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In serial transmission, the SCI operates as described below.
[1] The SCI monitors the TDRE flag in SSR, and if is 0, recognizes that data has been written to
TDR, and transfers the data from TDR to TSR.
[2] After transferring data from TDR to TSR, the SCI sets the TDRE flag to 1 and starts
transmission.
If the TIE bit is set to 1 at this time, a transmit data empty interrupt (TXI) is generated.
The serial transmit data is sent from the TxD pin in the following order.
[a] Start bit:
One 0-bit is output.
[b] Transmit data:
8-bit or 7-bit data is output in LSB-first order.
[c] Parity bit or multiprocessor bit:
One parity bit (even or odd parity), or one multiprocessor bit is output.
A format in which neither a parity bit nor a multiprocessor bit is output can also be
selected.
[d] Stop bit(s):
One or two 1-bits (stop bits) are output.
[e] Mark state:
1 is output continuously until the start bit that starts the next transmission is sent.
[3] The SCI checks the TDRE flag at the timing for sending the stop bit.
If the TDRE flag is cleared to 0, the data is transferred from TDR to TSR, the stop bit is sent,
and then serial transmission of the next frame is started.
If the TDRE flag is set to 1, the TEND flag in SSR is set to 1, the stop bit is sent, and then the
"mark state" is entered in which 1 is output continuously. If the TEIE bit in SCR is set to 1 at
this time, a TEI interrupt request is generated.
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