18.3.1 Modes of Operation
The USART operates in either asynchronous or synchronous mode.
In synchronous mode, a separate clock signal is transmitted with the data. This clock signal is generated by the bus master, and both
the master and slave sample and transmit data according to this clock. Both master and slave modes are supported by the USART. The
synchronous communication mode is compatible with the Serial Peripheral Interface Bus (SPI) standard.
In asynchronous mode, no separate clock signal is transmitted with the data on the bus. The USART receiver thus has to determine
where to sample the data on the bus from the actual data. To make this possible, additional synchronization bits are added to the data
when operating in asynchronous mode, resulting in a slight overhead.
Asynchronous or synchronous mode can be selected by configuring SYNC in USARTn_CTRL. The options are listed with supported
protocols in
Table 18.1 USART Asynchronous Vs. Synchronous Mode on page 529
. Full duplex and half duplex communication is
supported in both asynchronous and synchronous mode.
Table 18.1. USART Asynchronous Vs. Synchronous Mode
SYNC
Communication Mode
Supported Protocols
0
Asynchronous
RS-232, RS-485 (w/external driver), IrDA, ISO 7816
1
Synchronous
SPI, MicroWire, 3-wire
Table 18.2 USART Pin Usage on page 529
explains the functionality of the different USART pins when the USART operates in differ-
ent modes. Pin functionality enclosed in square brackets is optional, and depends on additional configuration parameters. LOOPBK and
MASTER are discussed in
respectively.
Table 18.2. USART Pin Usage
SYNC
LOOPBK
MASTER
Pin functionality
U(S)n_TX (MOSI)
U(S)n_RX (MISO)
USn_CLK
USn_CS
0
0
x
Data out
Data in
-
[Driver enable]
0
1
x
Data out/in
-
-
[Driver enable]
1
0
0
Data in
Data out
Clock in
Slave select
1
0
1
Data out
Data in
Clock out
[Auto slave select]
1
1
0
Data out/in
-
Clock in
Slave select
1
1
1
Data out/in
-
Clock out
[Auto slave select]
18.3.2 Asynchronous Operation
Reference Manual
USART - Universal Synchronous Asynchronous Receiver/Transmitter
silabs.com
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