747
SAM4CP [DATASHEET]
43051E–ATPL–08/14
Figure 36-8.
Preamble Patterns, Default Polarity Assumed
A start frame delimiter is to be configured using the ONEBIT bit in the US_MR. It consists of a user-defined pattern that
indicates the beginning of a valid data.
Figure 36-9
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 three 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 bit in the US_MR 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 bit in US_MR must be set to 1. In this case, the MODSYNC bit in the US_MR is bypassed and the sync
configuration is held in the TXSYNH in the US_THR. The USART character format is modified and includes sync
iinformation.
Figure 36-9.
Start Frame Delimiter
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
Manchester
encoded
data
Txd
SFD
DATA
One bit start frame delimiter
Preamble Length
is set to 0
Manchester
encoded
data
Txd
SFD
DATA
Command Sync
start frame delimiter
Manchester
encoded
data
Txd
SFD
DATA
Data Sync
start frame delimiter