Hitachi H8/329 Series Hardware Manual page 209

In transmitting serial data, the SCI operates as follows.
1. The SCI monitors the TDRE bit in SSR. When TDRE is cleared to "0" the SCI recognizes that
the transmit data register (TDR) contains new data, and loads this data from TDR into the
transmit shift register (TSR).
2. After loading the data from TDR into TSR, the SCI sets the TDRE bit to "1" and starts
transmitting. If the TIE bit (TDR-empty interrupt enable) in SCR is set to "1," the SCI requests
a TXI interrupt (TDR-empty interrupt) at this time.
If clock output is selected the SCI outputs eight serial clock pulses, triggered by the clearing of
the TDRE bit to "0." If an external clock source is selected, the SCI outputs data in
synchronization with the input clock.
Data are output from the TxD pin in order from LSB (bit 0) to MSB (bit 7).
3. The SCI checks the TDRE bit when it outputs the MSB (bit 7). If TDRE is "0," the SCI loads
data from TDR into TSR, then begins serial transmission of the next frame. If TDRE is "1," the
SCI sets the TEND bit in SSR to "1," transmits the MSB, then holds the output in the MSB
state. If the TEIE bit (transmit-end interrupt enable) in SCR is set to "1," a TEI interrupt (TSR-
empty interrupt) is requested at this time.
4. After the end of serial transmission, the SCK pin is held at the high level.
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