at91sam9263 ATMEL Corporation, at91sam9263 Datasheet - Page 665

• Manufacturer Part Number: at91sam9263
• Description: At91 Arm Thumb Microcontrollers
• Manufacturer: ATMEL Corporation

Figure 37-5. CAN Bit Timing
6249D–ATARM–20-Dec-07
CAN Clock
MCK
SYNC_
t
CSC
SEG
The SAMPLE POINT is the point in time at which the bus level is read and interpreted as the
value of that respective bit. Its location is at the end of PHASE_SEG1.
SJW: ReSynchronization Jump Width.
The ReSynchronization Jump Width defines the limit to the amount of lengthening or shortening
of the Phase Segments.
SJW is programmable to be the minimum of PHASE SEG1 and 4 TQ.
If the SMP field in the CAN_BR register is set, then the incoming bit stream is sampled three
times with a period of half a CAN clock period, centered on sample point.
In the CAN controller, the length of a bit on the CAN bus is determined by the parameters (BRP,
PROPAG, PHASE1 and PHASE2).
The time quantum is calculated as follows:
Note: The BRP field must be within the range [1, 0x7F], i.e., BRP = 0 is not authorized.
To compensate for phase shifts between clock oscillators of different controllers on the bus, the
CAN controller must resynchronize on any relevant signal edge of the current transmission. The
resynchronization shortens or lengthens the bit time so that the position of the sample point is
shifted with regard to the detected edge. The resynchronization jump width (SJW) defines the
maximum of time by which a bit period may be shortened or lengthened by resynchronization.
Example of bit timing determination for CAN baudrate of 500 Kbit/s:
PROP_SEG
t
PRS
MCK = 48MHz
CAN baudrate= 500kbit/s => bit time= 2us
t
t
BIT
t
CSC
t
t
PRS
PHS2
PHS1
t
SJW
=
=
=
t
CSC
=
=
=
(
t
BRP
CSC
NOMINAL BIT TIME
t
t
t
CSC
CSC
CSC
PHASE_SEG1
+
t
×
PRS
+
×
×
×
t
(
PHS1
1
PROPAG
(
(
(
) MCK
PHASE2
SJW
PHASE1
+
⁄
t
PHS1
+
1
+
)
+
+
+
t
1
PHS2
1
1
AT91SAM9263 Preliminary
)
)
)
Sample Point
PHASE_SEG2
t
PHS2
Transmission Point
665