Datasheet

773
SAM9X35 [DATASHEET]
11055E–ATARM–10-Mar-2014
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 39-9 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 39-9. 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 39-10 illustrates these patterns. If the start frame delimiter,
also known as the 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