Datasheet

756
SAM4S Series [DATASHEET]
11100F–ATARM–29-Jan-14
Figure 36-7. Transmitter Status
36.7.3.2 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 register 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 36-8 illustrates this coding scheme.
Figure 36-8. NRZ to Manchester Encoding
The Manchester encoded character can also be encapsulated by adding both a configurable preamble and a start frame
delimiter pattern. Depending on the configuration, the preamble is a training sequence, composed of a predefined pattern
with a programmable length from 1 to 15 bit times. If the preamble length is set to 0, the preamble waveform is not
generated prior to any character. The preamble pattern is chosen among the following sequences: ALL_ONE,
ALL_ZERO, ONE_ZERO or ZERO_ONE, writing the field TX_PP in the US_MAN register, the field TX_PL is used to
configure the preamble length. Figure 36-9 illustrates and defines the valid patterns. To improve flexibility, the encoding
scheme can be configured using the TX_MPOL field in the US_MAN register. If the TX_MPOL field is set to zero
(default), a logic zero is encoded with a zero-to-one transition and a logic one is encoded with a one-to-zero transition. If
the TX_MPOL field is set to one, a logic one is encoded with a one-to-zero transition and a logic zero is encoded with a
zero-to-one transition.
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