Atmel SAM9X25, Manual

Page: 838 / 1145

838
11054A–ATARM–27-Jul-11
SAM9X25
838
11054A–ATARM–27-Jul-11
SAM9X25
• The Baud Rate
• 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).
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
Baudrate_deviation 100
α [
8 2 Over –
( ) β
+
]
Baudrate
× × ×
8 F
SLAVE
×
-----------------------------------------------------------------------------------------------
×
⎝ ⎠
⎛ ⎞
%
=
Baudrate_deviation 100
α [
8 2 Over –
( ) 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 =