Datasheet

583
SAM9G45 [DATASHEET]
Atmel-6438O-ATARM-SAM9G45-Datasheet_08-Dec-15
Figure 32-6. Character Transmit
The characters are sent by writing in the Transmit Holding Register (US_THR). The transmitter reports two status
bits in the Channel Status Register (US_CSR): TXRDY (Transmitter Ready), which indicates that US_THR is
empty and TXEMPTY, which indicates that all the characters written in US_THR have been processed. When the
current character processing is completed, the last character written in US_THR is transferred into the Shift
Register of the transmitter and US_THR becomes empty, thus TXRDY rises.
Both TXRDY and TXEMPTY bits are low when the transmitter is disabled. Writing a character in US_THR while
TXRDY is low has no effect and the written character is lost.
Figure 32-7. Transmitter Status
32.7.3.2 Manchester Encoder
When the Manchester encoder is in use, characters transmitted through the USART are encoded based on
biphase Manchester II format. To enable this mode, set the MAN field in the US_MR to 1. Depending on polarity
configuration, a logic level (zero or one), is transmitted as a coded signal one-to-zero or zero-to-one. Thus, a
transition always occurs at the midpoint of each bit time. It consumes more bandwidth than the original NRZ signal
(2x) but the receiver has more error control since the expected input must show a change at the center of a bit cell.
An example of Manchester encoded sequence is: the byte 0xB1 or 10110001 encodes to 10 01 10 10 01 01 01 10,
assuming the default polarity of the encoder. Figure 32-8 illustrates this coding scheme.
Figure 32-8. NRZ to Manchester Encoding
D0 D1 D2 D3 D4 D5 D6 D7
TXD
Start
Bit
Parity
Bit
Stop
Bit
Example: 8-bit, Parity Enabled One Stop
Baud Rate
Clock
D0 D1 D2 D3 D4 D5 D6 D7
TXD
Start
Bit
Parity
Bit
Stop
Bit
Baud Rate
Clock
Start
Bit
Write
US_THR
D0 D1 D2 D3 D4 D5 D6 D7
Parity
Bit
Stop
Bit
TXRDY
TXEMPTY
NRZ
encoded
data
Manchester
encoded
data
10110001
Txd