PIC18F26K80-I Datasheet, PDF (447/622 Pages) Microchip Technology – 28/40/44/64-Pin, Enhanced Flash Microcontrollers with ECAN™ and nanoWatt XLP Technology

PIC18F66K80 FAMILY
27.9.1
EXTERNAL CLOCK, INTERNAL
CLOCK AND MEASURABLE JITTER
IN HS-PLL 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 (P_jitter) as
being 2% (Gaussian distribution, within 3 standard
deviations, see Parameter F13 in Table 31-7) and Total
Jitter (T_jitter) as being 2 * P_jitter.
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 resynchronization (compensation for jitter
or phase error).
Given the random nature of the added jitter error, 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 27-5.
FIGURE 27-5:
EFFECTS OF PHASE JITTER ON THE MICROCONTROLLER CLOCK
AND CAN BIT TIME
Nominal Clock
Clock with Jitter
Phase Skew (Jitter)
CAN Bit Time
with Jitter
CAN Bit Jitter
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 27-4: JITTER AND TOTAL
FREQUENCY ERROR
f = _1----0---^T-_--^j-ⁱ--^t_N-^t-^e--B^r----T-- = _1--2--0--^--_---^P--N--^j-ⁱ-B-^t-^t-^e-T-^r-
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 27-5: 16 MHz CLOCK FROM 4x
PLL JITTER:
2%  _{1---6-----M-1----H-----z-} = _{1---6-0---}^{.--0-1--2-0---}₆- = 1.25ns
and resultant frequency error is:
EQUATION 27-6: RESULTANT FREQUENCY
ERROR:
-2--1--^-0----^--1---_^.--2-8--5-_--^-1--1-0--0-_–--^–-₆--⁹_---^= 3.12510^–5= 0.0031%
 2010-2012 Microchip Technology Inc.
DS39977F-page 447