Bit-time = TSEG1 + TSEG2 + 1
Bit-time = (TSEG1
reg
+ 1) + (TSEG2
reg
+ 1) + 1
TimeBit
2/
rateBit
´
=
BRP
SYSCLKOUT
eCAN Configuration
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Controller Area Network (CAN)
13.7.1.2 CAN Bit Rate Calculation
Bit-rate is calculated in bits per second as follows:
Where bit-time is the number of time quanta (TQ) per bit. SYSCLKOUT is the device clock frequency,
which is the same as the CPU clock frequency. BRP is the value of BRP
reg
+ 1 (CANBTC.23-16).
Bit-time is defined as follows:
In the above equation TESG1
reg
and TSEG2
reg
represent the actual values written in the corresponding
fields in the CANBTC register. The parameters TSEG1
reg
, TSEG2r
eg
, SJW
reg
, and BRP
reg
are automatically
enhanced by 1 when the CAN module accesses these parameters. TSEG1, TSEG2 and SJW, represent
the values as applicable per Figure 13-7.
13.7.1.3 Bit Configuration Parameters for 75 -MHz CAN Clock
This section provides example values for the CANBTC bit fields for some bit rates and sampling points.
Note that these values are for illustrative purposes only. In a real-world application, parameters such as
the oscillator accuracy and the propagation delay introduced by various entities such as the network
cable, transceivers/ isolators must be taken into account before choosing the timing parameters.
Table 13-6 shows how the BRP
reg
field may be changed to achieve different bit rates with a BT of 15 for a
80% SP.
Table 13-6. BRP Field for Bit Rates (BT = 15, TSEG1
reg
= 10, TSEG2
reg
= 2, Sampling Point = 80%)
CAN Bus Speed BRP CAN Module Clock
1 Mbps BRP
reg
+1 = 5 15 MHz
500 kbps BRP
reg
+1 = 10 7.5 MHz
250 kbps BRP
reg
+1 = 20 3.75 MHz
125 kbps BRP
reg
+1 = 40 1.875 MHz
100 kbps BRP
reg
+1 = 50 1.5 MHz
50 kbps BRP
reg
+1 = 100 0.75 MHz
Table 13-7 shows how to achieve different sampling points with a BT of 15.
Table 13-7. Achieving Different Sampling Points With a BT of 15
TSEG1
reg
TSEG2
reg
SP
10 2 80%
9 3 73%
8 4 66%
7 5 60%
NOTE: For a SYSCLKOUT of 150 MHz, the lowest bit-rate that can be achieved is 11.719 kbps.
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