
ML66517 Family User’s Manual
Chapter 12
Serial Port Functions
12 – 36
Synchronous mode
[Master mode]
Figure 12-16 shows the timing diagram of operation during master mode reception.
The clock pulse from the baud rate generator (timer 4 for SIO1 and timer 3 for SIO6) is divided by 4 to
generate the external output clock. The 3rd pulse of the 1/4 divider (value 2 of the baud rate (1/4) counter in
figure 12-16) becomes the sampling clock for the receive data input pin (RXDn). The 4th pulse (value 3 of
the baud rate (1/4) counter in figure 12-16) becomes the receive shift clock.
In synchronization with the receive shift clock, the reception circuit controls reception of the receive data.
The falling edge of the receive shift clock immediately after SRnREN (bit 7) of SRnCON is set to “1”
triggers the reception operation to start and the external output clock is output from the receive clock I/O
pin (RXCn). At the next receive shift clock, receive data that was sampled at the prior sampling clock is
shifted into the receive shift register.
At the falling edge of the external output clock, the transmit side transmits data. That data is shifted into the
receive side at the falling edge of the transmit shift clock. Receive data is sampled only once. Thereafter,
data reception continues as specified by SRnCON. After the last receive shift clock is output, the contents
of the receive shift register are transferred to SnBUF, and a receive complete signal is generated in
synchronization with M1S1, the signal that indicates the beginning of an instruction. At this time, an
overrun error will be generated if the previously received data has not been read (the previously received
data will be overwritten).
Finally, SRnREN of SRnCON is automatically cleared to “0” to complete the reception series.
[Slave mode]
Figure 12-17 shows the timing diagram of operation during slave mode reception.
In the slave mode, the receive clock is input externally (from the receive clock I/O pin (RXCn)). This
external input clock is detected with the edge of CPU clock to generate the receive shift clock.
In synchronization with the receive shift clock that has been generated, the reception circuit controls
receiving the receive data.
Reception operation is triggered to begin when SRnREN (bit 7) of SRnCON is set to “1” and the external
input clock is input to the receive clock I/O pin (RXCn).
While the external input clock is at a Low level, the value of the receive data input pin (RXDn) is sampled.
The sampled receive data is shifted into the receive shift register at the next receive shift clock. Thereafter,
data reception continues as specified by SRnCON. After the last receive data is shifted in, the contents of
the receive shift register are transferred to SnBUF, and a receive complete signal is generated in
synchronization with M1S1, the signal that indicates the beginning of an instruction. At this time, an
overrun error will be generated if the previously received data has not been read (the previously received
data will be overwritten). This completes a one frame reception.
In the slave mode, SRnREN is not automatically cleared to “0” after completing the reception.
If the
receive shift clock continues to be input, the receive operation will restart.