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flag is automatically reset when software reads the character
from the UnRBUF register. The RSFT register is not user ac-
cessible.
18.2.2
The synchronous mode of the USART enables the device to
communicate with other devices using three communication
signals: transmit, receive, and clock. In this mode, data bits
are transferred synchronously with the USART clock signal.
Data bits are transmitted on the rising edges and received on
the falling edges of the clock signal, as shown in Figure 34.
Synchronous Mode
Data bytes are transmitted and received least significant bit
(LSB) first.
In the synchronous mode, the transmit shift register (TSFT)
and the transmit buffer (UnTBUF) double-buffer the data for
transmission. To transmit a character, a data byte is loaded
in the UnTBUF register. The data is then transferred to the
TSFT register. The TSFT register shifts out one bit of the cur-
rent character, LSB first, on each rising edge of the clock.
Figure 32.
USART Block Diagram
I
Sys_clk
Baud clock
Baud Clock
TDXn
RDXn
Transmitter
Receiver
Baud Rate Generator
Control and
Error Detection
Parity
Generator/Checker
CKXn
Figure 33.
USART Asynchronous Communication
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
1
16
Sample
Sample
STARTBIT
DATA (LSB)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
1
16
Sample
DATABIT