2.12 Bit-Timing Configuration Register (CANBTC)
(
)
1
1
+
´
=
reg
BRP
SYSCLKOUT
TQ
Bit-Timing Configuration Register (CANBTC)
www.ti.com
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
3.6.1).
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 3.1.1.
Figure 2-12. 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-
RWPI-0
RWPI-0
0
LEGEND: RWPI = Read in all modes, write in EALLOW mode during initialization mode only; -
n
= value after reset
Table 2-12. 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. Note that the CAN module is
clocked at the same frequency as the CPU (at SYSCLKOUT).
BRP
reg
denotes the "register value" of the prescaler; i.e., 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
Reserved
Reads are undefined and writes have no effect.
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;" i.e., 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]
40
eCAN Registers
SPRU074F – May 2002 – Revised January 2009
