STR720 datasheet(256/401 Pages) STMICROELECTRONICS

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STR720 - CONTROLLER AREA NETWORK (CAN)

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18.7.10.4 Oscillator Tolerance Range

The oscillator tolerance range was increased when the CAN protocol was developed from

version 1.1 to version 1.2 (version 1.0 was never implemented in silicon). The option to

synchronize on edges from dominant to recessive became obsolete, only edges from

recessive to dominant are considered for synchronization. The only CAN controllers to

implement protocol version 1.1 have been Intel 82526 and Philips 82C200, both are

superseded by successor products. The protocol update to version 2.0 (A and B) had no

influence on the oscillator tolerance.

The tolerance range df for an oscillator frequency fosc around the nominal frequency fnom is:

It depends on the proportions of Phase_Seg1, Phase_Seg2, SJW, and the bit time. The

maximum tolerance df is the defined by two conditions (both shall be met):

It has to be considered that SJW may not be larger than the smaller of the Phase Buffer

Segments and that the Propagation Time Segment limits that part of the bit time that may be

used for the Phase Buffer Segments.

The combination Prop_Seg = 1 and Phase_Seg1 = Phase_Seg2 = SJW = 4 allows the

largest possible oscillator tolerance of 1.58%. This combination with a Propagation Time

Segment of only 10% of the bit time is not suitable for short bit times; it can be used for bit

rates of up to 125 kBit/s (bit time = 8

µs) with a bus length of 40 m.

1df

–

() f

nom

•

osc

1df

() f

nom

•

≤≤

I: df

min Phase_Seg1 Phase_Seg2

()

13 bit_time

Phase_Seg2

–

⋅

()

⋅

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

≤

II: df

SJW

20 bit_time

⋅

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

≤

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