LM3S8938 Datasheet, PDF (404/550 Pages) List of Unclassifed Manufacturers – Microcontroller

Controller Area Network (CAN) Module
The minimum nominal length of Phase_Seg2 has to be regarded as well. Phase_Seg2 may not
be shorter than the CAN controller's Information Processing Time, which is, depending on the actual
implementation, in the range of [0..2] tq.
The length of the Synchronization Jump Width is set to its maximum value, which is the minimum
of 4 and Phase_Seg1.
The oscillator tolerance range necessary for the resulting configuration is calculated by the formula
given below:
(1 -df) x fnom <= fosc <= (1+ df) x fnom
where:
■ df = maximum tolerance of oscillator frequency
■ fosc = actual oscillator frequency
■ fnom = nominal oscillator frequency
Maximum frequency tolerance must take into account the following formulas:
df <= (Phase_Seg1,Phase_Seg2)min/ 2 x (13 x tbit - Phase_Seg2)
dfmax = 2 x df x fnom
where:
■ Phase_Seg1 and Phase_Seg2 are from Table 16-3 on page 402
■ tbit = Bit Time
■ dfmax = maximum difference between two oscillators
If more than one configuration is possible, that configuration allowing the highest oscillator tolerance
range should be chosen.
CAN nodes with different system clocks require different configurations to come to the same bit
rate. The calculation of the propagation time in the CAN network, based on the nodes with the
longest delay times, is done once for the whole network.
The CAN system's oscillator tolerance range is limited by the node with the lowest tolerance range.
The calculation may show that bus length or bit rate have to be decreased or that the oscillator
frequencies' stability has to be increased in order to find a protocol-compliant configuration of the
CAN bit timing.
The resulting configuration is written into the CAN Bit Timing (CANBIT) register :
(Phase_Seg2-1)&(Phase_Seg1+Prop_Seg-1)&(SynchronizationJumpWidth-1)&(Prescaler-1)
16.4.15.1 Example for Bit Timing at High Baud Rate
In this example, the frequency of CAN_CLK is 10 MHz, BRP is 0, and the bit rate is 1 Mbps.
tq 100 ns = tCAN_CLK
delay of bus driver 50 ns
delay of receiver circuit 30 ns
delay of bus line (40m) 220 ns
404
June 14, 2007
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