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MSM66577 Family User's Manual
Chapter 12 Serial Port Functions
12
[Slave mode]
Figure 12-13 shows the timing diagram of operation during slave mode transmission.
In the slave mode, the transmit clock is input from the transmit clock I/O pin (TXCn). This
external input clock is detected with the edge of CPU clock 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 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 the transmit shift clock that has been generated from
the detected edge of the externally input TXCn. Therefore, at the receive side, TXDn is
fetched at the rising edge of TXCn.
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 2 CPU clocks between the current
data transmission and the data next transmission, in which to set the next data. During
this interval, TXDn is forced to a High level. If an external clock is supplied, insert a margin
of 2 or more CPU clocks between the current data transmission and the next data
transmission.