing counter (1/16 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 SIO6 and SIO1 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 transmitted, if the
next data to be transmit has been written to SnBUF, the transmit complete signal will not be
generated.
Figure 11-14 shows the timing diagram of operation during continuous transmission.