
ML66517 Family User’s Manual
Chapter 12
Serial Port Functions
12 – 28
Synchronous mode (SIO1, SIO6)
[Master mode]
Figure 12-12 shows the timing diagram of operation during master mode transmission.
The clock pulse from the baud rate generator (timer 4 for SIO1 timer 3 for SIO6) is divided by 4 to generate
the transmit shift 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 (the 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 external output clock begins to be
output from the transmit clock I/O pin (TXCn). At the same time, transmit data is output LSB first from the
transmit data output pin (TXDn). Thereafter, as specified by STnCON and synchronized to the transmit
shift clock, transmit data is 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.
TXDn changes at the falling edge of TXCn. Therefore, at the receive side, TXDn is fetched at the rising
edge of TXCn.
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 first data is output, there is a delay of a maximum of 4 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.
[Note]
During continuous transmission, there is a time lag of 1 bit between the current data transmission and the
next data transmission, in which to set the next data. During this interval, TXDn is forced to a High level.