UM0306 Controller area network (bxCAN)
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Its operation may be explained simply by splitting nominal bit time into three segments as
follows:
● Synchronization segment (SYNC_SEG): a bit change is expected to occur within this
time segment. It has a fixed length of one time quantum (1 x t
CAN
).
● Bit segment 1 (BS1): defines the location of the sample point. It includes the
PROP_SEG and PHASE_SEG1 of the CAN standard. Its duration is programmable
between 1 and 16 time quanta but may be automatically lengthened to compensate for
positive phase drifts due to differences in the frequency of the various nodes of the
network.
● Bit segment 2 (BS2): defines the location of the transmit point. It represents the
PHASE_SEG2 of the CAN standard. Its duration is programmable between 1 and 8
time quanta but may also be automatically shortened to compensate for negative
phase drifts.
The resynchronization Jump Width (SJW) defines an upper bound to the amount of
lengthening or shortening of the bit segments. It is programmable between 1 and 4 time
quanta.
A valid edge is defined as the first transition in a bit time from dominant to recessive bus
level provided the controller itself does not send a recessive bit.
If a valid edge is detected in BS1 instead of SYNC_SEG, BS1 is extended by up to SJW so
that the sample point is delayed.
Conversely, if a valid edge is detected in BS2 instead of SYNC_SEG, BS2 is shortened by
up to SJW so that the transmit point is moved earlier.
As a safeguard against programming errors, the configuration of the Bit Timing Register
(CAN_BTR) is only possible while the device is in STANDBY mode.
Note: For a detailed description of the CAN bit timing and resynchronization mechanism, please
refer to the ISO 11898 standard.
Figure 130. Bit timing
SYNC_SEG BIT SEGMENT 1 (BS1) BIT SEGMENT 2 (BS2)
NOMINAL BIT TIME
1 x t
q
t
BS1
t
BS2
SAMPLE POINT TRANSMIT POINT
NominalBitTime 1 t
q
× t
BS1
t
BS2
++=
with:
t
BS1
= t
q
x (TS1[3:0] + 1) ,
t
BS2
= t
q
x (TS2[2:0] + 1),
t
q
= ( BRP[9:0] + 1 ) x t
PCLK
t
PCLK
= time period of the APB clock,
BRP[9:0], TS1[3:0] and TS2[2:0] are defined in the CAN_BTR Register.
BaudRate
1
NominalBitTime
----------------------------------------------=
where t
q
refers to the Time quantum
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