( )
1
2/
1
+´=
reg
BRP
SYSCLKOUT
TQ
eCAN Registers
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SPRUI07–March 2020
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Controller Area Network (CAN)
13.8.12 Bit-Timing Configuration Register (CANBTC)
The CANBTC register is used to configure the CAN node with the appropriate network-timing parameters.
This register must be programmed before using the CAN module.
This register is write-protected in user mode and can only be written in initialization mode (see
Section 13.7.1).
NOTE: To avoid unpredictable behavior of the CAN module, the CANBTC register should never be
programmed with values not allowed by the CAN protocol specification and by the bit timing
rules listed in Section 13.7.1.1.
Figure 13-21. Bit-Timing Configuration Register (CANBTC)
31 24 23 16
Reserved BRP
reg
R-x RWPI-0
15 10 9 8 7 6 3 2 0
Reserved SJW
reg
SAM TSEG1
reg
TSEG2
reg
R-0 RWPI-0 RWPI-
0
RWPI-0 RWPI-0
LEGEND: RWPI = Read in all modes, write in EALLOW mode during initialization mode only; -n = value after reset
Table 13-20. Bit-Timing Configuration Register (CANBTC) Field Descriptions
Bit Field Value Description
31:24 Reserved Reads are undefined and writes have no effect.
23:16 BRP
reg
.7:0 Baud rate prescaler. This register sets the prescaler for the baud rate settings. The length of one
TQ is defined by:
where SYSCLKOUT is the frequency of the CAN module clock.
BRP
reg
denotes the "register value" of the prescaler; that is, value written into bits 23:16 of the
CANBTC register. This value is automatically enhanced by 1 when the CAN module accesses it.
The enhanced value is denoted by the symbol BRP (BRP = BRP
reg
+ 1). BRP is programmable
from 1 to 256.
Note: For the special case of BRP = 1, the Information Processing Time (IPT) is equal to 3 time
quanta (TQ). This is not compliant to the ISO 11898 Standard, where the IPT is defined to be less
than or equal to 2 TQ. Thus the usage of this mode (BRP
reg
= 0) is not allowed.
15:10 Reserved Reads are undefined. Must be written with all zeroes only.
9:8 SJW
reg
1:0 Synchronization jump width. The parameter SJW indicates, by how many units of TQ a bit is
allowed to be lengthened or shortened when resynchronizing.
SJW
reg
denotes the "register value" of the "resynchronization jump width;" that is, the value written
into bits 9:8 of the CANBTC register. This value is automatically enhanced by 1 when the CAN
module accesses it. This enhanced value is denoted by the symbol SJW.
SJW = SJW
reg
+ 1
SJW is programmable from 1 to 4 TQ. The maximum value of SJW is determined by the minimum
value of TSEG2 and 4 TQ.
SJW
(max)
= min [4 TQ, TSEG2]
7 SAM This parameter sets the number of samples used by the CAN module to determine the actual level
of the CAN bus. When the SAM bit is set, the level determined by the CAN bus corresponds to the
result from the majority decision of the last three values. The sample points are at the sample point
and twice before with a distance of ½ TQ.
1 The CAN module samples three times and make a majority decision. The triple sample mode
shall be selected only for bit rate prescale values greater than 4 (BRP > 4).
0 The CAN module samples only once at the sampling point.
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