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follow immediately, or the communication line can return to the Idle (high) state. The figure below
illustrates the possible frame formats. Brackets denote optional bits.
Figure 34-2. Frame Formats
Frame
(IDLE) St 0 1 2 3 4 [5] [6] [7] [8] [P] Sp1 [Sp2] [St/IDL]
St Start bit. Signal is always low.
n, [n] Data bits. 0 to [5..9]
[P] Parity bit. Either odd or even.
Sp, [Sp] Stop bit. Signal is always high.
IDLE No frame is transferred on the communication line. Signal is always high in this state.
34.6.2 Basic Operation
34.6.2.1 Initialization
The following registers are enable-protected, meaning they can only be written when the USART is
disabled (CTRL.ENABLE=0):
Control A register (CTRLA), except the Enable (ENABLE) and Software Reset (SWRST) bits.
Control B register (CTRLB), except the Receiver Enable (RXEN) and Transmitter Enable (TXEN)
bits.
Baud register (BAUD)
When the USART is enabled or is being enabled (CTRLA.ENABLE=1), any writing attempt to these
registers will be discarded. If the peripheral is being disabled, writing to these registers will be executed
after disabling is completed. Enable-protection is denoted by the "Enable-Protection" property in the
register description.
Before the USART is enabled, it must be configured by these steps:
1. Select either external (0x0) or internal clock (0x1) by writing the Operating Mode value in the
CTRLA register (CTRLA.MODE).
2. Select either Asynchronous (0) or Synchronous (1) Communication mode by writing the
Communication Mode bit in the CTRLA register (CTRLA.CMODE).
3. Select pin for receive data by writing the Receive Data Pinout value in the CTRLA register
(CTRLA.RXPO).
4. Select pads for the transmitter and external clock by writing the Transmit Data Pinout bit in the
CTRLA register (CTRLA.TXPO).
5. Configure the Character Size field in the CTRLB register (CTRLB.CHSIZE) for character size.
6. Set the Data Order bit in the CTRLA register (CTRLA.DORD) to determine MSB- or LSB-first data
transmission.
7. To use parity mode:
7.1. Enable Parity mode by writing 0x1 to the Frame Format field in the CTRLA register
(CTRLA.FORM).
SAM D5x/E5x Family Data Sheet
SERCOM USART - SERCOM Synchronous and Asyn...
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Datasheet
DS60001507E-page 926