
18.3.2.1 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 18.2 USART Asynchronous Frame Format on
.
S
0
1
2
3
4
[5]
[6]
[7]
[8]
[P]
Stop
Start or idle
Stop or idle
Frame
Figure 18.2. USART Asynchronous Frame Format
The number of data bits in a frame is set by DATABITS in USARTn_FRAME, see
Table 18.3 USART Data Bits on page 530
, and the
number of stop-bits is set by STOPBITS in USARTn_FRAME, see
Table 18.4 USART Stop Bits on page 530
. 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.
Table 18.3. USART Data Bits
DATA BITS [3:0]
Number of Data bits
0001
4
0010
5
0011
6
0100
7
0101
8 (Default)
0110
9
0111
10
1000
11
1001
12
1010
13
1011
14
1100
15
1101
16
Table 18.4. USART Stop Bits
STOP BITS [1:0]
Number of Stop bits
00
0.5
01
1 (Default)
10
1.5
11
2
Reference Manual
USART - Universal Synchronous Asynchronous Receiver/Transmitter
silabs.com
| Building a more connected world.
Rev. 1.1 | 530