892
11054A–ATARM–27-Jul-11
SAM9X25
892
11054A–ATARM–27-Jul-11
SAM9X25
Figure 40-8.
Receiver Framing Error
40.5.3
Transmitter
40.5.3.1
Transmitter Reset, Enable and Disable
After device reset, the UART transmitter is disabled and it must be enabled before being used.
The transmitter is enabled by writing the control register UART_CR with the bit TXEN at 1. From
this command, the transmitter waits for a character to be written in the Transmit Holding Register
(UART_THR) before actually starting the transmission.
The programmer can disable the transmitter by writing UART_CR with the bit TXDIS at 1. If the
transmitter is not operating, it is immediately stopped. However, if a character is being pro-
cessed into the Shift Register and/or a character has been written in the Transmit Holding
Register, the characters are completed before the transmitter is actually stopped.
The programmer can also put the transmitter in its reset state by writing the UART_CR with the
bit RSTTX at 1. This immediately stops the transmitter, whether or not it is processing
characters.
40.5.3.2
Transmit Format
The UART transmitter drives the pin UTXD at the baud rate clock speed. The line is driven
depending on the format defined in the Mode Register and the data stored in the Shift Register.
One start bit at level 0, then the 8 data bits, from the lowest to the highest bit, one optional parity
bit and one stop bit at 1 are consecutively shifted out as shown in the following figure. The field
PARE in the mode register UART_MR defines whether or not a parity bit is shifted out. When a
parity bit is enabled, it can be selected between an odd parity, an even parity, or a fixed space or
mark bit.
Figure 40-9.
Character Transmission
40.5.3.3
Transmitter Control
When the transmitter is enabled, the bit TXRDY (Transmitter Ready) is set in the status register
UART_SR. The transmission starts when the programmer writes in the Transmit Holding Regis-
ter (UART_THR), and after the written character is transferred from UART_THR to the Shift
D0
D1
D2
D3
D4
D5
D6
D7
P
S
URXD
RSTSTA
RXRDY
FRAME
Stop Bit
Detected at 0
stop
D0
D1
D2
D3
D4
D5
D6
D7
UTXD
Start
Bit
Parity
Bit
Stop
Bit
Example: Parity enabled
Baud Rate
Clock
Summary of Contents for SAM9X25
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