PLL Clock Jitter Impact on CAN Precision in Auto MCU – KBA222406

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Question:

When PLL clock used for a CAN peripheral, how do I verify whether the PLL clock jitter meets the CAN precision specification?

 

Answer:

Use the following equation to evaluate the impact of PLL Jitter on CAN precision:

dfjitter = tjitter / (10×NBT),

where tjitter is the specified PLL clock jitter as the absolute interval ( CAN PLL Jitter provided in product datasheet) and NBT is the Nominal Bit Time (the inverse of a given CAN bit rate). The result of  dfjitter is the drift equivalent and should be added to the oscillator drift value of the crystal or resonator:

 

dftotal = dfjitter + dfoscillator .

 

Here is an example to check whether the PLL clock of S6J3120 meets the CAN precision specification.

 

Example:

CAN bus bit rate = 500 kbps,

CAN PLL Jitter = ±10ns  (tPJ from AC Characteristics of the S6J3120 series datasheet),

Oscillator drift of external crystal  dfoscillator = ±0.01% (generally provided by the crystal supplier),

Allowed CAN node frequency tolerance   df   = ±0.3%.

 

The relative PLL Jitter interval ±10 ns results in an absolute intervaltjitter = 20ns. The 500 kbps bit rate results in NBT = (1/500 kbps) = 2 µs.

 

  dfjitter= 20ns / 10×2us = ±0.1%,

  dftotal = 0.1% + 0.01% = ±0.11% < df  = ±0.3%,

 

Therefore, the CAN PLL Jitter spec of S6J3120 meets the CAN specification.