LM3S2939 Datasheet, PDF (566/749 Pages) Texas Instruments – Stellaris® LM3S2939 Microcontroller

Controller Area Network (CAN) Module
■ fnom = Nominal oscillator frequency
(^M1^ax−^(i1md^u−f^m)^dfr×^feq)f^u×n^eno^fcm^yntoo≤^lmerf^a≤o^ncs^efc^omsu≤^cst (^t1^a≤ke+⁽¹ⁱⁿd^t+of^a)^dc×^fco)f^un×n^to^tfhm^{ne ofomllowing formulas:}
df ^d≤^{f (P≤}₂^ha(_×^Ps₂^h₍^e₁^a_×3^s_₍^e_1×^s₃^e__tgb×^s1_i^et,_t^gbP1_i−^h,_t^aP_Ps−^h_he^a_aPss^__heea^s_s^e__{_e}^gs_S^2e_{_e}^)g_g^m_S²_2ie⁾₎ⁿ_g^m2i₎ⁿ
df ^dm^fax^ma=^x 2⁼ײd^×f ^d×^ff^×no^fm^nom
where:
■ Phase1 and Phase2 are from Table 15-3 on page 564
■ 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.
15.3.16.1 Example for Bit Timing at High Baud Rate
In this example, the frequency of CAN clock is 8 MHz, and the bit rate is 1 Mbps.
bit time = 1 µs = n * t_q = 8 * t_q
t_q = 125 ns
t_q = (Baud rate Prescaler)/CAN Clock
Baud rate Prescaler = t_q * CAN Clock
Baud rate Prescaler = 125E-9 * 8E6 = 1
tSync = 1 * t_q = 125 ns
\\fixed at 1 time quanta
delay of bus driver 50 ns
delay of receiver circuit 30 ns
delay of bus line (40m) 220 ns
tProp 375 ns = 3 * t_q
\\375 is next integer multiple of t_q
bit time = tSync + tTSeg1 + tTSeg2 = 8 * t_q
bit time = tSync + tProp + tPhase 1 + tPhase2
tPhase 1 + tPhase2 = bit time - tSync - tProp
tPhase 1 + tPhase2 = (8 * t_q) - (1 * t_q) - (3 * t_q)
tPhase 1 + tPhase2 = 4 * t_q
566
June 18, 2012
Texas Instruments-Production Data