Datasheet
SAM9G20
DS60001516A-page 454 2017 Microchip Technology Inc.
bility, 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 31-9: Preamble Patterns, Default Polarity Assumed
A start frame delimiter is to be configured using the ONEBIT field in the US_MR. It consists of a user-defined pattern that indicates the
beginning of a valid data. Figure 31-10 illustrates these patterns. If the start frame delimiter, also known as start bit, is one bit, (ONEBIT
at 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 at 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 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 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. 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