Datasheet

SAM9G45 [DATASHEET]
Atmel-6438O-ATARM-SAM9G45-Datasheet_08-Dec-15
608
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 32-43. 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 baud rate
 The oversampling (OVER = 0 => 16X or OVER = 1 => 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).
f
TOL_UNSYNCH
is the deviation of the real slave node clock from the nominal clock frequency. The LIN Standard
imposes that it must not exceed ±15%. The LIN Standard imposes also that for communication between two
nodes, their bit rate must not differ by more than ±2%. This means that the Baudrate_deviation must not exceed
±1%.
It follows from that, a minimum value for the nominal clock frequency:
Examples:
 Baudrate = 20 kbit/s, OVER = 0 (Oversampling 16X) => f
Nom
(min) = 2.64 MHz
 Baudrate = 20 kbit/s, OVER = 1 (Oversampling 8X) => f
Nom
(min) = 1.47 MHz
 Baudrate = 1 kbit/s, OVER = 0 (Oversampling 16X) => f
Nom
(min) = 132 kHz
 Baudrate = 1 kbit/s, OVER = 1 (Oversampling 8X) => f
Nom
(min) = 74 kHz
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 ID7Break Field
13 dominant bits (at 0)
Stop
Bit
Break
Delimiter
1 recessive bit
(at 1)
Synch Byte = 0x55
Baudrate_deviation 100
α[ 82Over–()β+ ] Baudrate×××
8f
SLAVE
×
----------------------------------------------------------------------------------------
×


%=
Baudrate_deviation 100
α[ 82Over–()β+ ] Baudrate×××
8
f
TOL_UNSYNCH
100
-------------------------------------


xF
Nom
×
----------------------------------------------------------------------------------------
×





%=
0.5– α +0.5 -1 β +1<<≤≤
f
Nom
min() 100
0.5 8 2 Over–()×× 1+[]Baudrate×
8
15–
100
---------
1+


× 1%×
-------------------------------------------------------------------------------------------
×





Hz=