Datasheet

783
SAM3X / SAM3A [DATASHEET]
Atmel-11057C-ATARM-SAM3X-SAM3A-Datasheet_23-Mar-15
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 pre-
defined 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 35-9 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.
Figure 35-9. Preamble Patterns, Default Polarity Assumed
A start frame delimiter is to be configured using the ONEBIT field in the US_MR register. It consists of a user-
defined pattern that indicates the beginning of a valid data. Figure 35-10 illustrates these patterns. If the start
frame delimiter, also known as start bit, is one bit, (ONEBIT to 1), a logic zero is Manchester encoded and
indicates that a new character is being sent serially on the line. If the start frame delimiter is a synchronization
pattern also referred to as sync (ONEBIT to 0), a sequence of 3 bit times is sent serially on the line to indicate the
start of a new character. The sync waveform is in itself an invalid Manchester waveform as the transition occurs at
the middle of the second bit time. Two distinct sync patterns are used: the command sync and the data sync. The
command sync has a logic one level for one and a half bit times, then a transition to logic zero for the second one
and a half bit times. If the MODSYNC field in the US_MR register is set to 1, the next character is a command. If it
is set to 0, the next character is a data. When direct memory access is used, the MODSYNC field can be
immediately updated with a modified character located in memory. To enable this mode, VAR_SYNC field in
US_MR register must be set to 1. In this case, the MODSYNC field in US_MR is bypassed and the sync
configuration is held in the TXSYNH in the US_THR register. The USART character format is modified and
includes sync information.
Manchester
encoded
data
Txd
SFD DATA
8 bit width "ALL_ONE" Preamble
Manchester
encoded
data
Txd
SFD
DATA
8 bit width "ALL_ZERO" Preamble
Manchester
encoded
data
Txd
SFD
DATA
8 bit width "ZERO_ONE" Preamble
Manchester
encoded
data
Txd
SFD
DATA
8 bit width "ONE_ZERO" Preamble