HITACHI 379
1. The SCI monitors the TDRE bit in the SSR. When TDRE is cleared to 0 the SCI recognizes
that the transmit data register (TDR) contains new data, and loads this data from the TDR into
the transmit shift register (TSR).
2. After loading the data from the TDR into the TSR, the SCI sets the TDRE bit to 1 and starts
transmitting. If the transmit-data-empty interrupt enable bit (TIE) in the SCR is set to 1, the
SCI requests a transmit-data-empty interrupt (TXI) at this time.
Serial transmit data is transmitted in the following order from the TxD pin (figure 13.11):
1. Start bit: one 0 bit is output.
2. Transmit data: seven or eight bits are output, LSB first.
3. Multiprocessor bit: one multiprocessor bit (MPBT value) is output.
4. Stop bit: one or two 1 bits (stop bits) are output.
5. Mark state: output of 1 bits continues until the start bit of the next transmit data.
6. The SCI checks the TDRE bit when it outputs the stop bit. If TDRE is 0, the SCI loads data
from the TDR into the TSR, outputs the stop bit, then begins serial transmission of the next
frame. If TDRE is 1, the SCI sets the TEND bit in the SSR to 1, outputs the stop bit, then
continues output of 1 bits in the mark state. If the transmit-end interrupt enable bit (TEIE) in
the SCR is set to 1, a transmit-end interrupt (TEI) is requested at this time.
TDRE
TEND
TXI
request
TXI interrupt
handler writes
data in TDR and
clears TDRE to 0
TXI
request
TEI
request
1 frame
0
1
1
1
0/1
0
1
Multi-
processor
bit
Serial
data
Start
bit
Data
Stop
bit
Start
bit
Data
Stop
bit
Idle (mark
state)
D
0
D
1
D
7
D
0
D
1
D
7
0/1
Multi-
processor
bit
Figure 13.11 Example of SCI Multiprocessor Transmit Operation (8-bit data with
multiprocessor bit and one stop bit)