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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 17.1 (p. 447) . Full duplex and half duplex
communication is supported in both asynchronous and synchronous mode.
Table 17.1. USART Asynchronous vs. Synchronous Mode
SYNC
0
1
Communication Mode
Asynchronous
Synchronous
Supported Protocols
RS-232, RS-485 (w/external driver), IrDA, ISO 7816
SPI, MicroWire, 3-wire
Table 17.2 (p. 447) explains the functionality of the different USART pins when the USART operates
in different modes. Pin functionality enclosed in square brackets is optional, and depends on additional
configuration parameters. LOOPBK and MASTER are discussed in Section 17.3.2.5 (p. 455) and
Section 17.3.3.3 (p. 462) respectively.
Table 17.2. USART Pin Usage
SYNC
0
1
1
1
1
1
LOOPBK MASTER
0
x
1
x
0
0
0
1
1
0
1
1
U(S)n_TX
(MOSI)
Data out
Data out/in
Data in
Data out
Data out/in
Data out/in
Pin functionality
U(S)n_RX (MISO) USn_CLK
Data in
-
Data out
Data in
-
-
-
-
Clock in
Clock out
Clock in
Clock out
USn_CS
[Driver enable]
[Driver enable]
Slave select
[Auto slave select]
Slave select
[Auto slave select]
17.3.2 Asynchronous Operation
17.3.2.1 Frame Format
The frame format used in asynchronous mode consists of a set of data bits in addition to bits for
synchronization and optionally a parity bit for error checking. A frame starts with one start-bit (S), where
the line is driven low for one bit-period. This signals the start of a frame, and is used for synchronization.
Following the start bit are 4 to 16 data bits and an optional parity bit. Finally, a number of stop-bits, where
the line is driven high, end the frame. An example frame is shown in Figure 17.2 (p. 447) .
Figure 17.2. USART Asynchronous Frame Format
St op or idle
S
0
1
2
3
Fram e
4
[5]
[ 6] [ 7] [ 8] ^{[ P]}
St op
St art or idle
The number of data bits in a frame is set by DATABITS in USARTn_FRAME, see Table 17.3 (p. 448)
, and the number of stop-bits is set by STOPBITS in USARTn_FRAME, see Table 17.4 (p. 448) .
Whether or not a parity bit should be included, and whether it should be even or odd is defined by
PARITY, also in USARTn_FRAME. For communication to be possible, all parties of an asynchronous
transfer must agree on the frame format being used.
2013-05-08 - Wonder Gecko Family - d0233_Rev0.50
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