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ML66525 Family User’s Manual
Chapter 11
Serial Port Functions
11 - 28
Synchronous mode
[Master mode]
Figure 11-12 shows the timing diagram of operation during master mode transmission.
The clock pulse from the baud rate generator (timer 3 for SIO6 and timer 4 for SIO1) 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, S1BUF”) acts as a trigger to start transmission.
One CPU clock after the write signal is generated, transmit data in S1BUF 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 S1BUF, 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 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.
[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.