STR720 datasheet(259/401 Pages) STMICROELECTRONICS

259 / 401 page

STR720 - CONTROLLER AREA NETWORK (CAN)

259/401

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 oscillator tolerance range of the CAN systems is limited by that node with the lowest

tolerance range.

The calculation may show that bus length or bit rate have to be decreased or that the stability

of the oscillator frequency has to be increased in order to find a protocol compliant

configuration of the CAN bit timing.

The resulting configuration is written into the Bit Timing Register:

(Phase_Seg2-1)&(Phase_Seg1+Prop_Seg-1)&

(SynchronisationJumpWidth-1)&(Prescaler-1)

Example for Bit Timing at High Baudrate

In this example, the frequency of APB_CLK is 10 MHz, BRP is 0, the bit rate is 1 MBit/s.

100

= tAPB_CLK

delay of bus driver

delay of receiver circuit

delay of bus line (40m)

220

tProp

600

= 6 • tq

tSJW

100

= 1 • tq

tTSeg1

700

= tProp + tSJW

tTSeg2

200

= Information Processing Time + 1 • tq

tSync-Seg

100

= 1 • tq

bit time

1000

= tSync-Seg + tTSeg1 + tTSeg2

tolerance for APB_CLK

0.39

min PB1 PB2

()

2x 13x bit time

PB2

–

()

---------------------------------------------------------------

0.1

µs

2x 13x 1

µs 0.2µs

–

()

-----------------------------------------------------------

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