Rainbow Electronics AT91CAP9S250A - Manual (Page 537)

537
6264A–CAP–21-May-07
AT91CAP9S500A/AT91CAP9S250A
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 charac-
ter 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 for-
mat is modified and includes sync information.
Figure 35-10. Start Frame Delimiter
Drift Compensation
Drift compensation is available only in 16X oversampling mode. An hardware recovery system
allows a larger clock drift. To enable the hardware system, the bit in the USART_MAN register
must be set. If the RXD edge is one 16X clock cycle from the expected edge, this is considered
as normal jitter and no corrective actions is taken. If the RXD event is between 4 and 2 clock
cycles before the expected edge, then the current period is shortened by one clock cycle. If the
RXD event is between 2 and 3 clock cycles after the expected edge, then the current period is
lengthened by one clock cycle. These intervals are considered to be drift and so corrective
actions are automatically taken.
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