
619
6437E–ATARM–23-Apr-13
SAM9M11
33.7.8.7
Slave Node Synchronization
The synchronization is done only in Slave node configuration. The procedure is based on time
measurement between falling edges of the Synch Field. The falling edges are available in dis-
tances of 2, 4, 6 and 8 bit times.
Figure 33-42. Synch Field
The time measurement is made by a 19-bit counter clocked by the sampling clock (see
SectionWhen the start bit of the Synch Field is detected the counter is reset. Then during the next 8
Tbits of the Synch Field, the counter is incremented. At the end of these 8 Tbits, the counter is
stopped. At this moment, the 16 most significant bits of the counter (value divided by 8) gives the
new clock divider (CD) and the 3 least significant bits of this value (the remainder) gives the new
fractional part (FP).
When the Synch Field has been received, the clock divider (CD) and the fractional part (FP) are
updated in the Baud Rate Generator register (US_BRGR).
Figure 33-43. Slave Node Synchronization
The accuracy of the synchronization depends on several parameters:
The nominal clock frequency (F
Nom) (the theoretical slave node clock frequency)
The Baudrate
The oversampling (Over=0 => 16X or Over=0 => 8X)
The following formula is used to compute the deviation of the slave bit rate relative to the master
bit rate after synchronization (F
SLAVE is the real slave node clock frequency).
Start
bit
Stop
bit
Synch Field
8 Tbit
2 Tbit
RXD
Baud Rate
Clock
LINIDRX
Synchro Counter
000_0011_0001_0110_1101
US_BRGR
Clcok Divider (CD)
0000_0110_0010_1101
US_BRGR
Fractional Part (FP)
101
Initial CD
Initial FP
Reset
Start
Bit
10101010
Stop
Bit
Start
Bit
ID0 ID1 ID2 ID3 ID4 ID5 ID6 ID7
Break Field
13 dominant bits (at 0)
Stop
Bit
Break
Delimiter
1 recessive bit
(at 1)
Synch Byte = 0x55