PIC18F4685-ISLASHSP datasheet(336/484 Pages) MICROCHIP

336 / 484 page

PIC18F2682/2685/4682/4685

DS39761B-page 334

Preliminary

23.9.1

EXTERNAL CLOCK, INTERNAL

CLOCK AND MEASURABLE JITTER

IN HSPLL-BASED OSCILLATORS

The microcontroller clock frequency generated from a

PLL circuit is subject to a jitter, also defined as Phase

Jitter or Phase Skew. For its PIC18 Enhanced micro-

controllers, Microchip specifies phase jitter (Pjitter) as

being 2% (Gaussian distribution, within 3 standard

deviations, see parameter F13 in Table 27-7) and Total

Jitter (Tjitter) as being 2 * Pjitter.

The CAN protocol uses a bit-stuffing technique that

inserts a bit of a given polarity following five bits with the

opposite polarity. This gives a total of 10 bits transmit-

ted without re-synchronization (compensation for jitter

or phase error).

Given the random nature of the jitter error added, it can

be shown that the total error caused by the jitter tends

to cancel itself over time. For a period of 10 bits, it is

necessary to add only two jitter intervals to correct for

jitter-induced error: one interval in the beginning of the

10-bit period and another at the end. The overall effect

is shown in Figure 23-5.

FIGURE 23-5:

EFFECTS OF PHASE JITTER ON THE MICROCONTROLLER CLOCK

AND CAN BIT TIME

Once these considerations are taken into account, it is

possible to show that the relation between the jitter and

the total frequency error can be defined as:

EQUATION 23-4:

where jitter is expressed in terms of time and NBT is the

Nominal Bit Time.

For example, assume a CAN bit rate of 125 Kb/s, which

gives an NBT of 8

μs. For a 16 MHz clock generated

from a 4x PLL, the jitter at this clock frequency is:

EQUATION 23-5:

The resultant frequency error is:

EQUATION 23-6:

Nominal Clock

Clock with Jitter

CAN bit Time

Phase Skew (Jitter)

CAN bit Jitter

with Jitter

Δf

jitter

NBT

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

jitter

NBT

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

16 MHz

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

0.02

×10

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

1.25ns

21.25

–

×10

()

–

×10

()

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

3.125

–

×10

0.0031%

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