Datasheet

312
AT32UC3A
Figure 26-9. Preamble Patterns, Default Polarity Assumed
A start frame delimiter is to be configured using the ONEBIT field in the MR register. It consists
of a user-defined pattern that indicates the beginning of a valid data. Figure 26-10 on page 313
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 seri-
ally 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 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 MR reg-
ister must be set to 1. In this case, the MODSYNC field in MR is bypassed and the sync
configuration is held in the TXSYNH in the THR register. The USART character format is modi-
fied 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
32058K
AVR32-01/12